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A note on aggression and cannibalism in laying hens following re-housing and re-grouping
Applied Animal Behaviour Science 76 (2002) 157±163
A note on aggression and cannibalism in laying hens following re-housing and re-grouping Sylvie Cloutier*, Ruth C. Newberry Departments of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Center for the Study of Animal Well-Being, Washington State University, P.O. Box 646520, Pullman, WA 99164-6520, USA Accepted 1 January 2002
Abstract We examined effects of being moved and mixed with strangers on the incidence of aggression and cannibalism in caged White Leghorn hens with intact beaks. We predicted that mixing of strangers would increase rates of aggression and cannibalism. Hens were reared in 16 stable groups of four birds together since hatch. At 18 months of age, eight groups were moved intact to new cages (unmixed). The remaining hens were moved to new cages and combined into eight new groups of four unfamiliar hens (mixed). Data on aggression and cannibalism were collected during the 7 months before and after mixing. Aggression was higher in mixed than unmixed groups during the ®rst week after mixing. By contrast, the frequency of aggression in unmixed groups was depressed on the day of moving but returned to pre-moving levels within 1 week. Contrary to our prediction, mixing with strangers had no effect on cannibalism. However, the incidence of cannibalism was altered during the ®rst month after moving hens to new cages, being higher than pre-moving levels for injuries to the head and neck and lower for injuries to other body parts. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laying hen; Xenophobia; Cannibalism; Aggression
1. Introduction Cannibalism in poultry involves the administration of beak-in¯icted injuries followed by consumption of blood and other tissues of conspeci®cs, either while they are still alive or after death. It constitutes a serious welfare and economic problem for poultry husbandry when live birds are injured and, sometimes, killed. Although aggression increases when unfamiliar laying hens are mixed (Guhl and Allee, 1944; Choudary and Craig, 1972; Cloutier and Newberry, 2000), re-grouping does not affect production or mortality in beaktrimmed hens (Hester and Wilson, 1986). However, to our knowledge, no one has * Corresponding author. Tel.: 1-509-335-2956; fax: 1-509-335-4650. E-mail address: [email protected] (S. Cloutier).
0168-1591/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 5 9 1 ( 0 2 ) 0 0 0 0 4 - 7
158
S. Cloutier, R.C. Newberry / Applied Animal Behaviour Science 76 (2002) 157±163
previously examined the impact of mixing strangers on the subsequent incidence of cannibalism in laying hens with intact beaks. Inclusive ®tness theory leads to the prediction that cannibalism should be directed towards unrelated individuals in preference to close relatives. There is evidence from some species that cannibals tend to avoid eating kin (Pfenning, 1999; Schausberger and Croft, 2001). Under natural conditions, chicks reared together are likely to be close kin. Thus, familiarity provides a proximal mechanism by which differential behaviour towards nonkin could occur. Although the advent of arti®cial hatching and rearing in recent years means that familiarity during rearing is no longer a reliable indicator of relatedness, domestic fowl continue to exhibit xenophobia. Therefore, we predicted that the incidence of cannibalism in laying hens would be higher among strangers than among familiar hens reared together from an early age. Because cannibalism could be dangerous to the perpetrator if the victim retaliates, we also predicted that victims of cannibalism would tend to be less aggressive, and to have shorter beaks, than other birds. 2. Methods 2.1. Subjects, housing and management We housed 80 female White Leghorn chicks of a cannibalistic strain in 16 groups of ®ve birds. Each bird was individually identi®ed with a numbered wing band and a leg band. At 5 weeks of age, the groups were transferred intact from brooder units to grow-out cages. Food (standard pullet mash) and water were provided ad libitum. A sand bath was placed in each cage twice weekly to accommodate dust bathing behaviour. At 16 weeks of age, the 16 groups were transferred to solid-sided layer cages (125 cm long 40 cm wide) having a 125 cm long food trough and four cup-drinkers. Group size was equalised at four birds by randomly removing one bird from groups of ®ve birds. Food (standard layer mash) and water were provided ad libitum. A sand bath was provided on 3 days per week. The duration of the photoperiod was 11.5 h and the average light intensity at the level of the feeder was 195 20:4 lx. At 18 months of age, all hens were moved to different cages in the same room. Eight groups were kept intact (unmixed) when moved. The remaining hens were recombined into eight new groups (mixed) by placing one randomly selected hen from each of four groups together simultaneously into a new cage. The birds were monitored frequently and any bird found with a bleeding injury was immediately separated from her cage mates behind a wire mesh partition in the same cage and given veterinary care until the wound had healed. Healed birds were closely monitored after reintroduction to their group. We were prepared to lower the light intensity if a severe outbreak of cannibalism occurred but this did not prove necessary. 2.2. Measurements Data on cannibalism were collected during the 7 months before (11±18 months of age) and after (18±25 months of age) mixing. All attacks resulting in a bleeding wound were
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159
reported as cannibalistic events. The identity of the victim was noted along with the body part(s) affected (head, neck, back, rump, wings, keel, abdomen, tail, oil gland, cloaca, legs and toes). A bird was considered to have participated in cannibalism whenever she was found with blood on her beak at the time of a cannibalistic incident or observed pecking at a bleeding wound on another bird. Data on aggression were collected during 16 observation sessions per group in the 7 months before and after mixing (190 min of observation per group; eight observation sessions before, and eight after, mixing). Data were also collected immediately after moving and mixing birds at 18 months (15 min of observation per group). During each observation session, we recorded all aggressive pecks, claws, leaps, threats, chases and ®ghts given and received by each bird as described by Cloutier and Newberry (2000). We made direct observations except for the observation session that immediately followed moving and mixing of hens which was recorded on videotape due to the anticipated high frequency of events. Data on body weight and beak length were collected from all hens in week 75 (i.e. during the month before mixing) and in weeks 86 and 108 (i.e. during the second and seventh month after mixing). Beak length (the distance from the cranial corner of the beak to the distal tip of the bird's upper mandible) was measured to the nearest 1 mm using a ruler. 2.3. Statistical analysis Mixed model analysis of variance (Proc Mixed, SAS Institute Inc., 1996), with a compound symmetry covariance structure and time as a repeated measure, was used to assess the effects of treatment (mixed versus unmixed), time and their interaction on group mean incidences of aggressive and cannibalistic behaviour, body weight and beak length. The different types of aggressive behaviour recorded (peck, claw, leap, threat, chase, ®ght) were pooled due to their usually low frequencies of occurrence. Cannibalistic events involving injury of the head and neck region were analysed separately from cannibalistic events involving other body parts (back, rump, wings, keel, abdomen, tail, oil gland, cloaca, legs and toes combined). The aggression and cannibalism data were not normally distributed even after transformation. For these variables, the mixed linear model was applied to both untransformed and ranked data. Similar results for the two analyses indicated a good reliability of the analysis on the ranked data (Zar, 1996). Only results from the analyses of ranked data are presented. Means comparisons between ``treatments within age'', and ``ages within treatment'', were made based on differences in least-squares means, with P-values adjusted for multiple comparisons using the Tukey option in Proc Mixed. Within bird correlations between aggressive and cannibalistic variables, beak length and body weight before and after moving were identi®ed using the Spearman rank-order correlation option in Proc Corr (SAS Institute Inc., 1989). The correlation results were adjusted for multiple comparisons using the Bonferroni correction. 3. Results The frequency of aggressive acts was higher in mixed than unmixed groups immediately after, and in the week following, moving but not 2 months later (F1;14 21:6, P 0:0004;
160
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Fig. 1. Mean (S.E.) frequency of aggressive acts delivered per hen per minute during observation sessions before, immediately after, and later after moving White Leghorn hens with intact beaks to new cages at 18 months of age. Half of the birds were mixed at this time. Significant differences between times within treatment are indicated by letters a, b and c for unmixed, and x, y and z for mixed, groups. Significant differences between mixed and unmixed groups within time are indicated by P < 0:001.
Fig. 1). Aggression differed across time (F4;56 7:25, P 0:0001) and there was a signi®cant treatment by time interaction (F4;56 22:89, P 0:0001). The frequency of aggression was higher at mixing in the mixed, and lower in the unmixed, groups compared to the levels recorded before moving. Subsequently, the level of aggression increased in the unmixed groups to reach a level comparable to that before moving. For the mixed groups, the level of aggression decreased but it was still signi®cantly higher 2 months after, than before, moving and mixing (Fig. 1). A total of 77 cannibalistic events occurred between 11 and 25 months of age, of which 52 involved injuries to the head/neck region and 25 involved other body parts. Cannibalism of the head/neck was not affected by mixing (treatment, F1;14 0:15, P 0:71; treatment time, F2;28 1:34, P 0:28) but it differed with time (F2;28 5:08, P 0:01), being elevated in the month following moving (Fig. 2). Cannibalism of other body parts was also not affected by mixing (treatment, F1;14 1:57, P 0:23; treatment time, F2;28 0:43, P 0:65) but differed with time (F2;28 7:34, P 0:003), being depressed in the month following moving (Fig. 3). No bleeding wounds occurred on the day of moving despite high levels of aggression in the mixed groups. Before moving, the frequencies with which individuals gave and received aggression were negatively correlated (r s 0:48, P 0:0001, N 64). After moving, mean beak length (mean of measurements at two ages) was positively correlated with mean body weight (r s 0:50, P 0:0001, N 59). No other signi®cant correlations were found between the aggression, cannibalism, body weight and beak length variables. Mixing did not affect body weight (treatment, F1;14 0:11, P 0:74; treatment time, F2;28 0:86, P 0:44) or beak length (treatment, F1;14 0:79, P 0:39; treatment time, F2;28 0:57, P 0:57). However, both body weight (F2;28 14:71, P < 0:0001) and beak length (F2;28 9:01, P 0:001) varied across time (Table 1).
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Fig. 2. Mean (S.E.) frequency of cannibalistic events resulting in injuries to the head/neck area in the 7 months before, in the month following, and in the subsequent 6 months after, moving White Leghorn hens with intact beaks to new cages at 18 months of age. Half of the birds were mixed at this time. There was a significant effect of time (P 0:01) but not mixing.
Fig. 3. Mean (S.E.) frequency of cannibalistic events resulting in injuries to body parts other than the head/ neck during the 7 months before, in the month following, and in the subsequent 6 months after, moving White Leghorn hens with intact beaks to new cages at 18 months of age. Half of the birds were mixed at this time. There was a significant effect of time (P < 0:001) but not mixing.
Table 1 Mean (S.E.) body weight (g) and beak length (mm) of White Leghorn hens with intact beaks before and after being moved to new cages at 18 months of age Trait
Two weeks before moving
Two months after moving
Seven months after moving
Beak Length Body Weight N
35 0.2b 1551 28.7a 64
34 0.2a 1741 35.4b 62
34 0.2a 1538 35.9a 59
a,b
Values within rows with different superscripts differ significantly (P < 0:05).
162
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4. Discussion Mixing hens at 18 months of age resulted in an increased frequency of aggression compared to that observed prior to mixing, and this elevated aggression was still noticeable 2 months after mixing. Choudary and Craig (1972) reported that aggression declined to a level similar to that before mixing 6±10 weeks after mixing but they did not analyse this effect statistically. Guhl and Allee (1944) observed reduced body weights in hens exchanged between ¯ocks on multiple occasions. Our results indicate that mixing hens with intact beaks on a single occasion did not affect body weight 2 or 7 months after mixing. In the unmixed groups, aggression dropped temporarily after moving, probably because the hens were busy exploring their new environment. The absence of a negative correlation between aggression given and received in the ®rst 2 months after moving probably re¯ects social instability in the mixed groups. Moving hens to new cages temporarily increased the incidence of cannibalism directed at the head and neck in both mixed and unmixed groups. However, aggression was elevated only in mixed groups, and was highest immediately after mixing on a day when no bleeding injuries were reported. Furthermore, we found no correlations between aggression and cannibalism either before or after moving. Therefore, it is unclear whether the injuries to the head and neck were initiated by aggressive behaviour. Moving hens to new cages without mixing temporarily decreased the incidence of cannibalism directed to other body parts suggesting that cannibalism to different body regions can be initiated by different factors. The correlation analysis provided no support for our prediction that victims of cannibalism would be less aggressive or have shorter beaks than other birds in the population. Mixing strangers is known to elicit a stress response (Anthony et al., 1988). However, mixing strangers did not produce elevated cannibalism in this study in which cannibalism remained relatively infrequent and did not escalate into fatal wounding. Our results indicate that familiarity from a young age provided no protection against cannibalism. Acknowledgements This work was supported by a grant from the Cooperative State Research, Education and Extension Service, US Department of Agriculture, under Agreement No. 97-35204-4812 to R.C. Newberry and C.M. Ulibarri. Dr. J.R. Alldredge provided statistical advice. References Anthony, N.B., Katanbaf, M.N., Siegel, P.B., 1988. Responses to social disruption in two lines of White Leghorn chickens. Appl. Anim. Behav. Sci. 21, 243±250. Choudary, M.R., Craig, J.V., 1972. Effects of early flock assembly on agonistic behaviour and egg production in chickens. Poultry Sci. 51, 1928±1937. Cloutier, S., Newberry, R.C., 2000. Recent social experience, body weight and initial patterns of attack predict the social status attained by unfamiliar hens in a new group. Behaviour 137, 705±726.
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163
Guhl, A.M., Allee, W.C., 1944. Some measurable effects of social organisation in flocks of hens. Physiol. Zool. 17, 320±347. Hester, P.Y., Wilson, E.K., 1986. Performance of White Leghorn hens in response to cage density and the introduction of cage mates. Poultry Sci. 65, 2029±2033. Pfenning, D.W., 1999. Cannibalistic tadpoles that pose the greatest threat to kin are most likely to discriminate kin. Proc. R. Soc. Lond. B 266, 57±61. SAS Institute Inc., 1989. SAS/STAT User's Guide, Version 6, 4th Edition, Vol. 2. SAS Institute Inc., Cary, NC, USA. SAS Institute Inc., 1996. SAS/STAT Software, Changes and Enhancements through Release 6.11, SAS Institute Inc., Cary, NC, USA. Schausberger, P., Croft, B.A., 2001. Kin recognition and larval cannibalism by adult females in specialist predaceous mites. Anim. Behav. 61, 459±464. Zar, J.H., 1996. Biostatistical Analysis, 3rd Edition. Prentice-Hall, Upper Saddle River, NJ, p. 270.
A note on aggression and cannibalism in laying hens following re-housing and re-grouping Sylvie Cloutier*, Ruth C. Newberry Departments of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Center for the Study of Animal Well-Being, Washington State University, P.O. Box 646520, Pullman, WA 99164-6520, USA Accepted 1 January 2002
Abstract We examined effects of being moved and mixed with strangers on the incidence of aggression and cannibalism in caged White Leghorn hens with intact beaks. We predicted that mixing of strangers would increase rates of aggression and cannibalism. Hens were reared in 16 stable groups of four birds together since hatch. At 18 months of age, eight groups were moved intact to new cages (unmixed). The remaining hens were moved to new cages and combined into eight new groups of four unfamiliar hens (mixed). Data on aggression and cannibalism were collected during the 7 months before and after mixing. Aggression was higher in mixed than unmixed groups during the ®rst week after mixing. By contrast, the frequency of aggression in unmixed groups was depressed on the day of moving but returned to pre-moving levels within 1 week. Contrary to our prediction, mixing with strangers had no effect on cannibalism. However, the incidence of cannibalism was altered during the ®rst month after moving hens to new cages, being higher than pre-moving levels for injuries to the head and neck and lower for injuries to other body parts. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laying hen; Xenophobia; Cannibalism; Aggression
1. Introduction Cannibalism in poultry involves the administration of beak-in¯icted injuries followed by consumption of blood and other tissues of conspeci®cs, either while they are still alive or after death. It constitutes a serious welfare and economic problem for poultry husbandry when live birds are injured and, sometimes, killed. Although aggression increases when unfamiliar laying hens are mixed (Guhl and Allee, 1944; Choudary and Craig, 1972; Cloutier and Newberry, 2000), re-grouping does not affect production or mortality in beaktrimmed hens (Hester and Wilson, 1986). However, to our knowledge, no one has * Corresponding author. Tel.: 1-509-335-2956; fax: 1-509-335-4650. E-mail address: [email protected] (S. Cloutier).
0168-1591/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 5 9 1 ( 0 2 ) 0 0 0 0 4 - 7
158
S. Cloutier, R.C. Newberry / Applied Animal Behaviour Science 76 (2002) 157±163
previously examined the impact of mixing strangers on the subsequent incidence of cannibalism in laying hens with intact beaks. Inclusive ®tness theory leads to the prediction that cannibalism should be directed towards unrelated individuals in preference to close relatives. There is evidence from some species that cannibals tend to avoid eating kin (Pfenning, 1999; Schausberger and Croft, 2001). Under natural conditions, chicks reared together are likely to be close kin. Thus, familiarity provides a proximal mechanism by which differential behaviour towards nonkin could occur. Although the advent of arti®cial hatching and rearing in recent years means that familiarity during rearing is no longer a reliable indicator of relatedness, domestic fowl continue to exhibit xenophobia. Therefore, we predicted that the incidence of cannibalism in laying hens would be higher among strangers than among familiar hens reared together from an early age. Because cannibalism could be dangerous to the perpetrator if the victim retaliates, we also predicted that victims of cannibalism would tend to be less aggressive, and to have shorter beaks, than other birds. 2. Methods 2.1. Subjects, housing and management We housed 80 female White Leghorn chicks of a cannibalistic strain in 16 groups of ®ve birds. Each bird was individually identi®ed with a numbered wing band and a leg band. At 5 weeks of age, the groups were transferred intact from brooder units to grow-out cages. Food (standard pullet mash) and water were provided ad libitum. A sand bath was placed in each cage twice weekly to accommodate dust bathing behaviour. At 16 weeks of age, the 16 groups were transferred to solid-sided layer cages (125 cm long 40 cm wide) having a 125 cm long food trough and four cup-drinkers. Group size was equalised at four birds by randomly removing one bird from groups of ®ve birds. Food (standard layer mash) and water were provided ad libitum. A sand bath was provided on 3 days per week. The duration of the photoperiod was 11.5 h and the average light intensity at the level of the feeder was 195 20:4 lx. At 18 months of age, all hens were moved to different cages in the same room. Eight groups were kept intact (unmixed) when moved. The remaining hens were recombined into eight new groups (mixed) by placing one randomly selected hen from each of four groups together simultaneously into a new cage. The birds were monitored frequently and any bird found with a bleeding injury was immediately separated from her cage mates behind a wire mesh partition in the same cage and given veterinary care until the wound had healed. Healed birds were closely monitored after reintroduction to their group. We were prepared to lower the light intensity if a severe outbreak of cannibalism occurred but this did not prove necessary. 2.2. Measurements Data on cannibalism were collected during the 7 months before (11±18 months of age) and after (18±25 months of age) mixing. All attacks resulting in a bleeding wound were
S. Cloutier, R.C. Newberry / Applied Animal Behaviour Science 76 (2002) 157±163
159
reported as cannibalistic events. The identity of the victim was noted along with the body part(s) affected (head, neck, back, rump, wings, keel, abdomen, tail, oil gland, cloaca, legs and toes). A bird was considered to have participated in cannibalism whenever she was found with blood on her beak at the time of a cannibalistic incident or observed pecking at a bleeding wound on another bird. Data on aggression were collected during 16 observation sessions per group in the 7 months before and after mixing (190 min of observation per group; eight observation sessions before, and eight after, mixing). Data were also collected immediately after moving and mixing birds at 18 months (15 min of observation per group). During each observation session, we recorded all aggressive pecks, claws, leaps, threats, chases and ®ghts given and received by each bird as described by Cloutier and Newberry (2000). We made direct observations except for the observation session that immediately followed moving and mixing of hens which was recorded on videotape due to the anticipated high frequency of events. Data on body weight and beak length were collected from all hens in week 75 (i.e. during the month before mixing) and in weeks 86 and 108 (i.e. during the second and seventh month after mixing). Beak length (the distance from the cranial corner of the beak to the distal tip of the bird's upper mandible) was measured to the nearest 1 mm using a ruler. 2.3. Statistical analysis Mixed model analysis of variance (Proc Mixed, SAS Institute Inc., 1996), with a compound symmetry covariance structure and time as a repeated measure, was used to assess the effects of treatment (mixed versus unmixed), time and their interaction on group mean incidences of aggressive and cannibalistic behaviour, body weight and beak length. The different types of aggressive behaviour recorded (peck, claw, leap, threat, chase, ®ght) were pooled due to their usually low frequencies of occurrence. Cannibalistic events involving injury of the head and neck region were analysed separately from cannibalistic events involving other body parts (back, rump, wings, keel, abdomen, tail, oil gland, cloaca, legs and toes combined). The aggression and cannibalism data were not normally distributed even after transformation. For these variables, the mixed linear model was applied to both untransformed and ranked data. Similar results for the two analyses indicated a good reliability of the analysis on the ranked data (Zar, 1996). Only results from the analyses of ranked data are presented. Means comparisons between ``treatments within age'', and ``ages within treatment'', were made based on differences in least-squares means, with P-values adjusted for multiple comparisons using the Tukey option in Proc Mixed. Within bird correlations between aggressive and cannibalistic variables, beak length and body weight before and after moving were identi®ed using the Spearman rank-order correlation option in Proc Corr (SAS Institute Inc., 1989). The correlation results were adjusted for multiple comparisons using the Bonferroni correction. 3. Results The frequency of aggressive acts was higher in mixed than unmixed groups immediately after, and in the week following, moving but not 2 months later (F1;14 21:6, P 0:0004;
160
S. Cloutier, R.C. Newberry / Applied Animal Behaviour Science 76 (2002) 157±163
Fig. 1. Mean (S.E.) frequency of aggressive acts delivered per hen per minute during observation sessions before, immediately after, and later after moving White Leghorn hens with intact beaks to new cages at 18 months of age. Half of the birds were mixed at this time. Significant differences between times within treatment are indicated by letters a, b and c for unmixed, and x, y and z for mixed, groups. Significant differences between mixed and unmixed groups within time are indicated by P < 0:001.
Fig. 1). Aggression differed across time (F4;56 7:25, P 0:0001) and there was a signi®cant treatment by time interaction (F4;56 22:89, P 0:0001). The frequency of aggression was higher at mixing in the mixed, and lower in the unmixed, groups compared to the levels recorded before moving. Subsequently, the level of aggression increased in the unmixed groups to reach a level comparable to that before moving. For the mixed groups, the level of aggression decreased but it was still signi®cantly higher 2 months after, than before, moving and mixing (Fig. 1). A total of 77 cannibalistic events occurred between 11 and 25 months of age, of which 52 involved injuries to the head/neck region and 25 involved other body parts. Cannibalism of the head/neck was not affected by mixing (treatment, F1;14 0:15, P 0:71; treatment time, F2;28 1:34, P 0:28) but it differed with time (F2;28 5:08, P 0:01), being elevated in the month following moving (Fig. 2). Cannibalism of other body parts was also not affected by mixing (treatment, F1;14 1:57, P 0:23; treatment time, F2;28 0:43, P 0:65) but differed with time (F2;28 7:34, P 0:003), being depressed in the month following moving (Fig. 3). No bleeding wounds occurred on the day of moving despite high levels of aggression in the mixed groups. Before moving, the frequencies with which individuals gave and received aggression were negatively correlated (r s 0:48, P 0:0001, N 64). After moving, mean beak length (mean of measurements at two ages) was positively correlated with mean body weight (r s 0:50, P 0:0001, N 59). No other signi®cant correlations were found between the aggression, cannibalism, body weight and beak length variables. Mixing did not affect body weight (treatment, F1;14 0:11, P 0:74; treatment time, F2;28 0:86, P 0:44) or beak length (treatment, F1;14 0:79, P 0:39; treatment time, F2;28 0:57, P 0:57). However, both body weight (F2;28 14:71, P < 0:0001) and beak length (F2;28 9:01, P 0:001) varied across time (Table 1).
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Fig. 2. Mean (S.E.) frequency of cannibalistic events resulting in injuries to the head/neck area in the 7 months before, in the month following, and in the subsequent 6 months after, moving White Leghorn hens with intact beaks to new cages at 18 months of age. Half of the birds were mixed at this time. There was a significant effect of time (P 0:01) but not mixing.
Fig. 3. Mean (S.E.) frequency of cannibalistic events resulting in injuries to body parts other than the head/ neck during the 7 months before, in the month following, and in the subsequent 6 months after, moving White Leghorn hens with intact beaks to new cages at 18 months of age. Half of the birds were mixed at this time. There was a significant effect of time (P < 0:001) but not mixing.
Table 1 Mean (S.E.) body weight (g) and beak length (mm) of White Leghorn hens with intact beaks before and after being moved to new cages at 18 months of age Trait
Two weeks before moving
Two months after moving
Seven months after moving
Beak Length Body Weight N
35 0.2b 1551 28.7a 64
34 0.2a 1741 35.4b 62
34 0.2a 1538 35.9a 59
a,b
Values within rows with different superscripts differ significantly (P < 0:05).
162
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4. Discussion Mixing hens at 18 months of age resulted in an increased frequency of aggression compared to that observed prior to mixing, and this elevated aggression was still noticeable 2 months after mixing. Choudary and Craig (1972) reported that aggression declined to a level similar to that before mixing 6±10 weeks after mixing but they did not analyse this effect statistically. Guhl and Allee (1944) observed reduced body weights in hens exchanged between ¯ocks on multiple occasions. Our results indicate that mixing hens with intact beaks on a single occasion did not affect body weight 2 or 7 months after mixing. In the unmixed groups, aggression dropped temporarily after moving, probably because the hens were busy exploring their new environment. The absence of a negative correlation between aggression given and received in the ®rst 2 months after moving probably re¯ects social instability in the mixed groups. Moving hens to new cages temporarily increased the incidence of cannibalism directed at the head and neck in both mixed and unmixed groups. However, aggression was elevated only in mixed groups, and was highest immediately after mixing on a day when no bleeding injuries were reported. Furthermore, we found no correlations between aggression and cannibalism either before or after moving. Therefore, it is unclear whether the injuries to the head and neck were initiated by aggressive behaviour. Moving hens to new cages without mixing temporarily decreased the incidence of cannibalism directed to other body parts suggesting that cannibalism to different body regions can be initiated by different factors. The correlation analysis provided no support for our prediction that victims of cannibalism would be less aggressive or have shorter beaks than other birds in the population. Mixing strangers is known to elicit a stress response (Anthony et al., 1988). However, mixing strangers did not produce elevated cannibalism in this study in which cannibalism remained relatively infrequent and did not escalate into fatal wounding. Our results indicate that familiarity from a young age provided no protection against cannibalism. Acknowledgements This work was supported by a grant from the Cooperative State Research, Education and Extension Service, US Department of Agriculture, under Agreement No. 97-35204-4812 to R.C. Newberry and C.M. Ulibarri. Dr. J.R. Alldredge provided statistical advice. References Anthony, N.B., Katanbaf, M.N., Siegel, P.B., 1988. Responses to social disruption in two lines of White Leghorn chickens. Appl. Anim. Behav. Sci. 21, 243±250. Choudary, M.R., Craig, J.V., 1972. Effects of early flock assembly on agonistic behaviour and egg production in chickens. Poultry Sci. 51, 1928±1937. Cloutier, S., Newberry, R.C., 2000. Recent social experience, body weight and initial patterns of attack predict the social status attained by unfamiliar hens in a new group. Behaviour 137, 705±726.
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163
Guhl, A.M., Allee, W.C., 1944. Some measurable effects of social organisation in flocks of hens. Physiol. Zool. 17, 320±347. Hester, P.Y., Wilson, E.K., 1986. Performance of White Leghorn hens in response to cage density and the introduction of cage mates. Poultry Sci. 65, 2029±2033. Pfenning, D.W., 1999. Cannibalistic tadpoles that pose the greatest threat to kin are most likely to discriminate kin. Proc. R. Soc. Lond. B 266, 57±61. SAS Institute Inc., 1989. SAS/STAT User's Guide, Version 6, 4th Edition, Vol. 2. SAS Institute Inc., Cary, NC, USA. SAS Institute Inc., 1996. SAS/STAT Software, Changes and Enhancements through Release 6.11, SAS Institute Inc., Cary, NC, USA. Schausberger, P., Croft, B.A., 2001. Kin recognition and larval cannibalism by adult females in specialist predaceous mites. Anim. Behav. 61, 459±464. Zar, J.H., 1996. Biostatistical Analysis, 3rd Edition. Prentice-Hall, Upper Saddle River, NJ, p. 270.