Feather Pecking Behavior Research Articles

Variance components and selection response for feather-pecking behavior in laying hens

Variance components and selection response for feather pecking behavior were studied by analyzing the data from a divergent selection experiment. An investigation indicated that a Box-Cox transformation with power lambda = -0.2 made the data approximately normally distributed and gave the best fit for the model. Variance components and selection response were estimated using Bayesian analysis with Gibbs sampling technique. The total variation was rather large for the investigated traits in both the low feather-pecking line (LP) and the high feather-pecking line (HP). Based on the mean of marginal posterior distribution, in the Box-Cox transformed scale, heritability for number of feather pecking bouts (FP bouts) was 0.174 in line LP and 0.139 in line HP. For number of feather-pecking pecks (FP pecks), heritability was 0.139 in line LP and 0.105 in line HP. No full-sib group effect and observation pen effect were found in the 2 traits. After 4 generations of selection, the total response for number of FP bouts in the transformed scale was 58 and 74% of the mean of the first generation in line LP and line HP, respectively. The total response for number of FP pecks was 47 and 46% of the mean of the first generation in line LP and line HP, respectively. The variance components and the realized selection response together suggest that genetic selection can be effective in minimizing FP behavior. This would be expected to reduce one of the major welfare problems in laying hens.

Quantitative trait loci for behavioural traits in chickens

The detection of quantitative trait loci (QTL) of behavioural traits has mainly been focussed on mouse and rat. With the rapid development of molecular genetics and the statistical tools, QTL mapping for behavioural traits in farm animals is developing. In chicken, a total of 30 QTL involved in pecking-related traits, open-field behaviour, tonic immobility, response to novel objects, and response to a restraint test were detected in different studies. In the search for a useful early predictor for feather pecking (FP) behaviour in adult laying hens, the following was found: FP in young animals is not a predictor for FP in adult animals, however, open-field behaviour in young animals is genetically correlated with FP in adult hens. Before the implementation of FP behaviour or open-field behaviour in breeding programmes, it is essential to know more about the correlation between these behavioural traits and also their relationship with production traits. Nevertheless, with the QTL for behavioural traits and the chicken genome sequence in progress, a better understanding of the underlying genetic mechanisms of behavioural traits will be feasible.

Chronic increase of dietary l-tryptophan decreases gentle feather pecking behaviour

Many studies show the involvement of the serotonergic (5-HT) system in the performance of abnormal behaviour in both human and animals. Recently, we showed that acute reduction of 5-HT turnover in the forebrain, increased gentle and severe feather pecking behaviour in chicks from a high (HFP) and low feather pecking (LFP) line of laying hens, suggesting that the performance of feather pecking behaviour involves low 5-HT neurotransmission. In the present study, we postulated that if low 5-HT is causally underlying feather pecking, increasing 5-HT turnover in the forebrain will decrease the development and performance of feather pecking. Augmentation of 5-HT neurotransmission in the brain was induced by chronically increasing dietary levels of the essential amino acid l-tryptophan (TRP) from which 5-HT is synthesised. From the age of 34 days, LFP and HFP chicks were fed a diet containing 2% TRP, whereas control birds of both lines were continuously fed with the normal rearing feed (0.16% TRP). From 35 days of age, litter was removed from the pens (10 pens/line-treatment) and all chicks (10 chicks/pen) were housed on a slatted floor until the end of the experiment. At 49 days of age, feather pecking behaviour was studied for 30 min. At 50 days of age baseline corticosterone, TRP and other large amino acids (LNAAs) were measured in the blood plasma of decapitated chicks (10 chicks per line-treatment). Furthermore, plasma corticosterone and central 5-HT turnover levels in response to manual restraint (5 min) were determined (10 chicks/line-treatment). For neither gentle nor severe feather pecking a significant line × treatment interaction was found. However, TRP treatment resulted in a significant [ P = 0.02] overall decrease of the frequency of gentle feather pecking. For severe feather pecking a similar but not significant pattern was found. Significant line effects were found for gentle and severe feather pecking. HFP birds showed significantly higher levels of gentle and severe feather pecking behaviour than LFP birds [ P < 0.001]. TRP treatment significantly increased the TRP/LNAA ratio in the plasma of the chicks. Furthermore, TRP treatment overall increased baseline and stress-induced levels of plasma corticosterone (although more pronounced in the LFP line). TRP supplementation significantly increased 5-HT turnover in the hippocampus and archistriatum and tended to do so in the remainder of the forebrain. The results confirm our hypothesis that feather pecking behaviour is triggered by low serotonergic neurotransmission, as increasing serotonergic tone, by increasing dietary TRP, decreases gentle feather pecking behaviour.

Genetic and Phenotypic Correlations Between Feather Pecking Behavior, Stress Response, Immune Response, and Egg Quality Traits in Laying Hens

The objective of the current study was to estimate genetic and phenotypic correlations among feather pecking (FP) behavior and stress response, immune response, and egg quality parameters. These traits have been measured in an F2 cross, coming from a cross between a high and a low FP line of laying hens. Heritabilities (h2) of stress response (32 wk), primary immune response to keyhole limpet hemocyanin (KLH) (36 wk) and Mycobacterium butyricum (39 wk), and egg quality parameters (35, 44, and 50 wk of age) were calculated. The h2 was 0.05 ± 0.05 (SE) for stress response, 0.15 ± 0.07 for antibody response to KLH, and 0.08 ± 0.06 for antibody response to M. butyricum. The h2 for egg quality traits were in the range of 0.12 to 0.30. Significant phenotypic correlations were found between gentle FP in adult hens and stress response, egg weight at 44 and 50 wk, and egg deformation at 50 wk. Significant additive genetic correlations were found between severe FP in adult hens and antibody response to KLH (0.79 ± 0.35), and between ground pecking in adult hens and egg deformation at 50 wk (0.63 ± 0.26), and between ground pecking and eggshell strength at 35, 44, and 50 wk of age (-0.86 ± 0.29, -0.81 ± 0.20, -0.76 ± 0.24, respectively).

Between breed genetic variation for welfare-related behavioural traits in domestic fowl

Feather pecking (FP) resulting in feather loss and cannibalism (C) is an important welfare problem in domestic fowl ( Gallus gallus domesticus). Behavioural hypotheses to explain this behaviour have generally been proposed on the basis of within breed experiments or by comparing two lines. We examined these hypotheses by utilising a multi-breed experimental design to estimate the between breed genetic variation for feather pecking and cannibalism, behavioural time budgets and measures of fear, sociality and coping strategy. There was extensive between breed (genetic) variation in feather pecking, cannibalism and general pecking behaviour. There was no evidence for between breed genetic variation in time budgets; that for fear and coping strategy was slight whereas some measures of sociality were moderately high. Between breed genetic correlations between behavioural traits and feather score, skin score and mortality were low and those between sociality and skin lesions and culling were labile. We conclude that there is a strong genetic basis for damaging feather pecking and cannibalism and that these behaviours are not strongly related genetically to other behavioural traits.

Prenatal light exposure affects early feather-pecking behaviour in the domestic chick

Recently we proposed that early feather pecking is a form of social exploration. Social recognition, important for exploration, is a lateralized function in the domestic chick. Lateralization of functions can be influenced by light exposure late in embryonic development. Therefore, we investigated whether this light exposure affected early posthatching feather-pecking behaviour in domestic chicks, Gallus gallus domesticus. White leghorn embryos either were exposed to light or remained in darkness in the last week of incubation. After hatching, they were housed in groups of two light-exposed and two dark-incubated chicks. Light-exposed chicks showed more feather pecking than did their dark-incubated cagemates. Dark-incubated chicks preferred to direct feather pecks to unfamiliar peers than to familiar peers; light-exposed chicks showed no preference. These effects were present in the first week after hatching and remained at least another 3 weeks. These results support the hypothesis that early gentle feather pecking is part of the normal behavioural repertoire of young chicks and influences social exploration. We discuss a possible mechanism underlying these results. We also suggest that it may be worthwhile not to expose embryos to light during the last week of incubation when housing hatchlings in commercial conditions, where feather pecking is a serious problem.

Effects of haloperidol, a dopamine D2 receptor antagonist, on feather pecking behaviour in laying hens

Two experiments investigating general behavioural activity and specific pecking behaviour in laying hens under the effect of a dopamine D2 receptor antagonist, haloperidol, were performed. In experiment 1, a total of 240 White Leghorn hens aged 70 weeks were housed in 30 floor pens. Fifty-five of these hens acted as subjects in a dose-response experiment. Behaviour was recorded 30 min before and 30 min after injection of haloperidol or saline. The changes in behaviour were corrected for the effect of saline treatment. The results showed that doses of 0.05, 0.10, 0.20 and 0.50 mg haloperidol/kg body weight (BW) did not give any clear sedative effect, while 1.0 mg haloperidol did so. Thus, the sedative effect seemed to arise at a dose between 0.50 and 1 mg haloperidol/kg and it was concluded that 0.50 mg haloperidol/kg could be used in White Leghorn hens without inducing sedation. In experiment 2, a total of 48 ISA Brown hens aged 118 weeks were used as subjects. The birds were paired with 82-week-old White Leghorn hens chosen at random from an experimental line selected against feather pecking and housed in battery cages. Feather pecking and aggressive pecking were recorded for a period of 50 min before and 50 min after injection of either saline or 0.50 mg haloperidol/kg BW. Feather pecking, but not aggressive pecking, was significantly reduced in the haloperidol treatment (from 1.7 to 0.29 bouts per bird/h, Mann–Whitney U-test N=18, P<0.05; 10.1 to −2.7 pecks per bird/h, P<0.001). The negative value was due to correcting for effect of injection. The results supported our hypothesis that feather pecking behaviour in the adult hen is influenced by dopamine.

Detection of different quantitative trait loci for antibody responses to keyhole lympet hemocyanin and Mycobacterium butyricum in two unrelated populations of laying hens

Quantitative trait loci involved in the primary antibody response to keyhole lympet hemocyanin (KLH) and Mycobacterium butyricum were detected in two independent populations of laying hens. The first population was an F2 cross (H/L) of lines divergently selected for either high or low primary antibody responses to SRBC, and the second population was an F2 cross between 2 commercial layer lines displaying differences in feather pecking behavior (FP). Both populations were typed with microsatellite markers widely distributed over the genome with similar intervals between markers. Titers of antibodies binding KLH and M. butyricum were measured for all individuals by ELISA. Two genetic models were applied to detect QTL involved in the humoral immune response: a half-sib model and a line-cross model, both using the regression interval method. In the half-sib analysis, 2 QTL (on GGA14 and GGA27) were detected for the antibody response to KLH for the H/L population, and 2 QTL (on GGA14 and GGA18) were detected for the FP population. Only 1 QTL was detected for M. butyricum on GGA14 in the FP population using the half-sib analysis model. Two QTL were detected for the FP population on GGA2 and GGA3 using the line-cross analysis model. A QTL for the primary antibody response to KLH detected on GGA14 was validated in both populations under the half-sib analysis model. The present data suggest differences in the genetic regulation of antibody responses to two different T-cell dependent antigens.

Identification of quantitative trait loci for receiving pecks in young and adult laying hens

Feather pecking (FP) is a major problem in cage and free-range housing systems. In free-range systems, FP is more difficult to control. It is not known why a victim is being pecked. It could be that a bird is genetically predisposed to be pecked. To study the genetics of FP behavior, a large F2 population of 630 hens was generated from a cross between two commercial laying lines differing in their propensity to feather peck. The traits measured at 6 and 30 wk of age were receiving gentle FP, receiving severe FP, and receiving aggressive pecking. In addition, receiving toe pecking (TP) was also measured at 30 wk of age. For receiving gentle FP at 6 wk of age, a significant QTL on GGA1 and three different suggestive QTL were identified on GGA2, GGA6, and GGA7, respectively. For receiving gentle FP at 30 wk of age, a suggestive QTL on GGA5 was detected. For receiving aggressive pecking, a suggestive QTL was detected on GGA3. For receiving TP, three suggestive QTL were detected on GGA1, GGA5, and GGA23, respectively. The different QTL detected for receiving gentle FP at different ages indicate that this trait is regulated by different genes in young and adult hens.

Reaction to frustration in high and low feather pecking lines of laying hens from commercial or semi-natural rearing conditions.

The effect of rearing conditions on feather pecking and reaction to frustration was studied in two lines of laying hens. From commercial rearing conditions (large group, no mother hen), seven birds from a high feather pecking line (HC birds) and eight birds from a low feather pecking line (LC birds) were used. From semi-natural rearing conditions (small group, mother hen present) seven birds from the high feather pecking line (HN birds) were used. Feather pecking behaviour of HC, LC, and HN groups was recorded for 30 min. After that, each bird was food deprived and trained to peck a key for a food reward in a Skinnerbox. After training, each bird was subjected to a frustration session in a Skinnerbox, where the feeder was covered with Perspex. Three HC birds showed severe feather pecking, compared with one HN bird and zero LC birds. Differences in reaction to frustration were found between birds from different lines, but not in birds from different rearing conditions. LC birds tended to put their head in the feeder more frequently than HC birds over all sessions. Although limited, this study indicates that rearing conditions influence feather pecking, but not reaction to frustration.

Mapping quantitative trait loci affecting feather pecking behavior and stress response in laying hens

In the European Union, legislation concerning animal housing is becoming stricter because of animal welfare concerns. Feather pecking (FP) in large group housing systems is a major problem. It has been suggested that corticosterone (CORT) response to manual restraint as a measure for stress is associated with FP behavior. The aim of the current study was to identify QTL involved in FP behavior and stress response in laying hens. An F2 population of 630 hens was established from a cross between two commercial lines of laying hens differing in their propensity to feather peck. The behavioral traits, measured at 6 and 30 wk of age, were gentle FP, severe FP, and aggressive pecking. Toe pecking was measured at 30 wk of age and CORT response to manual restraint was measured at 32 wk. All animals were genotyped for 180 microsatellite markers. A QTL analysis was performed using a regression interval mapping method. At 6 wk of age, a suggestive QTL on GGA10 was detected for gentle FP. At 30 wk of age, suggestive QTL were detected on GGA1 and GGA2 for gentle FP. A significant QTL was detected on GGA2 for severe FP. At 32 wk of age, a suggestive QTL was detected on GGA18 for CORT response to manual restraint. In addition, a suggestive QTL was detected on GGA5 with possible maternal parent-of-origin effect for CORT response.

Strain and age differences in behaviour, fear response and pecking tendency in laying hens

1. Behaviours associated with a high or low tendency to feather peck could be used as predictors of feather pecking behaviour in selective breeding programmes. This study investigated how strain and age at testing influenced responses in behavioural tests. 2. Four layer-type strains (ISA Brown, Columbian Blacktail, Ixworth and a high feather pecking (HP) and a low feather pecking (LP) line of White Leghorn) were reared in 6 same-strain/line pens of 8 birds from one day old. Birds in half the pens were given an open field test, a novel object test and a test with loose feather bundles between 4 and 12 weeks of age and a tonic immobility (TI) test at 13 weeks of age. All pens were tested with fixed feather bundles at 26 weeks, and undisturbed behaviour in the home pens was videoed at 1 and 27 weeks of age. Daily records of plumage damage were used as an indicator of feather pecking activity in the home pens. 3. Strain did not influence novel object test, open field test or loose feather test behaviour, although age effects in all three tests indicated a reduction in fearfulness and/or an increase in exploratory behaviour with increasing age. 4. White Leghorns showed longer TI durations than the other strains but less pecking at fixed feather bundles than ISA Browns and Columbian Blacktails. 5. There were few associations between behaviour in the 5 different tests, indicating that birds did not have overall behavioural traits that were consistent across different contexts. This suggests hens cannot easily be categorised into different behavioural 'types', based on their test responses and casts doubt on the usefulness of tests as predictors of feather pecking.

Heritability of feather pecking and open-field response of laying hens at two different ages

The objective of the current study was to estimate heritabilities (h2) of feather pecking and open-field response of laying hens at two different ages. An F2 cross, originating from a high and a low feather pecking line of laying hens, was used for the experiment. Each of the 630 birds of the F2 cross was subjected to an open-field test (individual, 10 min) at 5 and 29 wk of age and to a social feather pecking test (groups of five birds on wood shavings, 30 min) at 6 and 30 wk of age. Both tests were performed in a square open field (1.25 × 1.25 m). Behavior was recorded directly from a monitor. Heritabilities of feather pecking and open-field behaviors were calculated. In the open-field test at 5 wk of age, high h2 were found for most traits, ranging from 0.20 for the frequency of flying to 0.49 for number of steps. In the social test at 6 wk, gentle feather pecking (0.12) and ground pecking (0.13) were found to be heritable. When both tests were repeated at 29 and 30 wk of age, h2 estimates were lower for the open-field test, ranging from 0.10 for duration of sitting to 0.20 for latency to first step. In the social test, however, higher h2 estimates of 0.15 for gentle feather pecking and 0.30 for ground pecking were found compared with 6 wk of age. In conclusion, gentle feather pecking and open-field behaviors may be used in selection against feather pecking.

The development of feather pecking behaviour and targeting of pecking in chicks from a high and low feather pecking line of laying hens

Large individual differences between adult laying hens in their propensity for feather pecking are known to exist. However, not much research has been carried out into the individual differences concerning the development of feather pecking behaviour. The purpose of this study was to investigate whether contrasting levels of feather pecking, observed among adult birds from two lines of laying hens, already occur at an early age. Furthermore, an important question to be discussed was whether different behavioural systems may be related to the occurrence of feather pecking. Therefore, this study consisted of studying and comparing the behaviour of White Leghorn laying hens from a high (HFP) and low feather pecking line (LFP) during the first 8 weeks of life. Chicks were reared in litter-floor pens and were kept in groups of five animals per line (12 groups per line).HFP chicks showed significantly higher levels of gentle feather pecking (gentle FP) than LFP chicks at the age of 14 and 28 days. Furthermore, HFP chicks spent significantly more time preening than LFP chicks on days 14, 28 and 41. Duration of foraging behaviour and feeding behaviour was significantly higher in the LFP line compared to the HFP line on days 41 and 56 and days 28, 41 and 56, respectively. HFP chicks showed a significant negative correlation between gentle FP and preening on days 3 (r=−0.49) and 41 (r=−0.86). In the LFP line duration of feeding correlated negatively with gentle FP on day 3 (r=−0.63). A principal component analysis (PCA) revealed that in the HFP line, gentle FP and preening exhibited high and opposite loadings on the same component at all ages, whereas feeding consistently loaded on the other component. This outcome contrasted with that of the LFP line. In this line feeding predominantly loaded on the same principal component as gentle FP, with loadings opposite to those of gentle FP, whereas preening showed the same loadings as gentle FP, on days 3 and 41.In conclusion, differences in feather pecking behaviour between HFP and LFP chicks can already be observed at a very early age during development. Furthermore, our results indicate that HFP and LFP chicks differ in the way pecking behaviour is targeted. This difference could be related to the existence of a difference in underlying motivational system controlling the development of feather pecking between the two lines.

The social transmission of feather pecking in laying hens: effects of environment and age

Abnormal behaviours, such as feather pecking, can become an even greater problem if they spread through the flock. Domestic hens are a social species and it has been suggested that feather pecking behaviour can be socially transmitted from few feather pecking individuals to many. The purpose of this experiment was to investigate whether feather pecking is transmitted from an experimental strain of high feather peckers to an experimental strain of low feather peckers when housed together. The effects of environment and age were also investigated. Four hundred and twenty chicks were housed in either cages or pens in one of three treatment groups; all high feather peckers, all low feather peckers, and a mixed group of half high feather peckers and half low feather peckers. During the rearing phase, severe and gentle feather pecks were recorded at 13–15 weeks and feather plumage scored at 12 and 17 weeks. At 17 weeks of age, the groups of mixed hens were removed from the study and the remaining hens resorted into the same three treatment groups. During the laying phase, severe and gentle feather pecks were recorded at 30–32 weeks of age and feather plumage was scored at 29 weeks of age. There was significantly less feather pecking and plumage damage in the pens than in the cages. There was no evidence of severe feather pecking spreading from feather peckers to non-feather peckers, even when actual rates of feather pecking were examined rather than only considering the strain of hen. There was some evidence that gentle feather pecking was transmitted in laying hens housed in cages. However, as gentle feather pecking does not lead to plumage damage, this is less relevant to commercial practice.

Divergent selection on feather pecking behaviour in laying hens ( Gallus gallus domesticus)

A selection experiment was initiated in 1996 in which selection for (HP line) and against (LP line) feather pecking was performed. The foundation stock was a White Leghorn layer strain established in 1970 and maintained since then as a random bred control line at the Institute. Six hatches were produced over three generations. At the age of 68 weeks (generation 0, 1996), 35 weeks (generation 1, 1997), 30 weeks (generation 2, 1998), and 27 weeks (generation 3, 1999) female birds were transferred to observation pens and their feather pecking behaviour was recorded. In each generation, 30 females and 8 males were selected from approximately 200 females and 60 males. The selection criterion was breeding value estimated by animal model on the trait ‘number of bouts of feather pecking per bird per hour’. Feather pecking behaviour in adult hens was significantly higher in HP than in LP. In generation 2 the following was recorded: 3.10 versus 1.37 bouts per bird per hour ( P<0.01), 7.04 versus 3.58 pecks per bird per hour ( P<0.05) and the proportion of hens recorded feather pecking in the 180 min observation period was 67 versus 56% ( P<0.05). In generation 3 the following was recorded: 4.56 versus 0.63 bouts per bird per hour ( P<0.001), 13.9 versus 2.51 pecks per bird per hour ( P<0.001) and the proportion of hens recorded feather pecking in the 180 min observation period was 75 versus 49% ( P<0.001). In generation 3, plumage condition was better in LP on neck, breast, back, wings and tail, as well as overall ( P<0.001). Body weight did not differ between lines in generation 2, but in generation 3, HP hens were on average heavier than LP hens at the age of 27 weeks (1435 g versus 1371 g, P<0.001).

Can we predict which hens will feather peck?

Selective breeding, based on feather pecking bouts per hen, can be an effective method of reducing feather pecking in a controlled research environment (Kjaer 2000). For selective breeding against feather pecking to be applied on a commercial scale using similar methodology would be extremely time consuming. A simple, quick test which predicted a young bird’s likelihood of developing feather pecking would eliminate the need to observe feather pecking behaviour or expose birds to damaging pecking from conspecifics as part of the selection process. A predictive test could also increase the rate of genetic progress and reduce the cost of implementing a selection programme.

Cross-sectional study of the prevalence of feather pecking in laying hens in alternative systems and its associations with management and disease

A cross-sectional study of risk factors for feather pecking in layings hens in alternative systems was carried out in July 1998. A total of 637 questionnaires were sent out to...

Effect of manipulating feathers of laying hens on the incidence of feather pecking and cannibalism

Feather pecking is a problem in commercial laying hens, particularly in loose-housing systems, where many hens can be affected by only a few feather peckers. In addition, feather pecking can become an even larger problem if it spreads throughout the flock. There are several possible ways that feather pecking may spread. The simplest way is that one hen may damage the feathers of a hen, and another hen may find the damaged feathers an attractive pecking target. The aim of this experiment was to determine if damaged feathers were feather-pecked more than undamaged feathers on the same body area, and to determine whether some types of feather-body area manipulations were preferred over others as pecking stimuli. Manipulations involved damaging the feathers on the rump, tail or belly of different hens, with two or three levels of severity of manipulation at each body area. Sixteen groups of 11 Lohmann Brown hens between 26 and 28 weeks were observed with the recipient, the feather pecker and the body area that was pecked all being recorded. The feather pecks were classified separately as either gentle or severe. Damaged feathers received significantly more severe feather pecks than undamaged feathers. There were also more gentle feather pecks to damaged feathers, although this did not reach statistical significance. The feather-body area manipulations that received the greatest number of severe feather pecks were the tail feathers when they were cut very short, the rump feathers when they were trimmed, and the rump when feathers were removed. These results support the suggestion that feather pecking does indeed spread through flocks by damaged feathers becoming an attractive target for feather-pecking behaviour. An unexpected result of performing the feather manipulations was an outbreak of cannibalism in half of the experimental groups. Even though there was no visible damage to the skin of the hens after having the feathers manipulated, 13 of the 16 attacked hens were wounded on the part of the body where the feathers had been damaged in some way.

Relationship between feather pecking and ground pecking in laying hens and the effect of group size

The aim of this experiment was to study the relationship between feather pecking and ground pecking in laying hens and the effect of group size on feather pecking behaviour. Hisex White hens were kept in floor pens in group sizes of 15, 30, 60 and 120 birds, each with four replicates. Behavioural observations were performed at four different ages and focused on the number of feather pecks and aggressive pecks, both given and received. The part of the body pecked and the location of the bird was recorded as well as the number of pecks made to the floor, feeder and drinker. The results showed that most feather pecking activity occurred in the largest group size (120 birds) and there was some evidence of an increasing frequency of aggressive pecks with increasing group size. The parts of the body which were targets for feather pecking varied depending on the location of the bird giving the peck and the bird receiving it. When looking at the behaviour of individuals, birds doing a lot of feather pecking also showed more ground pecking.