breeding season habitat use of philippine hawk owl ninox

BREEDING SEASON HABITAT USE OF PHILIPPINE
HAWK OWL NINOX PHILIPPENSIS SPILONOTA IN
CEBU ISLAND, PHILIPPINES
PHILIP GODFREY JAKOSALEM
A Dissertation submitted to
The School
of BiologicalHABITAT
Sciences of the University
of East
Anglia
BREEDING
SEASON
USE OF
PHILIPPINE
in partial fulfillment of the requirements for the degree of
Master of Science in Applied Ecology and Conservation
HAWK OWL NINOX PHILIPPENSIS SPILONOTA IN
September 2011
CEBU ISLAND, PHILIPPINES
ACKNOWLEDGEMENTS
I sincerely appreciate the advice, encouragement, support, guidance and direction
provided by my supervisors Dr Jennifer Gill and Prof Nigel Collar. Their helpful comments
greatly improved this paper. It’s been a great pleasure and honor to work with both of you.
I thank my fellow course mates in Applied Ecology and Conservation for the friendship,
hardship, support and advised, with special thanks extended to Alaa Mohamed Ali, Maxim
Koshkin, Philip de Pous and Robert Hawkes for assisting me in some analysis, GIS and
smart graph.
This project was supported funded by the Chester Zoo Studentship 2011(Dr. Roger
Wilkinson and Ms. Yvette Foulds), The World Owl Trust (Tony Warburton) Philippine
Biodiversity Conservation Foundation, Birdlife International, Wetlands Trust and
University of East Anglia this project will not be successful without their support. I’m also
thankful to Dr Iain Barr for the additional transmitters and advice during the planning stage.
This work will not be accomplished without the support and permission granted from the
Department of Environment and Natural Resources, the municipalities of Alcoy, Dalaguete,
Argao, Boljoon and the community of Tabunan and Nug-as. I thanks Protected Areas and
Wildlife Department staff Reynaldo Yray for his assistance. My colleagues William L.R.
Oliver, Lisa Marie, Paguntalan, Kail Zingapan, Orlyn Orlanes-Roxas, Edgar Lillio and
Icarus, for their assistance, enthusiasm, and humor.
I am grateful to the numerous flied assistant who assisted me in this research, Pedro
Villarta, Fernando Anore, Ryan Anore, Teodoro Amaca and Tito Amaca for the skills and
finesse. I’m also thankful to various volunteers Fe Anore, Romele Sandot, Sherry Paul
Ramayla, Dexcem Pantinople, and Ara Patrice Kintanar for their assistance and dedication
during the study. I thank my family for helping me and facilitating additional data needed
for my study.
Finally, I would like to thank Ford International Fellowship Program for the
opportunity and unwavering encouragement and support on my fellowship.
ABSTRACT
1.
The largely deforested island of Cebu, Philippines, hosts a forest-dwelling hawkowl identified in the literature as Ninox philippensis spilonota but which is in fact a
Cebu island endemic species or subspecies, soon to be named.
2.
The five largest of 11 remaining forest patches on Cebu were surveyed between 28
March and 16 June 2011 involving 64 post-sunset 500-m long walked transects with
three playback stations. Within the same period radio-telemetry studies were
conducted on 10 owls but only for 3–5 days per owl as they removed the tailmounted transmitters. Habitat measurements were taken within a circular plot 10 m
in radius at standard points along transects and at sites where birds were recorded.
3.
A total of 52 owls were located across all five forests (at 16 sites two owls
responded together to playback) but only the largest forest, Alcoy, contained enough
transects for statistical analysis. Owls were detected in equal densities in forest
interior, forest edge and forest-plantation mix, and on ridges and in gullies. Wind
was the single variable measured that affected owl distribution, with the probability
of owl occupancy decreasing with increasing wind speed.
4.
Home ranges were calculated for 95% Minimum Convex Polygon, 95% kernel and
50% kernel, the 95% kernel yielding a mean 8.37 ha. Suitable forest studied covers
roughly 1,670 ha, with six unstudied forests totalling 250 ha, so assuming a pair
every 10 ha the global population of the Cebu Hawk-owl may be 192 pairs. If birds
failed to cover all their home ranges while being radio-tracked, home ranges may be
larger and the global population smaller.
5.
Continuing habitat degradation triggers IUCN Red List category Endangered.
Experiments with nest-box deployment are an obvious next step, provided this does
not increase predator pressure on other rare species.
TABLE OF CONTENTS
Introduction………………………………………………………………………
1
Decline of tropical forest……………………………………………………... 1
Decline of Philippine forest ………………………………………………….. 1
Fragmented forest of Cebu……………………………………………………
2
Philippine Hawk Owl in Cebu………………………………………………..
3
Methods…………………………………………………………………………..
4
Study species………………………………………………………………….
4
Philippine Hawk Owl Ninox philippnensis spilonota………………………..
4
Study sites…………………………………………………………………….
4
Alcoy Forest………………………………………………………………..
4
Upper Beciril Forest and Nangka Forest………………………………….. 5
Argao Forest……………………………………………………………….
5
Dalaguete Forest…………………………………………………………... 5
Tabunan Forest…………………………………………………………….
6
Species capture, marking and movements……………………………………
7
Owl detection transect…………………………………………………….
7
Habitat characteristic…………………...………………………………… 8
Radio-tracking of Philippine hawk-owls..………………………………..
9
Dietary composition and nest site characteristics………………………...
10
Population estimates……………………………………………………… 11
Data analysis………………………………………………………………….
11
Results……………………………………………………………………………
12
Distribution of hawk-owls on Cebu Island…………………………………..
12
Factors influence the distribution of Philippine Hawk Owl…………………. 14
Home range characteristics...……………………………………………..
15
Foraging habitat…………………………………………………………..
19
Roosting sites……………………………………………………………..
19
Nest site characteristic……………………………………………………. 20
Dietary composition………………………………………………………
21
Discussions………………………………………………………………………. 22
Conclusion……………………………………………………………………….. 25
Conservation implication…………………………………………………….
26
Reference………………………………………………………………………… 27
List of tables and figures
List of tables
Table 1. Name, unit and description of each of the habitat characteristics
recorded in survey plots (314 m2) on hawk-owl detection transects in five
forests on Cebu Island, Philippines. The surveys in which each variable was
recorded (T = transect surveys, R = locations of radio-tracked individuals) are
also shown……………………………………………………………………….. 9
Table 2. The total number of hawk owls and the estimated number of pairs
recorded on transects through different habitats in five forest on Cebu Island.....
12
Table 3. Habitat characteristics of the five forests on Cebu Island on which
hawk-owl surveys were carried out. The means (± SE) of each variable within
circular plots (314 m2) on separate transects are shown, and the number of
transects is given in parentheses………………………………………………… 13
Table 4. Differences in the habitat characteristics of the hawk-owl survey
transects located on ridges (22 transects) and gulleys (19 transects) within
Alcoy Forest, Cebu Island. The means of each variable within circular plots
(314 m2) on separate transects are shown. (see Table 1 for details of habitat
variables), and significant differences are highlighted in bold…………………
13
Table 5. . Estimates of home range size (from 95% minimum convex polygon
(MCP), 95% kernel and 50% kernel home range analyses) of adult male and
female and juvenile hawk owls radio-tracked within Alcoy Forest, Cebu Island.
Names in bold indicate those with home ranges in the forest interior, and mean
home range sizes for forest interior and forest edge are given…………………..
18
Table 6. Comparison between different radio fixes between different habitats
during radio telemetry……………………………………………………………
19
Table 7. Comparison between difference variable in forest and forest edge
home range within the study sites………………………………………………..
20
Table 8. Characteristics of the two hawk-owl nests that was located within
Alcoy Forest, Cebu Island, one of which was active in 2011, and the one which
had been used on 2005 but was inactive in 2011………………………………...
Table 9. Variation of potential prey composition of Philippine Hawk Owl as
21
observed as reported and observed by Alcoy Forest Wardens. x= reported;
x*=observed and – not reported…………………………………………………. 22
List of figures
Figure 1. Map of Cebu showing the five study sites……………………………..
6
Figure 2. The effect of wind-speed on the probability (dotted curve) of
occupancy of hawk-owls on transects in Alcoy Forest, Cebu Island predicted
from a logistic regression model. The wind speeds recorded on occupied (open
bars) and unoccupied (filled bars) transects are shown, together with their
frequencies……………………………………………………………………….. 14
Figure 3. Comparison of 95% minmum convex polygon (MPC) home range
between forest and forest edge radio tagged owl………………………………...
17
Figure 4. Home ranges of five adult male (pale grey), four adult female (dark
grey, within main forest) and one juvenile (dark grey out with main forest)
radio-tagged hawk-owls in Alcoy Forest, Cebu Island. Inner lines indicate 50%
and outer lines indicate 95% kernel estimates of home ranges…………………..
18
INTRODUCTION
Decline of tropical forest
Tropical forests are amongst the most threatened ecosystems in the world. Most tropical
forests are found in Asia, Americas and Africa. Asia has the most tropical moist forest
which totals 18%, while 58% and 25% are located in Americas and Africa, respectively.
However, the rate of deforestation has been increasing since 1990s, with 0.8-0.9% per year
in Asia and 0.4-0.5% in Americas and Africa. Among the Asian regions, Southeast Asia
has suffered the highest rate of deforestation with annual net losses of forest of up to 0.9
million hectares per year (Laurance 2007; FAO 2011). Logging is one of the major causes
of the decline of the forest in Southeast Asia, as well as conversion of forest into agriculture
which has even more damaging impact, particularly in countries like Singapore and
Philippines (Sodhi et al, 2004; Pulhin 2006).
Decline of Philippine Forest
The Philippines was once covered with tropical rainforest over 90% of the entire
archipelago. A total of 27 million hectares of tropical forest covered the entire country
during the start of the Spanish colonization in 1521. The country experienced widespread
degradation and deforestation over 19th century (Pulhin et al, 2006). The major causes were
increased growth of the human population and associated conversion of forest to agriculture
during the Spanish colonization (Kummer 1992). In addition to agriculture, the Spanish
regime established shipyards which required timber, particularly in the islands of Masbate,
Cavite, Pangasinan, Albay, Mindoro, Marinduque and Cebu (Vitug 1993). After the
Spanish colonization, America ruled the country and about 70% of the Philippines were
forested. In this period, the Americans introduced logging and exported Philippine timber
to the world market. After two decades in operation, about 163 saw mills operated
throughout the country, which contributed to the decline from 70% to 57% of the total
forest cover (Pulhin et al, 2006; ESSC 1999; de la Cruz 1941). The logging halted during
the Japanese occupation; however exploitation and extraction of forest products continued
and was concentrated in more accessible areas, particularly in the islands of Bohol and
Cebu in central Philippines.
1
Fragmented forest of Cebu
The island of Cebu is located in the centre of the Philippines. It is one of the largest islands
in the Philippines; with a total land area of 5,088 sq. km. Cebu has been one of the
progressive islands in the 19th country since 1900 (Rabor 1959), which has contributed to
the destruction of natural forest and high demand of wood fuels and household used. The
extent of forest exploitation meant that Cebu was viewed as the worst environmental
disaster in the country (Bensel 2008), and resulted in the extinction of many species of
plants and animals (McGregor 1904; Rabor 1952; Brooks et al, 1996). The remaining
natural forest habitats amounts to less than 0.3% of the original area and these are broken
into seven forest fragments. These forest fragments are widely distributed across the island
(Gonzalez et al, 1999; Paguntalan & Jakosalem 2008). The destruction of the forest is one
of the major causes of extinction of many species of wildlife in Cebu (Collar et al, 1999;
Brooks et al, 1996; Rabor 1959).
The island has been badly deforested since early 1900 (Brooks, et al, 1995; Collar
1998). The loss of forest has seriously affected the islands biodiversity. Cebu has three
endemic large mammals. Two of these shared with Negros Panay Faunal Region the
Visayan Spotted Deer Cervus alfredi and the Visayan Warty Pig Sus cebifrons were
declared extinct in the island (Grubb & Groves 1983; Gonzalez et al, 1999). Recent finding
of fossil remains in Cebu lead to the discovery a new species of buffalo, the Cebu Dwarf
Buffalo Bubalus cebuensis (Croft et al, 2006) which once inhabited the forest of Cebu.
There is also evidence of local extinction of birds in the island of Cebu; 39 of 79 forest
resident bird species are feared extinct on the island (Dickinson et al, 1991; Brooks et. al.
1995). However recent studies conducted in Cebu lead to the rediscovery of supposedly
extinct species, including the globally threatened Cebu Flowerpecker Dicaeum quadricolor
and Streak-breasted Bulbul I. s. monticola (Dutson 1993; Magsalay 1993; Brooks et al,
1995). Some larger species of doves, pigeons and raptors were also rediscovered, including
Pink-bellied Imperial-Pigeon D. poliocephala, Amethyst Brown Dove P. a. frontalis,
Philippine Serpent-Eagle S. holospilus, and the Philippine Hawk Owl N. philippensis
(Gonzalez et al, 1999; Paguntalan & Jakosalem 2008).
2
Philippine Hawk Owl in Cebu
The Philippine Hawk Owl Ninox philippensis is an endemic hawk owl (or boobooks) in the
Philippines. There are seven recognized subspecies which group in different categories
based on morphology. However, the species taxonomy is under review. Some authors
group these subspecies by location; the philippensis group comprised of N.p. philippensis
found in the island of Luzon, Polillio, Catanduanes, Marinduque, Samar and Leyte, the N.p.
centralis subspecies found in the islands of Siquijor, Panay, Guimaras and Negros and the
subspecies N.p. proxima and N.p. ticaoensis located in the island of Masbate and Ticao,
respectively. The second group is composed of the subspecies N.p. spilocephala are found
in the islands of Mindanao and Basilan, and the subspecies N.p. reyi is found in the island
of Sulu, Tawitawi and Bongao. The subspecies N.p. spilonota were found in the island of
Tablas, Sibuyan, Camiguin Sur and Cebu (König et al, 1999). Collar & Rasmussen (1998)
also suggested that this species should be grouped according to it plumage pattern namely
the bar-bellied group from the islands of Mindoro, Tablas, Sibuyan, Cebu Camiguin Sur
and Sulus and the streaked-breasted group composed of philippinesis and spilocephala.
Several authors have suggested that there are two or more species and additional new
subspecies in this group. König et al, (1999) suggested the subspecies spilocephala and
spilonata were viewed as separate species. Another taxonomical review suggested that
there are new species and new subspecies in this group like the subspecies spilonata may
be restricted to the three islands of Tablas, Sibuyan and Camiguin Sur while the species in
Cebu will be a full species (Rasmussen et al, in prep).
The subspecies spilonota was first described by Bourns & Worcester (1894). The
species is a forest dweller. It was recorded from lowland to montane forest (1600m ASL)
and remnant forest patches (Kennedy et al, 2000; Warburton 2009). It is locally common
all through its range. However, it was once feared extinct in Cebu where deforestation since
1900 has been severe. In surveys during 1950s and again in early 1990, the species was not
recorded in Cebu and it was considered extinct (Rabor 1959; Brooks et al, 1995). An
island-wide survey conducted in 1998 by Gonzalez et al. (1999) recorded the species in
three different forest patches in Cebu; Tabunan, Tuburan and Alcoy forest. A juvenile N.p.
spilocephala was caught by locals along the forest edge in Alcoy forest in 2003 and several
were heard calling in Alcoy and Tabunan forest confirmed the existence of the species in
the island (Paguntalan & Jakosalem 2008). Despite the rediscovery of this species there
only few study conducted about its taxonomy and ecological requirements. Very little
3
information exists about the ecology of N.p spilonota on Cebu, particularly its breeding
ecology, habitat associations, territory size, and population size. Such information is needed
to identify appropriate forest conservation measures. This study focused on patterns of
habitat use during the breeding season of N.p spilonota on Cebu Island, and aimed to
address four major objectives: 1) quantify the characteristics of habitat in sites with and
without owls; 2) estimate home range size if individual owls within the breeding season and
the extent to which the species uses forest edge habitat and secondary forest; 3) summarize
information on diet, particularly the contribution of prey from forest and farmland sources,
and nest site characteristics.
METHODS
Study species
Philippine Hawk Owl Ninox philippensis spilonota
The Philippine Hawk Owl N. philippensis spilonota is a nocturnal avian predator that is
widely distributed throughout the Philippine Islands, with the exception of Palawan. It is
one of the smallest owls in the Philippines. The species is locally common throughout its
range. It was listed as Least Concern by the IUCN but it is listed as a threatened species in
the Philippine Republic Act 9147 of 2001 (otherwise known as ―Wildlife Resources
Conservation and Protection Act‖).
Study sites
Alcoy Forest, Alcoy (9°52’N 123°30’E)
Alcoy Forest is lowland secondary limestone forest with elevations ranging from the
lowlands up to 960m ASL. The forest is comprised of native and endemic species of tree
including Artocarpus sp., Syzygium sp., Ficus spp., Casuarina rumphiana, Melia dubia,
Macaranga sp., Cinnamomum cebuense and Melastoma sp. (Paguntalan, 2009). The forest
is managed and protected by four local organizations through a Community-Based Forest
Management Agreement (CBFMA) with the Department of Environment and Natural
Resources (DENR). It covers more than 1600 ha including plantations and scrubland (Fig.
1). The largest forest patch totals up to 800 ha surrounded by plantations composed of
Mahogany Swietenia macrophylla, Gmelina arborea, Eucalyptus and Mountain Agoho
Casuarina rumphiana with Rattan Calamus sp. (Paguntalan 2009). Vegetable farms are
found at forest edges, in gullies and between exotic plantations.
4
Upper Beciril Forest (9°61’N 123°46’E) and Nangka Forest (9°68’N 123°43’E), Boljo-on
Boljo-on forest is composed of five fragments (Paguntalan 2009), with the largest fragment
located in Upper Beceril, Lower Beceril, Lunop and San Antonio range. This forest is
confined to river gullies and is estimated at 160 ha. Elevation ranges from 50 to 400m ASL.
The second largest fragment, estimated up to 130 ha, is located in Nangka and San Antonio
forest block connecting to Alcoy forest, with elevation ranges from 600 to 900m ASL.
Boljo-on is a secondary limestone forest composed of Ficus sp., Buchanania and Syzygium.
Agroforestry and vegetable farms are found along ridges and forest edges.
Argao Forest (9°54’N 123°32’E)
Mt Lantoy Forest in the Municipality of Argao is a lowland secondary forest with a typical
karst limestone formation with elevations up to 800m ASL (Fig. 1). There are some oldgrowth trees along the riverbanks and among the hills. Plantations of exotic trees like
Gmelina, Mahogany, Teak Tectona grandis and Acacia Samanea saman surround the
native vegetation. Fruit-bearing trees like Lanzones Lansium domisticum and Jackfruit
Artocarpus have also been planted along riverbanks and on slopes. Farmlands and grazing
areas dominate the landscape. Mt Lantoy is naturally bounded by the Argao River known
as Argao River Watershed Forest Reserve (Paguntalan 2009). The watershed reserve covers
some of the adjacent forest patches within the municipality.
Dalaguete Forest, Dalaguete (9°82’N 123°49’E)
Dalaguete Forest is made up of three separate forest fragments namely: Babayongan–
Bulak–Malones forest (500–800 m), Obo–Sacsac–Mantalongon (300–500 m) forest and the
Obong–Caliongan forest patch (60–200 m) (Paguntalan 2009). The natural limestone forest
is dominated by Vitex parviflora, Buchanania, Ficus and Syzygium spp. These forest types
are confined to very steep areas where farming is very difficult. The other dominant plant
species in the area are Nauclea, Pittosporum pentandrum, Ficus pseudopalma, Dracaena,
Sterculia philippensis, Leea manillensis, Leucosyke capitellata and Breynia. Large Ficus
species are commonly located on riverbanks. Vines are abundant, creating an illusion of
thick vegetation (Paguntalan 2009). Most of the forest patches are surrounded by vegetable
farms and scrubland (Fig. 1).
5
Tabunan Forest, Tabunan Cebu City (10°26’N 123°49’E)
The forest of Tabunan forms a thin, segmented strip of forest on steep limestone hillsides,
with closed-canopy areas of only less than 10 ha. It is part of the Central Cebu Protected
Landscape and managed by the Protected Area Management Board (PAMB). The elevation
of the area ranges from 400 to 880 m ASL. Common species of trees include Ficus species,
Homalanthus sp., Syzygium, Macaranga, Leea sp., Sterculia philippnensis, Dillenia sp.,
Leucosyke sp., Diospyrus philippensis and Mangifera sp. Farms abutting the forest
perimeter are mostly planted with high-value crops, vegetables, ornamentals and cut
flowers (Paguntalan 2009). A well-developed road from the Cebu Trans-central Highway
leads to a high concentration of village houses about 100 metres from the forest edge (Fig.
1).
Figure 1. Map of Cebu showing the five study sites.
6
Species capture, marking and movements
Fieldwork was conducted at five study sites on Cebu between 28 March and 16 June 2011
(with environmental data gathering by assistants into July), and was divided up into three
main components: nocturnal transects to determine owl presence or absence, specific
habitat assessments of known owl foraging and roosting sites, and home-range assessments
using radio-telemetry. In addition, incidental and anecdotal data were gathered on prey
items and nest-sites.
Owl detection transects
The presence or absence of owls was assessed in a total of 64 transects in five different
forests (41 in Alcoy, eight in Dalaguete, six in Boljo-on, five in Argao, four in Tabunan).
Transects were 500 m in length, were at least 100 m apart, and followed established trails
throughout the forests. Each transect was walked once soon after sunset with surveyors
listening constantly for owl calls, and three calling stations were established on each
transect (at 0, 250 and 500 m) at which playback of owl calls was carried out. Playback
consisted of a two-minute broadcast of pre-recorded calls of the Cebu Ninox (using an mp3
player with portable speakers); including the species’ double call, several screeches and
occasional chittering calls, followed by a two-minute listening period and finally a spotlight
search of the survey site. In Alcoy Forest, each transect was resurveyed on the following
night without playback, in order to confirm owl presence and absence (all owls were
relocated and no new owls were identified on these second transects). Transect locations
were stratified to ensure coverage of forest interior (at least 30 m from the forest edge; 27
transects), forest edge (within 30 m of the forest edge; 27 transects) and mixed plantation
(forest with some planting of fruit trees and commercial exotic species; 10 transects) sites.
All five forests are on steep-sided hills and trails tend be either in the gulley bottoms or the
ridge-tops, thus transect locations were also stratified to cover ridges (36 transects) and
gulleys (28 transects).
For each transect, measures of forest structure were quantified either at the locations
where owls had been recorded or, for transects without owls, at the mid-point of the
transect. A circular plot of 10 m radius, centred either on the tree in which owls were
recorded or on the nearest tree to the transect mid-point was established at each of these
locations, and the variables described in Table 1 were all recorded. In addition to measures
of mean tree height and density, we also recorded maximum tree height and the number of
7
tall (> 6 m) trees within each plot, as taller trees are more likely to provide suitable nesting
cavities. In addition, Alcoy Forest is subject to strong winds in the north-west and southeast regions, particularly during the monsoon season. Discussions with local wardens
suggested that wind speed may influence owl distribution, and thus each transect point was
visited once with an anemometer in order to measure wind speed.
Habitat characteristics
This study identified four habitat categories: foraging habitat (n=14), roosting habitat
(n=13), unoccupied habitat (n=11) and nesting habitat (n=2). The sites where hawk owls
were located during the radio telemetry and on transect were documented for certain habitat
characteristics. A 10m radius circular plot was created centred on the tree where each hawk
owl was first located during radio telemetry (Martin et al, 1997). The plot was divided into
four quadrants to facilitate the counting of trees and branches. Habitat features were
recorded following Martin et al, (1997).
Additional variables were measured during the line transects model. The site
variable was added as a random factor to examine the differences between sites (Table 1).
All the variables in the habitat models were included in the analysis since there was no
colinearity between variables. In addition, because owls were considered likely to forage
around the forest edges as well as inside forest, the habitat characteristics were recorded at
any foraging locations and at randomly located points around the edge of the forest. These
data allowed identification of the characteristics of locations used by the owls. The
description of the foraging and roosting site characteristics were quantified in terms of
canopy cover using densitometer, number and height of trees within the plot, and
understorey cover.
8
Table 1: Name, unit and description of each of the habitat characteristics recorded in survey
plots (314 m2) on hawk-owl detection transects in five forests on Cebu Island, Philippines.
The surveys in which each variable was recorded (T = transect surveys, R = locations of
radio-tracked individuals) are also shown.
Variable
Unit
Survey
Description
m
T, R
m
T, R
No. tall trees
number
T, R
Total no. trees
number
T, R
m
T, R
km/hour
T, R
categorical
number
R
R
Canopy cover
%
R
Mean dbh
cm
R
Understory cover
%
R
Visual estimate of the height of the
tallest tree within each plot
Mean height of all trees taller than 3
m…
Number of trees taller than 6 m within
each plot
Total no of trees taller than 3 m within
each plot
Estimate of the distance of the plot from
the nearest forest clearing or farm
Wind speed measured with an
anemometer at one location on each
transects.
Ridge-top or gulley
Total number of branches suitable for
owl perching (> 2 cm diameter) on trees
taller than 6 m
% canopy cover measured with a
densiometer
Diameter at breast height for all trees
taller than 6 m
% cover of understory vegetation (< 1 m
height)
Maximum tree
height
Mean tree height
Distance to clearing
Wind speed
Transect location
No. perching
branches
Radio-tracking of Philippine hawk-owls
Between 11 April and 11 June 2011, ten hawk-owls were captured using mist-nets and
playback and each was fitted with a tail-mounted Pip Ag393 transmitter (mass 2.3 g,
supplied by Biotrack Ltd). Although these tags have a standard expected lifespan of 10
weeks at 50 pulses per minute (Kenward 2004), hawk-owls proved quite adept at removing
the tags, and eight of the tags were found discarded within one week of deployment.
Consequently, individual owls were tagged consecutively and tracked repeatedly (for 3-5
days) until the tag was removed. All captured owls were measured and checked for the
presence or absence of a brood patch. Nine of the tagged owls were adults and four of these
had brood patches. These four individuals were consistently smaller (wing length range =
9
175 – 181 mm, tail length range = 91 – 99 mm, mass range = 120 – 129 g) than the
remaining five adults (wing length range = 184 - 190 mm, tail length range = 100-109 mm,
mass range = 124 – 142 g), and we therefore assumed the individuals with brood patches to
be female and those without to be male. The juvenile bird was within the size range of
females (wing length = 175 mm, tail length = 96 mm, mass = 118 g) but the sex of this
individual could not be determined with any certainty.
Each radio-tagged owl was tracked every night from dusk (one hour after sunset) to
dawn (two hours before sunrise), when they were deemed most likely to be active and
foraging. In addition, daytime tracking was carried out to locate potential nest and roost
sites, and to identify any diurnal activity. Locations of radio-tagged owls were assessed by
triangulation (White and Garrott 1990) using three-element Yagi antennae and Sica 138
mHz receivers (Biotrack Ltd) and handheld GPS. The field team worked on foot along
forest trails, roads, gulleys and ridgetops, handheld radios were used to facilitate
synchronized tracking and to keep the team in contact. The actual direction of each located
signal (azimuth) was determined using a compass, and triangulation of directional signals
was obtained with an interval of five minutes (Mazur et al.1998, Frosman et al. 2005). The
behavior of tagged individuals at each recorded location was classified as either foraging
(signals of individuals moving or perching for short occasions only) or roosting (nonmoving signals for more than 30 minutes, mostly diurnal records).
The habitat characteristics of the foraging and roosting locations of tagged individuals
(together with the characteristics of the three locations from which roosting owls were
disturbed during daytime fieldwork) were assessed. Circular plots of 10 m radius centred on
the tree in which each owl was first located during radio telemetry were established, and
divided into four quadrants to facilitate counting, and the variables listed in Table 1 were
recorded.
Dietary composition and nest site characteristics
In each territory where the owls were located, a list of potential prey was recorded. Prey
was classified according to likely origin (farmland vs. forest). Interviews with local forest
wardens on diet were also conducted to add more information on the owls’ dietary
requirements. Special efforts were made to find nests using the radio-tagged owls. Daytime tracking was conducted in order to identify whether these individuals were nesting.
The location of the two nests found was marked and the characteristics of the habitat
10
around them were measured following Martin et al, (1997). Nest tree species, height and
DBH, nest cavity height, nest hollow orientations, location, and names of dominant and
emergent species of tree were collected. Information on status and threats to the nest tree
was also noted.
Population Estimates
The actual and potential number of owl pairs on Cebu was estimated from a combination of
home range size estimates for tagged owls and assessment of availability of foraging
habitats. The forest of Alcoy and other remaining forests on the island have high degree of
homogeneity, which help with extrapolation of the results to other habitats and locations
around the island (Miller et al. 2004; Odum & Barrett 2005).
Data analysis
Differences in the measures of forest structure recorded on transect among the five forests
were compared with a Kruskal-Wallis test. To explore the differences in habitat structure
and wind speed on transects on which owls were and were not recorded, a logistic
regression was constructed which included maximum tree height, number of tall trees,
distance to the nearest clearing, location (ridge or gulley) and wind speed (Table 1). Only
the transects in Alcoy Forest were included within this model because too few transects
were available from the other sites (because of constraints imposed by forest area). The
collinearity between predictor variables was checked prior to analysis, and location (ridge
or gulley) was coded as a categorical predictor. In Alcoy Forest, differences between
transects on ridges and gulleys in the measures of forest structure and wind speed were
compared with t-tests.
Estimates of the home range size used by individuals during the period of radiotracking were carried out using ArcView GIS version 3.2a with the Animal Movement
Extension (Hooge & Eichenlaub 2000). We estimated home range using 95% minimum
convex polygons (MCP) and kernels from 50% and 95% probability models, as the latter
are less prone to the effects of outliers (Worton 1989, Mazur et al. 1998, Frosman et al.
2005). One location from each triangulation fix was included within the home range
estimates. The sizes of owl home ranges that did and did not encompass forest edges, and
the sizes of the home ranges of adult male and adult female owls were compared with
Mann-Whitney tests.
11
RESULTS
Surveys of the hawk owl were completed in five forest sites in Cebu Island; four in the
southern part of the island and one in central Cebu, and owls were located in all five sites
(owls detected on: Alcoy: 37/41 transects, Dalaguete Forest: 3/8 transects, Boljo-on Forest:
3/6 transects, Argao: 4/5 transects and Tabunan: 5/4 transects.
Distribution of hawk-owls on Cebu Island
Across the five forests, owls were detected in forest interior, forest edges and in areas of
mixed forest and plantation, where some commercially used species are grown within the
forest matrix (Table 2). In total, 52 owls were detected across these five forests, and at 16
locations there were two owls responding to playback together, suggesting that there were
at least 16 pairs of owls within this total.
Table 2. The total number of hawk owls and the estimated number of pairs recorded on
transects through different habitats in five forest on Cebu Island (see text for details).
Habitat
Forest interior
Forest edge
Mixed
plantation
Total
Alcoy
No.
No.
owls pairs
Dalaguete
No.
No.
owls pairs
Boljoon
No.
No.
owls pairs
Argao
No.
No.
owls pairs
Tabunan
No.
No.
owls pairs
20
7
10
9
2
2
0
1
2
0
0
1
0
1
2
0
0
0
0
1
3
0
0
1
2
2
1
1
0
0
37
13
3
1
3
0
4
1
5
1
The five forests were similar in mean tree heights, total numbers of trees and
numbers of tall tress recorded on the transect surveys, but the maximum tree heights were
significantly taller in Alcoy and Tabunan Forests (Table 3). Transects in the five forests
also did not differ significantly in their distances from clearings and farms, although the
larger distances recorded in Alcoy forest plots reflect the the much larger area of
continuous forest within Alcoy than within the other forest patches.
12
Table 3. Habitat characteristics of the five forests on Cebu Island on which hawk-owl
surveys were carried out. The means (± SE) of each variable within circular plots (314 m2)
on separate transects are shown, and the number of transects is given in parentheses (see
Table 1 for details of habitat variables).
Variables
Max tree
height
Mean tree
height
No. of tall
trees
Total no. of
trees
Distance to
clearing
Alcoy
(n=41)
10.24 ±
0.44
12.34 ±
0.68
27.00 ±
1.47
37.97 ±
1.88
136.8 ±
26.3
Dalaguete
(n=8)
8.5 ±
0.26
14.92 ±
1.86
34.67 ±
4.65
47.05 ±
4.98
85.0 ±
21.38
Boljoon
(n=6)
7.00 ±
0.86
14.7 ±
1.45
22.87 ±
1.11
41.03 ±
3.44
44.16 ±
26.4
Argao
(n=5)
9.40 ±
1.02
12.36
±1.73
26.36 ±
2.67
39.36 ±
4.64
47.0 ±
21.77
Tabunan
(n=4)
12.50 ±
1.55
17.47 ±
3.79
42.22 ±
6.79
50.22 ±
8.82
105.0 ±
55.0
X2
p
11.27
0.02
5.57
0.23
9.20
0.06
5.81
0.21
2.79
0.59
Within Alcoy forest, owls were located on both ridges (10/22 transects) and gullies
(12/19 transects). The mean tree height and total number of trees within survey plots was
significantly higher on ridges than gulleys, and ridge plots were significantly further from
clearings and farms. However, in general the structure of the forest in ridges and gulleys
was very similar (Table 4).
Table 4. Differences in the habitat characteristics of the hawk-owl survey transects located
on ridges (22 transects) and gulleys (19 transects) within Alcoy Forest, Cebu Island. The
means of each variable within circular plots (314 m2) on separate transects are shown. (see
Table 1 for details of habitat variables), and significant differences are highlighted in bold.
Variables
Maximum tree height
Mean tree height
Number of tall trees
Total number of trees
Distance to clearing
Wind speed
Ridges
Mean ± SE
9.77 ± 0.53
13.87 ± 0.97
28.81 ± 2.02
43.09 ± 2.38
190.9 ± 39.3
2.23 ± 0.45
Gullies
Mean ± SE
10.78 ± 0.73
10.57 ± 3.55
24.89 ± 2.08
31.78 ± 2.43
74.21 ± 28.9
1.15 ± 0.4
T
p
1.15
2.55
1.34
3.30
2.32
1.76
0.26
0.015
0.19
0.002
0.025
0.08
13
A logistic regression analysis of the factors influencing the presence and absence of
owls on transects within Alcoy forest showed that only wind speed had a statistically
significant influence on owl distribution (Wald = 11.71, df = 1, p < 0.001), with the
probability of owl occupancy declining significantly with increasing wind speed (Figure 2).
Wind speed correctly predicted 84.2% of the 19 transects without owls and 90.9% of the 22
transects with owls within the logistic regression model, with owls being very rarely
recorded on transects on which wind speed readings exceeded 2 km/hour (Figure 2) .
1
0
Figure 2. The effect of wind-speed on the probability (dotted curve) of occupancy of hawkowls on transects in Alcoy Forest, Cebu Island predicted from a logistic regression model.
The wind speeds recorded on occupied (open bars) and unoccupied (filled bars) transects
are shown, together with their frequencies.
14
Home range characteristics
A total of 11 owls were mist-netted and 10 owls were radio tracked. This study compared
the home range between the 10 radio tracked hawk owls using ground-based radio
telemetry. Each hawk owl was tracked for 5-6 days between April 11 and June 11, 2011
during the breeding season. Only hawk owls with more than 30 locations during
triangulation were included in estimations of home range size (Table 5). The home range
analysis indicates that estimates varied between 95% MCP, 95% kernel and 50% core
kernel analyses (Table 5). The 95% kernel method estimates of home range size were
larger than 95% MCP (Table 5, Fig. 2). The mean hawk owl home range size estimates
during the breeding season were 7.24 ha (± 4.10 SD) 95% MCP, 8.31 ha (± 4.19 SD) 95%
kernel and 2.67 ha (± 1.34 SD) 50% kernel core area (Fig. 3).
For the 6 hawk owls that were located in the forest edge, mean home range size
estimates were 6.0 5ha (± 4.61SD) 95% MCP, 7.33 ha (± 4.66 SD) 95% kernel and 2.67 ha
(± 1.71) 50% kernel. By contrast, the home range size estimates for the four owls located
within the forest were 9.01 ha (± 2.82 SD) 95% MCP, 9.28 ha (± 2.97 SD) 95% kernel and
2.68 ha (± 0.72 SD) 50% kernel (Appendix 1). The forest interior home ranges of hawk
owls were therefore larger than those of owls in the forest edge (Table 5). Most of the
location of the hawk owl’s location are clumped in distribution rather than spread
throughout the territory (Appendix 2). The home range located in small forest patches has
smaller home range size forest counterpart (Table 5). This three were located two to three
kilometre away from the larger forest patch. This forest patch is composed of mixed native
trees, exotic trees and agroforestry.
The peak activity period during radio telemetry of hawk owls was from 1800-0100
hours, and activity slowed down between 0200 to 0400 hours. The species were recorded in
different bearings within its home range in the early dusk. The most of the radio tagged
hawk owls that were located between 0200 and 0400 were all motionless during this period.
15
Table 5. Estimates of home range size (from 95% minimum convex polygon (MCP), 95%
kernel and 50% kernel home range analyses) of adult male and female and juvenile hawk
owls radio-tracked within Alcoy Forest, Cebu Island. Names in bold indicate those with
home ranges in the forest interior, and mean home range sizes for forest interior and forest
edge are given
Owl
Radio 95% MCP
Tracking Period Tracking
Fixes (n)
(ha)
Days
95%
Kernel
(ha)
50%
Kernel
(ha)
Adult males
Ryan
8-11 May 2011
4
207
7.95
8.43
2.71
Godo
3-7 May 2011
5
140
4.54
5.67
3.23
Lisa
1-3 June 2011
3
54
1.13
1.59
.089
Doro
11-12 April 2011
3
31
5.29
9.45
5.09
Tito
22-25 May 2011
4
131
11.91
12.49
3.11
Mean ± SD
6.2 ± 4.0 7.52 ± 4.1 3.0 ± 1.5
Adult females
Orlyn
11-14 May 2011
4
94
9.63
11.43
3.55
Elline
25-27 May 2011
3
53
10.63
12.87
3.25
Pedro
19-21 May 2011
3
64
12.62
12.98
2.44
Sidro
20-23May 2011
4
163
5.58
5.98
1.65
Mean ± SD
9.61 ± 3.0 10.81 ± 3.3 2.72 ± 0.8
Juvenile
James
26-30April 2011
5
263
2.14
2.89
1.63
Overall mean ±
7.04 ± 4.0 8.37 ± 4.2 2.75 ± 1.2
SD
Forest Interior
3.83 ± 1.0 107.66 5.89 ± 4.4 7.33 ± 4.3 2.80 ± 1.5
mean ± SD
Forest Edge mean
138.5 ±
4.0 ± 0.8
9.01 ± 2.8 9.94 ± 2.7 0.72 ± 0.07
± SD
64.9
16
Figure 3. Comparison of 95% minmum convex polygon (MPC) home range between forest
and forest edge radio tagged owl
17
Figure 4. Home ranges of five adult male (pale grey), four adult female (dark grey, within
main forest) and one juvenile (dark grey out with main forest) radio-tagged hawk-owls in
Alcoy Forest, Cebu Island. Inner lines indicate 50% and outer lines indicate 95% kernel
estimates of home ranges.
18
Foraging habitat
There were six radio tagged owls observed foraging in different habitats with good. The
species was mostly recorded foraging within the secondary forest 60% (59.5±38.7) and
forest edge19% (19.0±16.1) (Table 6). However, they were also recorded in farmland (3%)
with tall trees present. Two hawk owls were also tracked along the exotic plantation 6.0%
adjacent to the secondary forest and scrubland along the forest edge. No territory overlap
was observed during the study. The hawk owls were also recorded in mix plantation (12%)
and small forest patches with large trees and dense branches. There was also a significant
difference in term of canopy cover between forest and forest edge foraging habitat. The
canopy was more open at mean 14.3 as compared to 11.1. However, the understorey within
the forest edge was denser (mean 68.3%) than in the forest (59.0%). There was no
significant difference between the number of trees, dbh, and number of branches.
Table 6. Comparison between different radio fixes between different habitats during radio
telemetry.
Habitat use
Percent
Mean
Std. Deviation
Forest
60
59.5
38.7
Forest edge
19
19.0
16.1
Mix Forest
12
12.0
19.1
Plantation
6
6.0
8.0
Farm
3
3.5
4.1
Roosting sites
All of the roosting (n=13) territory was positioned with in their home range. A total
of 69% of the roosting sites were located within the forest and 31% were observed along
the forest edge. Roosting sites were composed of thick foliage cover like vines, leaves and
epiphytes. There is no difference between roosting sites in the forest and forest edge in term
for foliage cover. However, the forest edge owl’s resident has shorter tree height with an
average at 9.61 m while the forest resident owl was taller with a mean of 11.7 m (Table 7).
The roosting tree was average dbh was 58.1 cm in the forest edge and 45 cm with the forest
owls. Most of the roosting sites were characterized by high number of woody vines, and the
presence of owl fecal on the ground.
19
Table 7. Comparison between difference variable in forest and forest edge home range
within the study sites.
Number of
Understorey
Canopy
DBH
tree
Tree
Height
Forest
Mean
35.5
55.0
11.1
45.0
11.7
Std Deviation
14.7
11.7
4.4
10.0
.9
29.8
57.6
14.8
58.1
9.6
9.2
20.1
13.2
22.3
3.2
Forest Edge
Mean
Std Deviation
Nest site characteristics
Nest finding efforts were concentrated on the 10 radio tagged owls supplemented by reports
from forest wardens. Only 2 nests were recorded, one active and one in-active. The two
nests were located in small mixed forest patches approximately 2-3 km from the main
forest block. These patches were surrounded by vegetable farms and exotic tree plantations.
The patch with the active nest was estimated to cover an area of 1.5 hectares that indicates
the nest cover c.400 square metres (0.04 ha). The nest sites were located in the secondary
forest a gully (active nest) and on a steep slope (inactive nest). The distance between the
nests to forest edge was around 5 metres and 3 metres (Table 8).
The nest trees were native species Melanolepis multiglandulosa and Vitex
parviflora, the latter being a threatened species. The DBH of the two nesting tree was
around 104 cm and estimated mean height of nest tree was 16.5 m. The mean height of the
hollow from the ground was 2.28 m. The average size of the entrance to hollow was 17.78
x 11.43cm (Table 5). In the two nesting tree entrance was slightly smaller than the cavity
with an average cavity depth 25.4 cm.
The two nest sites were located in clumps of trees within the plot, with a mean
canopy cover of 8.7 (using a densiometer). The mean height of the canopy was 6.23 metres.
The active nest had denser understorey with 80% in the plot while the inactive nest had
10% understorey cover. The active nest site was dominated by Duabanga moluccana while
the inactive nest site was dominated by Vitex parviflora. The active nest hollow was located
20
on the main truck. The hawk owl’s nest hollow was observed in mid height less than 3 m
above the ground (Table 8). The entrances of the nest hollows were facing northeast and
southwest.
Table 8. Characteristics of the two hawk-owl nests that was located within Alcoy Forest,
Cebu Island, one of which was active in 2011, and the one which had been used on 2005
but was inactive in 2011.
Variables
Nest site characteristics
Tree species
Tree status
Height of nesting tree (m)
DBH (cm)
Nest hollow height (m)
Nest hollow orientation
Nest hollow entrance height (cm)
Nest hollow entrance width (cm)
Nest hollow depth (cm)
Surrounding habitat characteristics
Forest type
Estimated forest patch size (ha)
Canopy cover (%)
Estimated canopy height (m)
Dominant canopy species
Tallest emergent tree species
Understorey cover (%)
Distance to clearing (m)
Distance to main forest area (m)
Topography
Slope (%)
Nest 1 (Active)
Nest 2 (Inactive)
Melanolepis
multiglandulosa
Alive
20
98
2.43
Northeast
20.32
12.7
33.2
Vitex parviflora
Secondary plantation
1.4
9.67
7.04
Duabanga moluccnna
Blume
Melanolepis
multiglandulosa
80
5
700
Gulley
75
Secondary forest
0.005
7.73
5.43
Vitex parviflora
Dead
13
110
2.13
Southwest
15.24
10.16
17.78
Vitex parviflora
10
3
400
Near ridge
75
Dietary composition
To quantify prey density in different habitat along forest and farmland, reports from the
forest wardens and observations from the hawk owls nesting site were compiled. This study
identified 15 potential prey items as reported by the forest wardens and four prey types
were observed during the study (Table 9).The Philippine Hawk Owl N. p. spilonota diet is
comprised of a range of different prey types, including small mammals, small birds, reptiles
and insects (Table 9). They species appear to particularly prey on rats and small reptiles.
This study observed the hawk owl preys on Cicada and Cricket. The hawk owl has also
21
been reported preying on amphibians along the forest edge or farmland particularly on
Rana sp and Bufo sp. These amphibians are mostly observed in the forest edge and
farmland or along creeks and water sources
Table 9. Variation of potential prey composition of Philippine Hawk Owl as observed as
reported and observed by Alcoy Forest Wardens. x= reported; x*=observed and – not
reported.
Prey Species
Taxa
Family/Order
Common Name
Forest
Farmland
Mammals
Muridae
Rats
x
x*
Birds
Nectariniidae
Sunbird
x
x*
Paridae
Tits
x
x
Dicaeidae
Flowerpecker
x
x
Muscicapidae
Flycatcher
x
x
Colubridae
Snake
x
x
Agamidae
Gliding lizards
x
x
Scincidae
Common skinks
x
x
Gekkonidae
Gecko
x
x
Ranidae
Frog
--
x
Bufonidea
Toad
--
x
Gryllidae
Crickets
x
x*
Lepidotera
Moth
x
x
Phasmatodea
Stick insects
x
x
Mantodea
Mantis
x
x
Cicadidae
Cicada
x
x*
Reptiles
Amphibian
Insects
DISCUSSION
The Philippine Hawk Owl breeding season habitat use is wide ranging. The study revealed
that hawk owls can persist in different habitat. There are more hawk owls sighted within
forest than at the forest edge. More hawk owls were recorded along gullies than on ridges,
and most of larger stands of trees were located within the gullies. The three significant
predictors that influence the presence and absence of hawk owls are the mean tree height,
location between ridge and gully and wind—are all important in identifying suitable habitat
22
for the species. Hawk owls appear to prefer gully locations, with low levels of wind and tall
trees. Wind significantly affects the probability of recording owl in all locations. In some
instances hawk owls seemed to avoid windy areas within the forest or at forest edge. In
particular, ridge top forests with apparently suitable tall trees may be too windy to be
occupied by the species.
The absence of owls in some areas may not necessarily indicate that the site was not
suitable for the species. The study identified 11 sites where owl was recorded. The hawk
owls avoid this areas since the site are windy especially during monsoon season. The
understorey cover was 3.6% slightly less than occupied habitat. However, most of these
sites are located along the ridge top forest. This implies that ridge forest has lower
probability of holding owls.
However, the presence of suitable nesting cavities within the home range may be
important to the species. Tree cavities were observed in some of the radio-tagged owls’
territories. The number of cavities can limit the population of cavity-nesting species like
owls (Newton 1994, 1998, Cockle et al, 2011). Consequently, lack of suitable nesting
cavities could be one possible reason that the species declined to the point of being feared
extinct in the island in 1950s (Rabor 1959; Brooks et al, 1995). The island had been
deforested by early 1990 and only few old growth forests were found. It is therefore
important to locate and protect tall trees with cavities for the owls and other species within
the different forest patches. I recommend continuing long-term study on the species,
particularly in respect of nest cavity availability and its effects on the population status of
Cebu’s owls
The home range of the Philippine Hawk Owl during the breeding season was
extensive. Most of the larger home ranges were located within the forest rather than forest
edge. Studies conducted on a similar species, Ninox connivens in Australia have shown that
they have much larger territories of > 226 hectares (Taylor et al, 2004). This is typical of
larger species of Ninox. The difference in home range size maybe due to several factors like
size of the forest and human disturbance. The hawk owl home range tends to be smaller
compared to the other species of the hawk owls. The species home range Some Philippine
hawk owl home ranges were located within mixed mature exotic plantations with
regenerating secondary forest along the forest edge. At the landscape level, the radio-tagged
23
hawk owl locations were mostly recorded within the forest than at the forest edge.
However, the species also was also observed perching in the middle of the farmland and
clearings perching in tall trees. However, all roosting sites were located within forested
areas. The density of foliage seems to be a key characteristic of roosting sites. Dense
foliage may provide protection from other predators, wind, rain, protection and
disturbances from some species of birds particularly bulbuls.
Radio telemetry within the forest is a real challenge because of radio signals
bouncing off trees. It is important to look for a good position during homing to get a good
radio signal. Another challenge was transmitter retention. The hawk owl can easily remove
the transmitter or it will partially moult. The transmitters might be too big for the species
and they have a strong enough beak to remove the tag. It is also important to consider other
attachment methods like backpack for future study. It is better to look for other methods
for the future studies in attaching radio transmitters with the owl. Furthermore, studies of
the home range during the non-breeding season are needed, to check for differences
between breeding and non-breeding territories. This could provide additional information
on home range size, roosting and foraging habitat. This can provide more information on
the conservation status of the species. This study is vital for the conservation of the
potentially new endemic species of hawk owls in Cebu (Rasmussen et al, in prep.).
Most of the remaining forests on Cebu are protected except Boljoon forest. Local
legislation for the protection of wildlife was enacted particularly in Tabunan Forest,
Dalaguete and Alcoy Forest where the species has been recorded. The home ranges located
in small forest patches are more susceptible to habitat destruction. This study recommends
protection of small forest patches potentially holding hawk owl territories. Planting native
tree between forest patches will facilitate movement of individuals between forest sites.
The hawk owls are generalist predators on a variety of vertebrates and invertebrates.
Based on reports from the forest wardens and a few recorded observations, the owls
exhibited considerable dietary breadth. However I was unable to conduct prey density and
preference studies in territories. Hence I recommend more detailed study on prey
abundance and preferences of the hawk owl within the different habitat use. It is also
important to know whether dietary preferences exist. It is also important to know if there is
24
a significant relationship between the abundance of prey and the breeding success of the
species.
The results of the transect surveys support the concept that the hawk owl prefers
forest over forest edge. The hawk owl also prefers gullies over ridge top forest. There were
more proportionately records of owls along gullies than ridges. A total of 30 territories
were recorded along the gully compared to 22 territories within the ridge (Fig. 11).
Moreover, more territories of hawk owls were observed within the forest than forest edge.
Among the radio-tagged owls nine had clearly defined territories, demonstrated by the fact
that they responded to playback immediately.
It is essential to understand its habitat requirements during the non-breeding season
and compare the home range size.
It is also important to explore other small forest
patches in the island possible holding some population of hawk owls. This study
recommends determining the differences in the ecological requirements of the other
subspecies on the other islands. The use of radio telemetry is essential to know the different
habitat requirements of the species before it is too late. Habitat restoration should be done
in all the study sites using native species of tree and with thought given to the carrying
capacity of current habitats. The results of this study can also provide appropriate
information for conservation planning and management of the species and the improvement
of its current status.
CONCLUSION
The Philippine Hawk Owl N.p. spilonota require breeding season home ranges of at least 8
to 12 hectares (using 95% maximum convex polygon and 95% kernel estimates). The
species uses a variety of habitat categorized by different forest structures and types. In this
study breeding hawk owls were located along the forest edge with a few large trees and
suitable nest cavities. An estimate of breeding habitat used in forest edge almost the same
as compared to the forest habitat. The hawk owl prefers forest over forest edge and
plantation forest. There are more hawk owl observed with in the forest interior compared to
forest edge. However the can survive in a different variety of habitat. There is more
sighting of species territories within the forest. The territories within the forest are larger
than expected home range.
25
The habitat characteristic provided more understanding of the species habitat
requirements. Hence conservation of the species in a landscape-based approach is possible.
This is important for maintaining the hawk owl population on the island. Small forest
patches ranging from 1 to 2 hectares with good stands of tree are important to the species.
Three of the radio-tagged owls were located in a small forest patch and this suggests that
the species can survive in disturbed habitats. Most of their territories were surrounded by
farmlands and exotic tree plantation. Thus the conservation of small forest patches needs to
be promoted on the island.
This study is an important step in conserving the last remaining forest in island.
Several species of conservation interest were rediscovered bring back it population. The
Philippine Hawk Owl was the first endemic raptor to be rediscovered in the island. Raptors
are good indicators species for changes in the ecosystems (Rodriguez-Estralla et al. 1998).
They are good indicators of habitat quality because of their sensitivity to human
disturbance (Taylor, 1984). On the other hand the species was observed preying on small
mammals in a vegetable farms near the forest edge. This could serve as natural pest control
as most of the forest edge sites are owned by farmers producing vegetables for the town of
Cebu.
This study explored Philippine Hawk Owl breeding habitat use, home range, diet
preference, and nest characteristics. The results indicate that habitat use varied in different
scales. The species prefers forest over other habitat type. However, it manages to survive in
mixed habitats and small forest patches. Proportionately more owls recorded in Alcoy
forest than the rest of the sites.
Conservation implication
Taking this account together, they prefer larger forest patches like Alcoy forest. Most of the
radio telemetry locations were concentrating with in the forest. Protecting the forest will
save the species from extinction. Most of the forest sites are protected. However, the
pressure from the booming economy means an increasing expansion of agricultural land in
the island and the increasing human population creates more pressure on the last remaining
forest in the island. There should be a good and realistic plan for the management and
conservation of the species or perhaps the island’s biodiversity to ease the pressure on its
habitat. The conservation programmes within the southern forest have been existing for
26
almost a decade particularly in the municipality of Alcoy, Dalaguete and Argao (Paguntalan
2009).
The program is in partnership with community organization, non-government
organizations, local government and the environment department. Tabunan forest within
Central Cebu Protected Landscape manage by the local the department of environment and
natural resources and protected areas management board should be retain and protected
since it is the only important the last remaining old growth forest in the island. Yet there
are still illegal activities within this forest sites. Strict implantation of laws is needed for the
success of conservation and proper guidance to the community organization and local
government unit. They should be guided in the right way in working for conservation.
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