(Full Article)

Arch.Geflügelk., 77 (1). S. 10–14, 2013, ISSN 0003-9098. © Verlag Eugen Ulmer, Stuttgart
The effect of selection for high and low feather pecking on
aggression – related behaviours of laying hens
Der Einfluß der Selektion auf hohes und niedriges Federpicken auf das aggressive Verhalten
von Legehennen
W. Bessei, Hanna Bauhaus and Stefanie Bögelein
Manuscript received 27 February 2012, accepted 12 May 2012
Introduction
Feather pecking and aggressive pecking in chickens are considered to have different underlying motivations. Threatening, aggressive pecking, and fighting behavior is performed
in the context of establishing and maintaining the social
hierarchy in poultry flocks (GUHL, 1968). Aggressive pecks
are delivered in an upright body posture and mainly directed
towards the head of the birds, where they cause feather
damage (BILCIK and KEELING, 1999). In some cases, when a
submissive bird flees from the attack of a dominant bird,
aggressive pecks are delivered to the back. Most feather damages of the body, however, are due to vigorous feather
pecking. Therefore it is possible to differentiate between aggressive pecking and vigorous feather pecking. The latter is
often followed by pulling and eating the feathers (MCKEEGAN
and SAVORY, 1999). There is no clear tendency in the interrelationship between aggression or the position in the social
hierarchy in a flock and feather pecking. WENNRICH (1975)
suggested an interrelationship between feather pecking
and aggressive pecking in so far as a high social rank may
facilitate both, aggressive and feather pecking. Laying hens
in 4-bird-cages, which showed poor feather conditions as a
result of feather pecking, had lower social rank scores than
those with better plumage conditions. The results, however,
were not consistent in all groups (HUGHES and DUNCAN,
1972). Males in a flock have usually a higher social rank than
females. But they are preferred targets for feather pecking
of hens (BRAASTAD and LANGSTRAND, 1994). Indeed, the males
high in rank received more feather pecks by hens than low
ranking males (KATHLE et al., 1997). KJAER et al. (2001)
found no relationship between feather pecking and aggressive pecking in lines selected for high and low feather pecking. The present study was carried out to compare aggressive interactions within and between White Leghorn lines
selected for high and low feather pecking in a social environment.
Materials and methods
A total of 224 laying hens of a White Leghorn line, which
have been divergently selected for high (HFP) and low
(LFP) feather pecking behavior for 9 generations were
Dept. of Farm Animal Behavior and Poultry Science, University Hohenheim,
Stuttgart, Germany
used in the present experiment. The selection criterion was
the number of bouts of vigorous feather pecking in adult
layers kept in groups. The selection was started in the Danish
Institute of Animal Sciences, Foulum, Denmark, for the
first 5 generations (KJAER et al., 2001) and then for 4 more
generations at the Institute of Animal Husbandry and Breeding, University of Hohenheim, Stuttgart-Hohenheim, Germany. The pullets of the lines were raised in separate deep
litter pens under conventional lighting and feeding programmes. At 26 weeks of age 114 HFP and 110 LFP birds
were randomly assigned to five pens of 16 m2 each with half
the birds of each pen of the HFP and LFP line. The pens
were separated by a wire mesh grid and the birds could see
and hear the birds of the other pens. All pens were equipped
with round feeders, nipple drinkers, nests, perches and a
litter area (1/3 of the total space). The litter area was covered with wood-shavings (10 cm depth). The litter was dry
and friable throughout the experiment. Two third of the pen
area were covered by a perforated plastic floor. The slats
were raised at a height of 38 cm from the litter area. Artificial light was provided through incandescent bulbs from 3 am
to 5 pm (14 hrs). There was additional natural light through
transparent plastic material at the upper part of the side
wall of the pen. Light intensity provided by the electric
bulbs was 20 lux, and, depending on the irregular fluctuation of the natural light, increased occasionally up to 2500
lux on sunny days. Fresh air was supplied through a thermostatically controlled ventilation system. Room temperature was kept between 18 and 22°C. A conventional layer
mash containing 16.6% crude protein and 11.4 ME/kg was
provided ad libitum. The birds were weighed and individually identified by numbered plastic tags on their back before the start of the observations (25 weeks of age). The
tags of soft plastic material (14 × 5.5 cm, length × width)
were fixed with plastic bands at the humerus of both wings
one week before the start of the observations. The birds
adapted to the tags within a few hours and showed undisturbed locomotor behaviour including flying to the perches
and nests. Gentle pecking of the tags by the pen mates was
observed at a low frequency during the first days and disappeared thereafter. Each pen was visually observed in 15
sessions of 20 min each for aggressive behaviour and in 10
sessions of 20 min each for feather pecking behaviour. The
observations were carried out from 26 to 35 weeks of age
from 6:30 to 15:00 hrs. They were balanced for observation day and time of day. Criteria for aggression were the
number of threats and aggressive pecks delivered and
received. Threats were recorded when a hen visually fixed
a pen mate in an upright body posture. Threats were followed either by avoidance or escape behaviour of the subArch.Geflügelk. 1/2013
Bessei et al.: Interrelationships between aggressive pecking and feather pecking in hens
ordinate hen, by aggressive pecking or by fighting. Aggressive pecks are defined as pecks towards the head and the
body of conspecifics in an aggressive posture of the aggressor or during a fight among hens. The winner and loser of the
fights were recorded.
With regard to feather pecking, only vigorous pecks or
pulls delivered and received were recorded. This behavior
is carried out in a non-aggressive posture of the feather
pecking hen. The hen to be pecked either tolerates pecking
or tries to escape.
The differences of aggressive pecks delivered and received
and feather pecks delivered and received were calculated
and referred here as feather pecking index and aggression
index. The latter has been described by LEE and CRAIG (1982)
as a criterion of social rank in laying hens.
Statistical analysis
The JMP SAS, Version 7 programme was used for all statistical procedures (SAS, 2007: JMP 7 Statistics and Graphics
Guide, SAS Institute Inc., Cary, NC, USA). The mean number
per 20 minutes session of threats delivered and received,
aggressive pecks delivered and received, bouts of vigorous
feather pecks delivered and received of each bird were calculated over the 15 observation sessions for feather pecking
and 10 observation sessions for aggressive pecking. Data
were subjected to a two-factorial analysis of variance with
pen and line as factors. The residuals were tested for normal
distribution. All criteria with the exception of body weight,
feather pecking index and aggression index significantly
deviated from the Gaussian distribution. As the effect of
pen was not significant for any of the criteria the line effects
were analysed by a one-factorial design using non-parametric procedure (Wilcoxon-Test) for not normally distributed criteria and ANOVA and student’s t-test for criteria
which followed a normal distribution. The Bartlett-Test
was used to test the divergence of variances of the index of
feather pecking and Aggression Index. The correlations
between the criteria were calculated within lines by Spearmans rank correlation coefficient.
11
ered and received more aggressive pecks than LFP birds.
There was no significant difference between the lines in the
mean aggression index, while the feather pecking index
was significantly higher in the HFP as compared to the LFP
line. There was greater variation for both the aggression and
feather pecking indices between individual birds in HFP than
LFP (Figure 1a, b). HFP were significantly heavier than LFP,
though the difference was relatively small.
The correlations of the different behavioural criteria and
body weight are shown in Table 2. There were significantly
positive correlations between threats delivered, aggressive
pecks and aggression index within both lines. The correlations between the aggressive pecks received and threats
received were also significant within both lines (0.65 and
0.25). The correlations between aggressive pecks delivered
and received, and threats delivered and received were close
to zero in both lines. The aggression index was positively
correlated with threats and aggressive pecks delivered and
negatively correlated with threats and aggressive pecks
received. The correlation coefficients were significant in
all cases. The correlations of feather pecks delivered and
received were low and not significant. Feather pecks delivered were positively and feather pecks received negatively
correlated with the feather pecking index. The correlations
were significant in both lines. The correlations between
feather pecks delivered and received and the aggression
index were not significant in the HFP and in the LFP. Aggressive pecks received and feather pecking index were negatively correlated, the effect was significant in the LFP only
(–0.23 for LFP and –0.09 for HFP). The correlation of feather
pecking index and aggression index was significant in the
HFP only (0.19). Body weight was significantly positively
correlated with aggressive pecks delivered, aggression index
and feather pecks received within the HFP. Negative correlations appeared between body weight and feather pecks
delivered, aggressive pecks received and feather pecking
index within the HFP. All correlations between body weight
and the behavioural traits were low and not significant in
the LFP.
Discussion
Results
The lines differed significantly in the number of threats and
aggressive pecks delivered and received (Table 1). HFP deliv-
The selection for high and low number of bouts of feather
pecking has led to highly divergent lines in this criterion.
The HFP birds in the present study delivered 7 times more
feather pecks than the LFP birds (Table 1). But the number
Table 1. Mean values *) of lines selected for high (HFP) and low (LFP) feather pecking for threats delivered and received, aggressive
pecking delivered and received and feather pecks delivered and received; feather pecking index, aggression index and body
weight (BW);
Mittelwerte für ausgeteilte und erhaltene Drohungen (threats), Federpickschläge Fp)und aggressive Pickschläge (Aggr.) sowie für den
Federpick-Index und Aggressionsindex (Differenz zwischen ausgeteilten und erhaltenen Pickschlägen) und Körpergewicht der auf
hohes (HFP) und niedriges (LFP) Federpicken selektierten Linien
Line
N
Threats
delivered
Threats
received
Aggressive
pecks
delivered
Aggressive
pecks
received
Aggression
index**)
Feather
pecks
delivered
Feather
pecks
received
Feather
pecking
index**)
BW (kg)
HFP
LFP
114
110
224
0.99
0.50
p < 0.002
0.98
0.49
p < 0.004
3.46
1.19
p < 0.001
3.15
1.35
p ≤ 0.001
0.49
0.43
n.s.
7.57
1.05
p < 0.001
3.96
4.17
n.s.
3.61
–3.12
p ≤ 0.001
1.52
1.45
p ≤ 0.001
*) Mean number of events per 20 minutes based on 15 observation sessions of 20 minutes each
Mittlere Anzahl von Ereignissen über 15 Beobachtungen zu 20 Minuten
**) Number of aggressive pecks (resp. feather pecking bouts) delivered minus number of aggressive pecks (resp. feather pecking bouts) received
Mittlere Anzahl ausgeteilter aggressiver Pickschläge (bzw. Federpickserien) abzüglich Anzahl erhaltener aggressiver Pickschläge.
Arch.Geflügelk. 1/2013
12
Bessei et al.: Interrelationships between aggressive pecking and feather pecking in hens
a
30
20
10
0
-10
-20
-30
b
HFP
LFP
HFP
LFP
110
90
70
50
30
10
-10
-30
Figure 1. a,b. Variation in the Aggression-Index (a) and Feather
Pecking Index (b) between the high (HFP) and low (LFP) feather
pecking lines (points show for each bird the difference between
pecks delivered and pecks received)
Variation des Aggressionsindex (a) und des Federpick-Index (b) der
auf hohes (HFP) und geringes(LFP) Federpicken selektierten Linien
(die Punkte bezeichnen die Differenz zwischen ausgeteilten und
erhaltenen Federpickschlägen bzw. aggressiven Pickschlägen)
of feather pecks received was nearly identical in the HFP and
LFP lines and the correlation between feather pecks delivered
and received was low and not significant within both lines
(0.01 and –0.07). The interrelationships between feather
pecks delivered and feather pecks received are not consistent
in the studies reported in the literature. CUTHBERTSON (1978)
assumed that high feather pecking hens be less frequently
the target of being pecked. KJAER et al. (2001) observed the
lines used in the present study in an earlier generation. As
in the present study there was no difference between the
lines in the number of pecking bouts received. The genetic
correlation between feather pecking and being pecked within
lines was not significant (KJAER and SØRENSEN, 1997). Although a genetic basis for feather pecks received has been
reported in different studies (CUTHBERTSON, 1978; BESSEI,
1985) feather pecking and being feather pecked are obviously determined by different genetic mechanisms. Selection for
high feather pecking has obviously not changed the risk of being feather pecked in the lines used in the present study.
There was a high consistency among the aggression related
criteria within both lines with the HFP showing higher means
for threats and aggressive pecks delivered and received
(Table 1). These results are in contrast to observations of
KJAER et al. (2001) who found no difference in aggression
between the same lines in earlier generations, and to other
lines differing in feather pecking (BLOKHUIS and ARKES, 1984).
The divergence in aggression between the HFP and LFP
may have developed during the recent selection generations.
It is also possible that the extended time spent in the observation of aggressive behaviour, and the concentration of
the observer on aggression related criteria has uncovered
differences in aggression which have not been recognized in
the earlier studies. High rank in the social hierarchy may
facilitate the approach to target birds for feather pecking.
This could explain the higher aggression in the HFP lines
of this experiment. MCKEEGAN and SAVORY (1999) reported
a low non-significant correlation between non-aggressive
feather pecks/pulls and aggressive pecks in layer flocks. The
correlation between aggressive pecking, feather damages
and feather eating events was significant. This correlation
Table 2. Spearman‛s rank correlation coefficients between aggression and feather pecking related criteria and body weight of a
high (HFP; above the diagonal) and low (LFP, below the diagonal) feather pecking line. Significant coefficients (p < 0.05) are
marked with underlined figures.
Spearman`s Rangkorrelationskoeffizienten zwischen den Merkmalen die mit Federpicken (ausgeteilte und erhaltene Federpickschläge,
Federpick-Index) und Aggression (ausgeteilte und erhaltenen Drohungen, aggressiven Pickschlägen und Aggressionsindex) verbunden
sind sowie dem Körpergewicht innerhalb der auf hohes und geringes Federpicken selektierten Linien. Signifikante Korreletionskoeffizienten (p < 0,05) in unterstrichenen Zahlen)
Threats del
Threats Rec.
Aggr. Del
Aggr. Rec.
Aggr. Ind
Fp del
Fp rec
Fp Ind
BW
Threats
delivered
Threats
received
Aggressive
pecks
delivered
Aggressive
pecks
received
Aggr.
Index
Feather
pecks
delivered
Feather
pecks
received
Feather
Pecking
Index
Body
weight
–
–0.12
0.00
0.48
–0.03
0.39
0.09
–0.06
0.09
0.10
–
0.60
–0.07
–0.09
0.25
–0.21
0.04
–0.10
0.10
–0.06
–
–0.13
0.65
–0.15
–0.05
0.81
0.06
–0.04
0.06
0.09
–
0.49
–0.45
0.78
–0.74
–0.62
–0.14
0.19
–0.23
–0.15
–
0.01
–0.08
–0.05
–0.12
0.04
0.13
–0.15
0.19
0.15
–
0.03
–0.03
0.20
–0.07
0.18
0.01
–0.07
0.57
0.04
–
0.00
–0.07
0.11
–0.09
–0.02
0.95
–0.31
–0.86
0.09
–
0.12
–0.03
0.28
–0.18
0.30
–0.20
0.20
–0.25
–0.06
–
Arch.Geflügelk. 1/2013
Bessei et al.: Interrelationships between aggressive pecking and feather pecking in hens
was mainly based on the behaviour of two out of 12 groups,
which showed the highest level of feather pecks and the
highest level of aggression. The HFP of the present study
may represent the conditions of these extreme groups of
the MCKEEGAN and SAVORY (1999) experiment. The higher
body weight of the HFP (Table 1) may also have assisted
the birds to establish a higher social rank within the mixed
groups. If high social rank and aggression facilitate feather
pecking, we would expect that the HFP deliver more feather
pecks than the LFP and that the LFP receive more feather
pecks than the HFP. We would also expect a positive correlation between feather pecking and aggressive actions within
the lines, and in the HFP in particular. However, the correlations between aggressive pecks delivered and feather pecks
delivered were close to zero. In addition, there was a significantly positive correlation between aggressive pecks
delivered and feather pecks received in the HFP line (0.20)
which shows that the aggressive HFP received more feather
pecks than expected under the above mentioned assumption. The present results show that the social behaviour in
the groups of laying hens is not a univariate system of aggression and submission. The LFP, though less aggressive than
the HFP, obviously are able to avoid aggressive encounters.
This is clearly demonstrated in the variation of the aggression index (Figure 1). While the means of both lines are
similar, the aggression index of the HFP birds showed a
wider range than of the LFP and the LFP are neither presented at the higher nor at the lower end of the distribution. The fact that the LFP experience a significantly lower
number of threats shows that the birds are able to anticipate and avoid social encounters.
Aggression avoidance behaviour of the LFP may have
developed in response to the large group keeping system.
BESSEI et al. (1984) observed the behaviour of Leghorn hens
after five generations of selection for high egg production
under cage and floor pen systems. When tested in both environments, the hens were less aggressive in the system where
they had been selected. The results have been explained
by the development of aggression avoidance strategies.
ENGELMANN (1968) suggested layers reduce aggression
through avoiding visual fixation of pen mates. The behavioural strategies of avoiding aggressive interactions of the
LFP it not known and requires further studies. The avoidance behaviour of the LFP was obviously not effective with
regard to prevent receiving feather pecks. LFP received as
many feather pecks as HFP. This can be explained by the
non-aggressive posture that feather pecking hens when
they approach target birds. The feather pecked hens not
only tolerate the approach of the feather pecker, but also
vigorous and damaging feather pulling.
Conclusions
Selection for high feather pecking has obviously led to
increased aggression in the lines used in the present study.
Selection for high feather pecking did not reduce the risk
of being feather pecked. The LFP, though less aggressive
than the HFP, obviously are able to avoid aggressive encounters. The avoidance behaviour of the LFP was not effective
with regard to feather pecking behaviour.
Acknowledgement
The authors acknowledge the financial support of the studies
on feather pecking of the German Research foundation
(DFG) and Dr. J.B. Kjaer for supply of the lines selected for
high and low feather pecking.
Arch.Geflügelk. 1/2013
13
Summary
Feather pecking and aggressive pecking of laying hens are
generally considered to underlie different motivations. Divergent selection of a White Leghorn line for high (HFP) and
low (LFP) feather pecking in a social group has lead to increased aggression in the HFP line. The aggressive behaviour (threats, aggressive pecking delivered and received)
and vigorous feather pecks delivered and received have
been observed in a total of 114 HFP and 110 LFP hens kept
in 5 intermingled groups with half of the birds of each line.
The difference between feather pecks and aggressive pecks
delivered and received, referred here as feather pecking
index and aggressive pecking index, was calculated. The
HFP birds delivered significantly more feather pecks than
LFP birds. There was no significant difference between the
lines for the feather pecks received. The HFP lines showed
higher frequencies in threats and aggressive pecks delivered and received than LFP birds. The feather pecking index
was higher in the HFP than in the LFP. There was no difference between the lines in the mean aggression index. High
positive correlations within lines were obtained for the
threats, aggressive pecks delivered and fights won. The correlations between aggressive pecks delivered and received
and feather pecks delivered and received were small and
not significant. The means of aggression index were close
to zero for both, HFP and LFP. But the variation was significantly higher in the HFP than in the LFP. This indicated
that the overwhelming share of aggressive encounters happened among the HFP. The LFP not only showed a lower
level of active aggression, but also successfully avoided aggressive encounters with the more aggressive HFP. The difference of the feather pecking index of the HFP and LFP
and the similar frequency of feather pecks received in both
lines indicated that the LFP were less successful in avoiding
feather pecks. The results confirm the hypothesis that feather pecking and aggressive pecking are controlled by different motivations. The selection for high feather pecking not
only increased the frequency in the selection trait but also in
active aggressive behaviours.
Key words
Laying hens, feather pecking, aggression, genetics
Zusammenfassung
Die Beziehungen zwischen aggressivem Picken und
Federpicken bei Legehennenlinien, die auf hohes
und geringes Federpicken selektiert wurden
Es wird allgemein angenommen, dass Federpicken und
aggressives Picken bei Legehennen unterschiedlichen Motivationen folgen. In der vorliegenden Arbeit soll geprüft
werden, ob die Selektion auf hohes Federpicken (HFP) und
niedriges Federpicken (LFP) die Aggressionen beeinflusst.
Das aggressive Verhalten (erhaltene und ausgeteilte aggressive Pickschläge und Drohungen) und heftige Federpickschläge (erhalten und ausgeteilt) wurden bei insgesamt
114 HFP und 110 LFP Hennen beobachtet. Die Tiere wurden in fünf gemischten Gruppen mit je zur Hälfte HFP und
LFP gehalten. Die Differenz zwischen erhaltenen und ausgeteilten Federpickschlägen sowie aggressiven Pickschlägen wurde als Federpick-Index bzw. als Aggressionsindex
bezeichnet. Die HFP teilten signifikant mehr Federpickschläge aus als die LFP. Es gab jedoch keine signifikante
Differenz zwischen den Linien für erhaltene Federpick-
14
Bessei et al.: Interrelationships between aggressive pecking and feather pecking in hens
schläge. Die HFP zeigten signifikant mehr Drohverhalten
und aggressives Picken als die LFP. Gleichzeitig verzeichneten die HFP mehr empfangene Drohungen und aggressive
Pickschläge als die LFP. Der Federpickindex war bei den
HFP höher als bei den LFP. Die Mittelwerte des Aggressionsindexes lagen in beiden Linien nahe bei Null und unterschieden sich nicht signifikant. Aber die Variation des Aggressionsindexes war bei den HFP signifikant größer als bei
den LFP. Innerhalb der Linien wurden hohe positive Korrelationen zwischen den Merkmalen Drohen und aggressives
Picken vorgefunden. Die Korrelationen zwischen ausgeteilten und erhaltenen aggressiven Pickschläge sowie
ausgeteilten und erhaltenen Federpickschlägen waren
gering und nicht signifikant. Die Ergebnisse zeigen, dass
der überwiegende Anteil der aggressiven Handlungen sich
zwischen den HFP Tieren abspielte. Die LFP zeigten nicht
nur einen geringeren Aggressionsspiegel, sondern konnten
offensichtlich erfolgreich aggressive Handlungen von Seiten der HFP vermeiden. Die LFP waren jedoch weniger
erfolgreich im Vermeiden von Bepickt-werden. Die Ergebnisse bestätigen die Hypothese, wonach Federpicken und
aggressives Picken durch unterschiedliche Motivationen
kontrolliert werden. Die Selektion auf hohes Federpicken
erhöhte nicht nur die Häufigkeit im Selektionsmerkmal,
sondern auch die Häufigkeit aggressiver Handlungen.
Stichworte
Legehennen, Federpicken, Aggression, Genetik
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Correspondence: Prof. Dr. Dr. h.c. Werner Bessei: Dept. of Farm Animal Behavior and Poultry Science (470c), University Hohenheim, Stuttgart, Germany;
e-mail: [email protected]
Arch.Geflügelk. 1/2013