Nocturnal Acoustic Monitoring Project

Nocturnal flight call analysis as a method for monitoring
density and species composition of migratory songbirds
(Order Passeriformes) across southern Vancouver Island,
British Columbia in 2004.
Photo: Ralph Hocken
Prepared for – TD Friends of the Environment Foundation
Prepared by – Jessica Murray, Director
Rocky Point Bird Observatory
TABLE OF CONTENTS
PAGE
PROJECT OVERVIEW ................................................................................................. 2
INTRODUCTION .......................................................................................................... 2
METHODS .................................................................................................................... 4
Site location ............................................................................................................ 4
Microphone design ................................................................................................. 5
Flight call analysis ................................................................................................. 5
RESULTS....................................................................................................................... 6
Number of detections.............................................................................................. 6
Weather and detection ability ................................................................................. 6
Time of night .......................................................................................................... 7
Highlights................................................................................................................ 7
DISCUSSION .............................................................................................................. 10
Methodology ......................................................................................................... 10
Swainson’s Thrush............................................................................................... 11
Considerations for analysis .................................................................................. 11
Species complexes................................................................................................. 12
Infrequently banded species ................................................................................. 12
Weather ................................................................................................................ 12
CONCLUSION ............................................................................................................ 13
ACKNOWLEDGEMENTS........................................................................................... 13
LITERATURE CITED................................................................................................. 14
1
PROJECT OVERVIEW
Rocky Point Bird Observatory (RPBO) is a registered, non-profit organization
located west of Victoria, British Columbia. The primary role of RPBO is to monitor the
long-term population trends of migratory songbirds (order Passeriformes) through the
method of bird banding. Banding provides critical information about the productivity,
survivorship, arrival and departure dates, and long-term population trends of many
songbird species. In addition to banding, an emerging method for monitoring the
migratory movements of songbirds is nocturnal acoustic monitoring.
Acoustic
monitoring used in conjunction with banding at RPBO would provide a valuable data set
on migration trends to support the conservation of songbird species. The main objective
of this study was to determine whether nocturnal acoustic monitoring could be used as a
tool at RPBO to estimate migration and population trends of songbirds over time. The
nocturnal flight calls of migrating songbirds were recorded each night between August 6
and September 20, 2004. 38 nights were successfully monitored and allowed for 572
individual flight calls of 12 different songbird species to be recorded. The most
frequently recorded species were Swainson’s Thrush, Yellow Warbler, Wilson’s Warbler,
and Hermit Thrush. This project demonstrated the potential of using nocturnal acoustic
monitoring as a conservation tool for many migratory songbird species, especially the
Swainson’s Thrush.
INTRODUCTION
Rocky Point Bird Observatory (RPBO) is one of 23 member stations composing
the Canadian Migration Monitoring Network and is the only bird observatory situated on
the Canadian Pacific coast (Figure 1).
North
Rocky Point
CANADA
Bird Observatory
23
FIGURE 1. The 23 banding stations that compose the Canadian Migration Monitoring Network.
Source – Bird Studies Canada
2
The observatory is uniquely situated on Department of National Defence land,
located at the southern-most tip of Vancouver Island, British Columbia. The observatory
encompasses a mosaic of habitat types including mixed coniferous and deciduous forests,
freshwater ponds and marshes, marine tidal flats and one of the few remaining areas of
intact Garry oak (Quercus garryana) habitat in Canada.
In 1994, the potential of Rocky Point as a songbird migration station was
recognized and the Rocky Point Bird Observatory Society officially formed a non-profit,
volunteer organization in association with the Victoria Natural History Society and the
Canadian Wildlife Service. Since RPBO’s inception, over 24,650 birds of 95 different
species have been banded, and the master checklist of all birds observed at RPBO
includes 293 species. Over the past 11 years we have learned a great deal about the
migratory habits of many songbird species including information on arrival and departure
dates, the relationship between weather conditions and migration, and the degree of
regularity with which individual birds return. This has provided RPBO and the Canadian
Wildlife Service with a valuable data set to enable the long-term monitoring and
conservation of many avian species.
For numerous songbird species, banding is a valuable tool for obtaining
information about morphological features and timing of migration. However, at RPBO
certain bird species that forage high in the forest canopy are infrequently observed at
ground level. For these species, population trends cannot be accurately assessed through
the traditional method of banding because their capture rate in mist-nets is so infrequent.
An emerging approach for species that cannot be monitored by traditional banding is the
method of nocturnal acoustic monitoring. Each year millions of songbirds migrate across
North America throughout the night. Several species of warblers (Parulinae), sparrows
(Emberizinae) and thrushes (Catharus) are well known for giving reliable nocturnal flight
calls while migrating (Evans et. al 2000), and on a good migration night numerous bird
calls have been heard by listeners on the ground at RPBO. The majority of songbird
species each have a unique nocturnal flight call and therefore a uniquely associated
spectrogram. Spectrograms are visual representations that illustrate the frequency,
modulation, and duration of each flight call.
By recording and analyzing these
3
spectrograms, we can identify the species composition and numbers of birds migrating
over Rocky Point.
The objectives of this study are to determine whether acoustic monitoring can be
used as a conservation tool at RPBO to estimate migration and population trends of
songbirds over time, and to determine whether acoustic monitoring can be used to
monitor species that are infrequently encountered through traditional banding methods to
support the long-term monitoring of songbird species.
METHODS
Site location
An open meadow located west of the banding station (Figure 2) was determined
to be the most conducive area to record nocturnal flight calls.
2
North
1
3
FIGURE 2. An aerial view Rocky Point Bird Observatory, the extreme southernmost tip of
Vancouver Island. 1 = Rocky Point Bird Observatory banding station; 2 = west meadow
(selected location of microphone for nocturnal monitoring in 2004); 3 = proposed microphone
locations for future monitoring.
Source – Department of National Defence - Rocky Point
4
The site was free from any nearby interference noises (frogs at marshes, ocean waves,
human activity) and overhanging canopy which could potentially muffle flight calls
passing overhead.
Microphone design
A single, pressure zone microphone was used to record the flight calls of
nocturnal migrants. Microphone construction followed the guidelines laid out by Old
Bird Inc., a non-profit organization dedicated to facilitating acoustic monitoring and
providing expert advice to acoustic monitoring stations (www.oldbird.org). A pre-amp
was added to the design to increase detection sensitivity; the microphone recorded an
approximate 1,000 m2 cross section of sky. The microphone was waterproofed with two
layers of plastic wrap and was mounted on the roof of a storage trailer located in the west
meadow. A wooden pallet and bungee cords were used to secure the microphone to the
roof in case of strong winds. An audio cable was run from the rooftop to inside the trailer
and a laptop computer where the flight calls were recorded directly to the hard drive each
night.
Based on known migration chronology of songbirds at RPBO from previous
banding seasons (Derbyshire 1999 and 2000, Gibson 2001, Woodward 2002, Jantunen
2003), nocturnal flight calls were recorded each night from August 6 until September 20,
2004, from sunset to sunrise.
Flight call analysis
Flight calls were detected using ‘Tseep’ and ‘Thrush’ software – programs
specifically designed by OldBird Inc. for detecting the short, high-pitched call notes of
most North American songbird species. Flight calls were then run through ‘GlassOFire’
software, also designed by OldBird Inc., which produced an accompanying spectrogram
for each call. Resulting spectrograms were closely examined and identified to species
where possible through comparison with a flight call spectrogram library provided by Old
Bird (Evans and O’Brien, 2002), and also by comparing nocturnal spectrograms to
diurnal spectrograms for each species, as the general structure of some diurnal and
nocturnal call notes are similar.
5
RESULTS
Number of detections
Monitoring was conducted over 38 nights resulting in 572 individual flight calls
of 12 songbird species to be recorded. Of these flight calls, 340 calls were identified to
species and the remaining 232 were identified to either a ‘complex’ (a group of species
which were problematic to identify due to almost identical nocturnal flight calls,) or
‘unidentifiable’ (more sophisticated analysis software needed). The most frequently
recorded species were Swainson’s Thrush, Yellow Warbler, Wilson’s Warbler and
Hermit Thrush (Appendix 1). Detection of flight calls generally peaked twice in the
season: once in mid-August and once in mid-September (Figure 3).
120
Number flight calls recorded per night
100
80
60
40
20
0
6-Aug04
10-Aug- 14-Aug- 18-Aug- 22-Aug- 26-Aug- 30-Aug04
04
04
04
04
04
3-Sep04
7-Sep04
11-Sep04
15-Sep04
19-Sep04
Date of night
FIGURE 3. Nightly abundance of migratory flight calls detected throughout the monitoring
season (August 6 – September 20, 2004), (no flight calls recorded August 21, 24, 26, 27, 31,
September 1, 2, and 3 due to weather interference).
Weather and detection ability
No flight calls were detected on August 21, 24, 26, 27, 31 and September 1, 2,
and 3. These nights had strong winds and/or rain, which interfered with recording.
Among the remaining nights of monitoring, there was considerable variation in weather
and the number of migrants recorded per night.
6
Time of night
Detections tended to build in frequency throughout the night, and the highest
frequencies of calls were detected approximately 30-60 minutes before sunrise (Figure 4).
120
Number of flight call detections
100
80
60
40
20
0
8:00:00
PM
9:00:00
PM
10:00:00
PM
11:00:00
PM
12:00:00
AM
1:00:00
AM
2:00:00
AM
3:00:00
AM
4:00:00
AM
5:00:00
AM
6:00:00
AM
Time of night (hrs)
FIGURE 4. Average time of night nocturnal flight calls were detected.
Highlights
Due to the fact that very little acoustic monitoring has been conducted on the west
coast of North America, this exploratory research resulted in a number of interesting
observations being made.
Hermit Thrush set a new earliest arrival record at RPBO on August 9; the
historical arrival record was August 27, 1998.
MacGillivray’s Warbler was one of the target species to record, as very few of
them are banded each season at RPBO. Two calls were recorded: one on August 7 and
the other on August 17. There were no reference recordings available for the nocturnal
flight call of this western warbler species, so unidentified spectrograms were compared to
nocturnal flight calls of its eastern counterpart, the Mourning Warbler. These two species
have nearly identical vocalizations, and as expected the spectrograms for both species
7
matched very closely. This detection/discovery and recording of the MacGillivray’s
Warbler nocturnal flight call has provided essential baseline data to allow for future
monitoring of this western species.
Additionally, a Northern Waterthrush, which is a rare bird for Rocky Point
(usually occurring less than five times per year) was recorded on August 12.
Spectrograms
The majority of songbird species have a unique nocturnal flight call and therefore
a uniquely associated spectrogram. Spectrograms are visual representations that illustrate
the frequency, modulation, and duration of each flight call. By recording and analyzing
these spectrograms, we can identify the species of birds migrating over Rocky Point.
MacGillivray’s Warbler – August 18th, 0421 hrs
10
F
r
e
q
u
e
n
c
y
(kHz)
0
Photo credit: Ralph Hocken
0
100
Call duration (mS)
8
Yellow Warbler – August 15th, 0235 hrs
10
F
r
e
q
u
e
n
c
y
(kHz)
0
Photo credit: Ralph Hocken
0
100
Call duration (mS)
Chipping Sparrow – September 10th, 0455 hrs
10
F
r
e
q
u
e
n
c
y
(kHz)
0
0
100
Photo credit: Ralph Hocken
Call duration (mS)
9
Lincoln’s Sparrow – September 7th, 0500 hrs
10
F
r
e
q
u
e
n
c
y
(kHz)
0
Photo credit: Ralph Hocken
0
100
Call duration (mS)
(*Note – all nocturnal flight calls are “presumed”, even if 99.9% confident of the species
identification because there is no visual confirmation of the bird).
DISCUSSION
Methodology
Initially, an open, grassy meadow (Figure 2) was thought to be the most
conducive location to recording nocturnal flight calls. However, it appears that a more
productive location may be a transect running parallel to the ocean shoreline. Because
landbirds are generally reluctant to cross large bodies of water, birds tended to call much
more frequently at this land-water interface before embarking on their flight across the
Straight of Juan de Fuca. The majority of birds followed the shoreline for hundreds of
meters, before seeming to reluctantly cross over the water. This significant increase in
flight call numbers was not detected from our microphone location which was only 1 km
away. If multiple microphones were put in series running along the ocean shoreline a
few hundred metres apart, many more flight calls could be detected in this zone of
increased call activity.
10
Swainson’s Thrush
In 1995, the Canadian Wildlife Service (CWS) conducted a four-night pilot study
at RPBO to determine if the number of nocturnal migrants was comparable to the
banding data. CWS detected 495 migrating Swainson’s Thrush during this four night
period, whereas only 20 Swainson’s Thrush were banded the entire fall migration period
from July 21 - October 18. This nocturnal acoustic migration project conducted by
RPBO in 2004 revealed similar results. Using the acoustic technique, we detected 251
Swainson’s Thrush over 20 nights; whereas only 148 were banded over the entire three
month season.
Over the acoustic monitoring season (August 6 – September 20, 2004) the highest
numbers of Swainson’s Thrush were recorded on the night of September 11.
The
microphone recorded 86 flight calls within the last 20 minutes (0556 – 0616 hrs) of
recording. At the land-water interface over the same time period (0556 – 0616 hrs) 2,326
Swainson’s Thrush calls were counted by observers. However, this does not imply that
each call was a distinct bird (i.e.-this does not imply 2,326 individual birds). Of the
2,326 individual calls, a conservative estimate would be 300-400 birds on that single
morning. Near dawn, nocturnal migrants descend to the ground to spend the daytime
feeding and resting. During their descent there is a natural inclination for each bird to
call several times (Farnsworth et al., 2004).
Due to our numerous detections of
Swainson’s Thrush, nocturnal recording would likely be an extremely useful alternative
to banding as a monitoring tool for this species.
Considerations for analysis
Several studies have shown that the peak in vocalizations of nocturnal migrants
occur in the hours just before dawn (Farnsworth et al., 2004). One factor to consider was
how to separate the year-round, resident birds that vocalized near sunrise from migrant
birds of the same species. For instance, species such as White-crowned Sparrow and
Song Sparrow are year-round residents, but there are most likely migratory populations
as well. To avoid erroneously counting residents as migrants, I only included flight calls
that were made throughout the night up until 30 minutes before sunrise, which is
approximately when the resident birds at RPBO began to call.
Another consideration was that a single bird can call several times. I determined
that if multiple flight calls were made within a short time span they probably originated
11
from a single bird. However, if the calls were separated by several minutes it was
assumed they came from different birds (Evans and Rosenberg, 2000).
Species complexes
There are some groups of birds that have almost identical nocturnal flight calls,
and therefore are difficult to distinguish to species level. The nocturnal flight calls of
Yellow-rumped Warbler, Orange-crowned Warbler, Townsend’s Warbler, Nashville
Warbler and White-crowned Sparrow are all very similar and often not separable (Evans
and O’Brien, 2002). Approximately 30 % of the flight calls detected over the season fell
into this complex, and therefore were not separated to species. However, with more
study of the slight differences in duration, pitch and character of these calls using more
sophisticated software, the majority of calls should be able to be identified. Additionally,
Fox Sparrow and Song Sparrow were sometimes difficult to separate and some were left
as a ‘sparrow complex’.
Infrequently banded species
One of the objectives of this project was to determine whether acoustic monitoring
could be used as a method to monitor species that are infrequently encountered through
banding.
For example, infrequently banded species such as the Black-throated Gray
Warbler, Varied Thrush and Black-headed Grosbeak are target species for monitoring and
should be detected through the acoustic method if the previously mentioned improvements
to protocol such as microphone placement are observed.
For species such as the
Townsend’s Warbler this model may be unrealistic without more sophisticated software,
as its nocturnal flight call is too similar to other species to distinguish confidently. The
acoustic method was successful in detecting the infrequently banded MacGillivray’s
Warbler.
Weather
Weather may have played a role in nightly variation in number of nocturnal
migrant detections. Birds tend to gauge weather when initiating migratory movements
(New Jersey Audubon Society, 2004). Bird migration is dependent on a number of
weather factors such as rain, wind direction and speed, and location of frontal boundaries
(Able, 1973). In general, the busiest migration nights tended to be on cold (~8 Co), clear
12
nights. Wind speed was close to zero, and winds were generally south-southwesterly.
Migration is complex and other factors besides weather may be responsible for nightly
variations in migration numbers.
CONCLUSION AND RECOMMENDATIONS
This project effectively demonstrated the use of nocturnal flight call analysis as a
conservation tool for many songbirds, especially the Swainson’s Thrush. Due to our
numerous detections of Swainson’s Thrush, nocturnal recording would likely be an
extremely useful alternative to banding as a monitoring tool for this species.
With a few minor changes in protocol, I believe this method of nocturnal
monitoring could potentially record thousands of nocturnal migrants passing over RPBO.
This project was successful in demonstrating that acoustic monitoring is an effective
method of supplementing the banding data and the technique also demonstrated its use
for detecting infrequently encountered species such as the MacGillivray’s Warbler.
Nocturnal monitoring may be more cost-effective and less intrusive than conventional
monitoring techniques which are subject to numerous biases such as detecting only
understory birds. Continuing this project in future migration periods will enable the
Rocky Point Bird Observatory to accurately assess population dynamics and support the
conservation of songbird species for many generations to come.
ACKNOWLEDGEMENTS
First and foremost, thank you to the TD Friends of the Environment Foundation
for providing the funding and support to this conservation project. Thank you to Bill
Evans and OldBird Inc for their expert advice on spectrogram analysis, microphone
design and analysis software. A special thanks to Bob Chappell for his dedication to
constructing and monitoring the project’s microphone and experience in other electrical
aspects. Thanks to Jeremy Gatten for his help and advice on the project. Thanks to the
board members of Rocky Point Bird Observatory, especially to David Allinson for his
advice, support and assistance in setting up the project, and to Paul Levesque and Katie
Christie for their comments and advice on this report. Thank you to Jukka Jantunen for
the observational flight call data he provided to the project. Thank you to Ralph Hocken
for his excellent bird photographs. Thank you to Bird Studies Canada for their map of
13
the Canadian Migration Monitoring Network. A final thanks to the Department of
National Defence for their aerial photographs and most importantly for providing access
to Rocky Point Bird Observatory.
LITERATURE CITED
Able, K.P. 1973. The role of weather variables and flight direction in determining the
magnitude of nocturnal bird migration. Ecology, Vol. 54. 5: 1031-1041.
Derbyshire, D. 1999. Migration Monitoring at Rocky Point, Fall 1999. Rocky Point Bird
Observatory, Metchosin, B.C.
Derbyshire, D. 2000. A Report on Migration Monitoring at Rocky Point: Fall 2000.
Rocky Point Bird Observatory, Metchosin, B.C.
Evans W.R. and M. O’Brien. 2002. Flight Calls of Migratory Birds Eastern North
American Landbirds. Old Bird, Inc. [CD-ROM]
Evans, W.R. and K.V. Rosenberg. 2000. Acoustic monitoring of night-migrating birds: a
progress report. Proceedings of the 3rd Partners in Flight Workshop; 1995.
Farnsworth, A., S.A. Gauthreaux and D. van Blaricom. 2004. A comparison of nocturnal
call counts of migrating birds and reflectivity measurements on Doppler radar.
Journal of Avian Biology. 35: 365-369.
Gibson, C.G. 2001. Migration Monitoring at Rocky Point Bird Observatory in 2001.
Rocky Point Bird Observatory, Metchosin, B.C.
Jantunen. J. 2003. Migration monitoring at Rocky Point Bird Observatory in fall 2003.
Rocky Point Bird Observatory, Metchosin, B.C.
New Jersey Audubon Society. 2004. Preserving oases along the flyway.
http://www.njaudubon.org/Education/Oases/
Woodward, D. 2002. Fall Banding Report for Rocky Point Bird Observatory. Rocky
Point Bird Observatory, Metchosin B.C.
14
APPENDIX 1. Total number of nocturnal flight calls detected per species per night from August
6 to September 20. Note - no flight calls were detected on August 21, 24, 26, 27, 31, September
1, 2, and 3 due to strong winds and/or rain which interfered with recording.
SWTH = Swainson’s Thrush (Catharus ustulatus), YWAR = Yellow Warbler (Dendroica petechia),
WIWA = Wilson’s Warbler (Wilsonia pusilla), HETH = Hermit Thrush (Catharus guttatus), MGWA =
MacGillivray’s Warbler (Oporornis tolmiei), LISP = Lincoln’s Sparrow (Melospiza lincolnii), CHSP =
Chipping Sparrow (Spizella passerina), SAVS = Savannah Sparrow (Passerculus sandwichensis), SOSP =
Song Sparrow (Melospiza melodia), FOSP = Fox Sparrow (Passerella iliaca), COYE = Common
Yellowthroat (Geothlypis trichas), NOWA = Northern Waterthrush (Seiurus noveboracensis), ‘complex’ =
a group of birds having almost identical nocturnal flight calls, and therefore difficult to distinguish to
species level
DATE
06Aug
07Aug
08Aug
09Aug
10Aug
11Aug
12Aug
13Aug
14Aug
15Aug
16Aug
17Aug
18Aug
19Aug
20Aug
22Aug
23Aug
25Aug
28Aug
29Aug
30Aug
04Sep
05Sep
06Sep
07Sep
08Sep
09Sep
10Sep
11Sep
12Sep
13Sep
14Sep
15Sep
16Sep
17Sep
18Sep
19Sep
SWTH
WIWA
HETH
MGWA
LISP
CHSP
SAVS
SOSP
FOSP
COYE
NOWA
1
1
1
1
4
1
1
1
1
1
3
16
3
2
4
8
3
3
1
4
2
28
69
6
1
15
1
86
5
3
1
2
1
1
3
1
1
4
4
3
2
13
10
1
3
4
1
2
1
171
Unidentifiable
511
61
GRANDTOTAL
572
1
19
11
11
9
17
13
19
10
6
3
2
1
1
1
9
10
13
1
2
5
1
1
2
2
1
1
4
1
1
1
1
3
1
18
5
1
1
1
1
1
1
2
4
2
2
1
2
251
Totals
1
1
1
1
'complex'
5
1
7
0
17
5
5
7
9
0
3
3
1
5
0
3
29
75
7
1
17
2
94
5
2
1
6
10
2
0
4
2
1
1
1
1
6
9
1
20Sep
Totals
YWAR
40
15
13
2
1
6
5
3
1
2
1
15
16