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Effect of group size on aggressive reactions to an introduced conspecific in groups of domestic turkeys (Meleagris gallopavo)
Applied Animal Behaviour Science 93 (2005) 251–258 www.elsevier.com/locate/applanim
Effect of group size on aggressive reactions to an introduced conspecific in groups of domestic turkeys (Meleagris gallopavo) T. Buchwalder a,*, B. Huber-Eicher b a
Department of Biology, Division of Evolutionary Ecology, Zoological Institute, University of Berne, 3012 Berne, Switzerland b Centre for Proper Housing: Poultry and Rabbits, Swiss Federal Veterinary Office, Burgerweg 22, 3052 Zollikofen, Switzerland Accepted 18 November 2004 Available online 7 March 2005
Abstract Aggressive encounters and injuries due to head pecking seriously threaten the welfare of domestic turkeys and also result in economic losses for the turkey industry. Animals of different species show an increase in aggression when unfamiliar conspecifics are mixed but a marked drop in aggression occurs as they become familiar with each other. We conducted an experiment to investigate the effect of group size on the frequencies of different types of aggressive interactions performed by 12-week-old broad-breasted turkey toms. We introduced non-group members for 30 min into 12 small (6 birds) and 12 large groups (30 birds; 432 birds in total) and observed the effect of this short term introduction on the subsequent incidence of aggressive behaviour. Half of the introduced animals originated from small groups and half from large ones. Members of the small groups initiated more fights (0.9/0.1) and delivered more aggressive pecks (20.3/5.0) to the introduced bird than members of the large groups. Also, birds originating from small groups initiated more fights (1.4/0.6) and delivered more aggressive pecks (7.4/3.2) than birds from large groups. Group members showed virtually no aggression towards reintroduced members of their own group. We demonstrate that groups of turkey toms react differently to
* Corresponding author. Tel.: +41 31 915 35 19; fax: +41 31 915 35 14. E-mail address: [email protected] (T. Buchwalder). 0168-1591/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2004.11.020
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an introduced conspecific depending on their group size and whether the introduced bird is a group member or not. # 2005 Elsevier B.V. All rights reserved. Keywords: Turkeys; Aggression; Injurious pecking; Familiarity; Group size
1. Introduction Wild turkeys have a highly competitive social system in which fights and other aggressive interactions between males are common (Buchholz, 1997). Half a year after hatching, young males of a brood flock will break away as a sibling group. They generally reject external males and form a closed unit for life. A very strict hierarchy is established by the rank fights both within and between sibling groups (Healy, 1992; Buchholz, 1997). Fights between wild turkeys are usually brief and hardly ever result in serious injuries (Healy, 1992). At the lekking sites, where the sibling groups perform vocal and visual displays during the breeding season, the dominant male of the dominant group does nearly all of the mating. As turkey males form groups with their brothers or half-brothers, males that are subdominant may still gain an evolutionary advantage by boosting the mating success of their dominant relative (Sherman, 1999) by supporting the group’s fight for a dominant position on the lek. Like wild turkeys, domestic turkeys also show agonistic behaviour and perform aggressive head pecking (Moinard et al., 2001). But with broad-breasted turkeys fattened under commercial conditions, aggressive interactions and injuries due to head pecking often become so severe that death occurs or culling is necessary (Hester et al., 1987; Classen et al., 1994; Sherwin and Kelland, 1998; Moinard et al., 2001). This aggressive behaviour is of considerable animal welfare and economic concern. In an attempt to lower its negative effects, turkey industry birds are frequently beaktrimmed and reared at low light intensities. With regard to animal welfare, neither measure is a satisfying solution to the problem of injuries caused by aggressive interactions. Individuals of different species, both mammals and birds, show increased aggression when mixed with unfamiliar conspecifics (Adams, 1981, 1991; Chase, 1985; Schaffner and French, 1997; Wiley et al., 1999; Estevez et al., 2002). Lindberg and Nicol (1996) reported that upon introduction into a group, laying hens behaved differently depending on the group being familiar or not. Domestic hens were highly aggressive to one another when first put together in small groups, but gradually became familiar and formed stable dominance hierarchies in which aggression levels were much lower. This suggests that members of a small stable group of laying hens can distinguish between group members and non-group members. In a recent study, we showed that domesticated broad-breasted fattening turkeys also distinguish between group members and non-group members (Buchwalder and Huber-Eicher, 2003). They antagonised unfamiliar birds markedly more than familiar ones, at least in groups of four. With respect to animal welfare and economic problems caused by the aggressive behaviour of domestic turkeys in commercial husbandry, the question is raised to which maximum group size turkeys are
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still able to identify their group members. Douglis (1948) found that laying hens could recognise and accordingly react to at least 27 other individual hens. No data are available for domestic turkeys. However, wild turkeys in groups of 2–20 were observed fighting as a group and rejecting non-group members (Watts and Stockes, 1971; Williams, 1981). In this work, we examined the effect of introducing individual birds into small or large test groups of turkey toms on the subsequent incidence of aggressive interactions. The group size was 6 or 30 birds, respectively. Under natural conditions, chicks reared together are very likely to be close kin. Familiar conspecifics are, therefore, relatives. Inclusive fitness theory (Hamilton, 1970) predicts that turkeys should distinguish between familiar and unfamiliar conspecifics and that aggression should be directed towards unfamiliar rather than towards familiar birds, i.e. close relatives. In addition, it is plausible that the more birds a group counts, the more difficult it will be for an individual to distinguish between familiar and unfamiliar ones. Consequently, we predicted that the reaction to an introduced bird should be more aggressive in small groups than in large groups.
2. Methods 2.1. Subjects and housing The birds used in this experiment were turkey toms of the breeding line B.U.T. 9 from British United Turkeys (BUT). The experiment was carried out in two consecutive runs, each lasting 9 weeks. For each run, 216 six-week-old, not beak-trimmed male poults were bought from a commercial breeder. They were assigned randomly to six groups of 6 (small groups) and six groups of 30 (large groups) animals and housed in pens of 2 m 3 m (1 bird per m2) and 3 m 4 m (2.5 birds per m2) for small and large groups, respectively. As some animals had to be separated (seven birds) or died (nine birds) because of aggressive interaction, leg breaks, wing breaks or heart-malfunction during rearing, group sizes at testing time were slightly different. Large groups consisted of 26–30 members (1 26, 1 27, 1 28, 6 29, 3 30) and small groups consisted of 5–6 members (1 5, 11 6). Pens were arranged on each side of two corridors in a poultry house and separated by plywood walls. The sides facing the corridors were made of wire mesh and reinforced with plywood up to one meter from the ground to prevent visual contact between the groups. Pens were littered with wood shavings and long-cut straw and contained a suspended bell-drinker (diameter, 40 cm) and feeder (diameter, 50 cm), a bale of straw and a perch (200 and 300 cm in small and large pens, respectively, height, 60 cm). The average daily temperature was 16 8C (minimum 11 8C, maximum 20 8C). Artificial light was provided with a light intensity of approximately 30 lx at the height of the animals (mean of five measurements: to the left, right, front, rear and upwards). Lights were on from 6 a.m. to 8 p.m. with a 15-min twilight phase at the beginning and end of each day. The animals were kept beyond the duration of the experiment until the end of the fattening period (week 16) and then slaughtered at a conventional slaughterhouse.
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2.2. Procedures Three tests were performed on each of the 12 large (30 birds) and the 12 small (6 birds) test groups when the turkeys were 12 weeks old: (i) introduction of a bird from a large group, (ii) introduction of a bird from a small group and (iii) reintroduction of a group member into its own group to check the effect of the introduction procedure per se. From each group, we picked two birds to be introduced into other groups and one bird to be reintroduced. Per test there was one group used and one animal introduced or reintroduced, this gives a total of seven and six birds for introduction- or reintroduction-tests, respectively, in small groups and 31 and 30 birds for introduction- or reintroduction-tests, respectively, in large groups per test. Birds were selected randomly with the restriction that their body weight did not differ more than 10% from the mean body weight of the group tested. Per day, the above experiments (i)– (iii) were performed in random order on one small and one large group. In the morning of the testing day, all the animals were weighed and a bar was sprayed across the tail feathers with black hair spray. The birds to be introduced received an additional black bar of the tail feather’s tips in order to stay identifiable among the birds of the tested group. In a pilot study, we had ascertained that the additional colouring was ignored by the birds and particularly that it did not attract pecking. In this pilot study, we saw that outside the test-phase, there was no aggression observed (0 fights, 0 pecks and 0 leaps while observing 2 birds per pen during 30 min in 16 different pens). The bird to be introduced or reintroduced was taken out of its home pen, kept in a covered box (55 cm 55 cm 60 cm) in the corridor for 10 min and then introduced into the group to be tested according to the following protocol: the observer carried the bird on her arms to the centre of the pen and placed it carefully on the floor. She then walked slowly to the rear wall and back to the entrance of the pen, focussing the introduced bird in order to start immediately with observations when leaving the pen. All the social interactions between the introduced bird and test group members were recorded during the following 30 min. After the test, the introduced bird was brought back to its group. 2.3. Behavioural observations Observations were made by one observer sitting on a chair in the corridor in front of the pen using the ‘observer’ program (Noldus Information Technology, NL). ‘‘All occurrences’’ sampling (Altmann, 1974) was used to record aggressive interactions between residents and introduced or reintroduced birds, distinguishing the initiator of the interaction. Aggressive interactions were subdivided into ‘fights’ (number and duration), ‘pecks’ (number) and ‘leaps’ (number). Before fighting, the initiating turkey tom provokes the opponent by standing in front of him, raising the head and giving ‘fighting purrs’ (Healy, 1992). ‘Fights’ actually start when the opponent is responsive and the two males face each other with the raised heads both giving ‘fighting purrs’. During the fight, opponents try to attack each other by pecking or leaping. ‘Fights’ end when one bird turns away from the opponent. Aggressive ‘pecks’ are defined as pecks to the head, neck or snood of the opponent; ‘leaps’ are defined as leaping at the opponent’s breast, extending the feet forward with the toes spread. ‘Pecks’ and ‘leaps’ were recorded independently of the fighting context. 70% of all the aggressive pecks and 32% of all the leaps were observed in a non-fighting context. In addition, tests were videotaped to allow verification later on
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when necessary. The occurrence of injuries larger than 4 mm2 was taken as a criterion to immediately stop a test. Using this criterion, four tests had to be aborted after 13, 14, 17 and 21 min. However, the results were included in the analysis. The results of the aborted tests might slightly underestimate the level of aggression in these groups because of the reduced observation time. 2.4. Statistical analysis The effects of run (first, second) and pen (1–12) were examined by analysis of variance (Siegel and Castellan, 1988). Since no effects were detected, data from the first and second run were merged and treated as one dataset for the statistical analysis. The mean number of aggressive interactions between test group members and introduced bird per test and the mean duration of fights between test group members and introduced bird per test were analysed using a repeated-measures analysis of variance (Zar, 1998). All tests are twosided. As the data of the three different behaviours (fights, pecks, leaps) might be interrelated, we applied Bonferroni Correction and set alpha level to 1.7%.
3. Results Members of the small groups initiated significantly more fights with the introduced bird and delivered more aggressive pecks to it than members of the large groups did. The
Table 1 Behavioural reactions of residents to an introduced bird (a and b) and of the introduced bird to the residents (c) in small and large groups of turkey toms (analysis of variance with repeated measures, N = 24) F1
p-Value
17.19 0.04 13.59 0.14
<0.001 0.85 0.001 0.71
(b) Reaction from the residents to the introduced bird, subject to its origin 0.3 (0.8) 0.7 (0.9) Fightsa Fightsb 63 (72.1) 37 (21.7) Pecksa 13.5 (13.0) 11.8 (15.1) 2.1 (2.8) 1.7 (3.3) Leapsa
2.20 0.43 0.43 0.22
0.15 0.56 0.52 0.64
(c) Reaction from the introduced bird to the residents 1.4 (1.4) 0.6 (1.0) Fightsa 52 (26.9) 36 (12.8) Fightsb Pecksa 7.4 (6.3) 3.2 (5.8) Leapsa 1.8 (3.0) 1.3 (2.9)
9.19 0.00 7.03 0.33
0.006 1.0 0.01 0.57
Small groups
Large groups
(a) Reaction from the residents to the introduced bird (S.D.) Fightsa 0.9 (1.0) 0.1 (0.4) Fightsb 47 (46.9) 30 (–) 20.3 (15.6) 5.0 (5.7) Pecksa Leapsa 2.0 (2.8) 1.7 (3.3) Originating from small groups
a b
Mean number per 30 min. Mean duration in second.
Originating from large groups
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duration of these fights and the number of leaps to the breast of the introduced bird were not significantly different in small and large groups (Table 1a). The reaction of the group to the introduced bird was not significantly affected by the introduced birds origin, i.e. whether it came from a small or large group (Table 1b). However, there were some differences in the response of the introduced birds to the test situation. Introduced birds originating from small groups initiated significantly more fights and delivered more aggressive pecks than birds from large groups. No significant differences were found in the duration of fights and in number of leaps delivered (Table 1c). The introduction procedure had no effect on aggression as group members showed virtually no aggression towards reintroduced members of the own group (fights: 0; pecks: 0.2; leaps: 0).
4. Discussion Our results show that groups of turkey toms, reared together from early age, react differently to an introduced conspecific depending on the group size. Members of small groups (6 animals) initiated more fights and delivered more aggressive pecks against an introduced unfamiliar bird than members of large groups (30 animals). Accordingly, introduced birds originating from small groups showed more aggressive behaviour against members of the test group than birds originating from large groups. The fact that birds of small groups antagonised unfamiliar birds, confirms earlier findings with small groups (Buchwalder and Huber-Eicher, 2003). According to the observations, wild turkeys show aggressive behaviour towards unfamiliar conspecifics in order to drive them out of the group (Watts and Stockes, 1971; Williams, 1981). In the case of wild turkey toms, group members are related conspecifics while non-group members are not and they are, therefore, not tolerated in the group for competitive reasons (Sherman, 1999). The same should also hold for larger groups. In our study, however, large groups of 30 birds show significantly less aggression towards an introduced unfamiliar bird. This is what we would expect under the assumption that with an increase of group size, it gets more difficult for an individual to distinguish between familiar and unfamiliar birds. Thirty birds seem to be above the group size limit, which allows recognition of familiar birds. The fact that there is apparently a limiting group size, which allows distinction of conspecifics, leads us to the conclusion that there is not a uniform group characteristic which serves for distinction. Instead, distinction of conspecifics must be based on individual recognition. The observed aggression in small groups may be interpreted in terms of evolutionary ecology but it may be argued that due to domestication, turkeys do not form such exclusive groups anymore. Considering this, the shown aggression of small groups towards introduced unfamiliar birds may be interpreted as an attempt to integrate a ‘newcomer’ into the hierarchy. There is a good evidence that wild turkeys and small groups of domesticated turkeys form stable hierarchies (Healy, 1992; Buchholz, 1997). Under commercial conditions, domesticated turkeys live in much larger groups than their ancestors did and the question arises as to what extent the birds are able to switch to a strategy more suited for large groups. According to the considerations mentioned above, the high level of aggression in commercial flocks of turkeys may be interpreted as an unsuccessful attempt
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to establish dominance hierarchies with each of the many individuals or the inability of adopting a strategy that reduces aggressive encounters in large groups and which is not based on individual recognition. In our study, we found high levels of aggression in small groups and low ones in large groups upon introduction of unfamiliar birds. This may prima facie seem contradictory to the fact that we find high overall levels of aggression in large commercial groups (Hester et al., 1987; Classen et al., 1994; Sherwin and Kelland, 1998; Moinard et al., 2001) and low ones in small groups (Engelmann, 1969; Healy, 1992). It is important to note that in this study we were looking at the immediate reaction of the group to the introduced bird. In terms of Pagel and Dawkins (1997), we were looking at ‘establishment fights’ in which an initial high level of aggression results in a low overall level in the long run and vice versa. To summarise, our results show that 12-week-old broad-breasted turkey toms in small groups, in which stable dominance hierarchies are established (Healy, 1992; Buchholz, 1997), distinguish between resident group members and introduced bird and react aggressive towards an introduced non-group member. In contrast, turkey toms in large groups react not at all or significantly less aggressive to an introduced non-group member, suggesting, that turkeys in large groups hardly are able of distinguishing between resident group members and introduced bird. This inability of individual recognition of group members in large groups of turkey toms makes it difficult to establish stable dominance hierarchies and might be an important reason for the excessive aggression in large commercial flocks of fattening turkey toms.
Acknowledgements This work was supported by grants of the Federal Veterinary Office. We thank our colleagues at the Centre for Proper Housing: Poultry and Rabbits and the staff of the Swiss Poultry Husbandry School for their co-operation. Finally, we thank two anonym referees for their helpful comments on the manuscript. The experiment reported here is part of a dissertation at the Faculty of Natural Sciences of the University of Berne, Switzerland.
References Adams, D., 1981. Motivational systems of social behavior in male rats and monkeys: are they homologous? Aggress. Behav. 7, 5–18. Adams, D., 1991. Biology does not make men more aggressive than women. In: Biorkvist, K., Niemla, P. (Eds.), Of Mice and Women: Aspects of Female Aggression. Academic Press, pp. 17–25. Altmann, J., 1974. Observational study of behaviour: sampling methods. Behaviour 49, 227–267. Buchholz, R., 1997. Male dominance and variation in fleshy head ornamentation in wild turkeys. J. Avian Biol. 28, 223–230. Buchwalder, T., Huber-Eicher, B., 2003. A brief report on aggressive interactions within and between groups of domestic turkeys (Meleagris gallopavo). Appl. Anim. Behav. Sci. 84, 75–80. Chase, I., 1985. The sequential analysis of aggressive acts during hierarchy formation: an application of the ‘jigsaw puzzle’ approach. Anim. Behav. 33, 86–100. Classen, H., Riddel, C., Robinson, F., Shand, P., McCurdy, A., 1994. Effect of lighting treatment on the productivity, health, behaviour and sexual maturity of heavy male turkeys. Br. Poult. Sci. 35, 215–225.
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Douglis, M., 1948. Social factors influencing the hierarchies of the domestic hen: interactions between resident and part-time members of organised flocks. Physiol. Zool. 21, 147–182. Engelmann, C., 1969. Verhalten des Geflu¨ gels: Puten. In: Porzig, E. (Ed.), Das Verhalten Landwirtschaftlicher Nutztiere. VEB Deutscher Landwirtschaftsverlag, Berlin, pp. 389–400. Estevez, I., Newberry, R., Keeling, L., 2002. Dynamics of aggression in the domestic fowl. Appl. Anim. Behav. Sci. 67, 307–325. Hamilton, W., 1970. Selfish and spiteful behaviour in an evolutionary model. Nature 228, 1218–1220. Healy, W., 1992. Behaviour. In: Dickson, J. (Ed.), The Wild Turkey, Biology and Management. Stackpole Books, pp. 46–65. Hester, P., Sutton, A., Elkin, R., 1987. Effect of light intensity, litter source, and litter management on the incidence of leg abnormalities and performance of male turkeys. Poult. Sci. 66, 666–675. Lindberg, A., Nicol, C., 1996. Effects of social and environmental familiarity on group preferences and spacing behaviour in laying hens. Appl. Anim. Behav. Sci. 49, 109–123. Moinard, C., Lewis, P., Perry, G., Sherwin, C., 2001. The effects of light intensity and light source on injuries due to pecking of male domestic turkeys (Meleagris gallopavo). Anim. Welf. 10, 131–139. Pagel, M., Dawkins, M., 1997. Peck order and group size in laying hens: ‘futures contracts’ for non-aggression. Behav. Process. 40, 13–25. Schaffner, C., French, J., 1997. Group size and aggression: ‘recruitment incentives’ in a cooperatively breeding primate. Anim. Behav. 54, 171–180. Sherman, P., 1999. Birds of a feather lek together. Nature 401, 119–120. Sherwin, C., Kelland, A., 1998. Time-budgets, comfort behaviours and injurious pecking of turkeys housed in pairs. Br. Poult. Sci. 39, 325–332. Siegel, S., Castellan, N., 1988. Nonparametric Statistics for the Behavioural Sciences. McGraph Hill, New York. Watts, C., Stockes, A., 1971. The social order of turkeys. Sci. Am. 224 (6), 112–118. Wiley, R., Steadman, L., Chadwick, L., Wollerman, L., 1999. Social inertia in white-throated sparrows results from recognition of opponents. Anim. Behav. 57, 453–463. Williams, l., 1981. The Book of the Wild Turkey. Winchester Press, pp. 54–73. Zar, J., 1998. Biostatistical Analysis, fourth ed. Prentice Hall, pp. 255–260.
Effect of group size on aggressive reactions to an introduced conspecific in groups of domestic turkeys (Meleagris gallopavo) T. Buchwalder a,*, B. Huber-Eicher b a
Department of Biology, Division of Evolutionary Ecology, Zoological Institute, University of Berne, 3012 Berne, Switzerland b Centre for Proper Housing: Poultry and Rabbits, Swiss Federal Veterinary Office, Burgerweg 22, 3052 Zollikofen, Switzerland Accepted 18 November 2004 Available online 7 March 2005
Abstract Aggressive encounters and injuries due to head pecking seriously threaten the welfare of domestic turkeys and also result in economic losses for the turkey industry. Animals of different species show an increase in aggression when unfamiliar conspecifics are mixed but a marked drop in aggression occurs as they become familiar with each other. We conducted an experiment to investigate the effect of group size on the frequencies of different types of aggressive interactions performed by 12-week-old broad-breasted turkey toms. We introduced non-group members for 30 min into 12 small (6 birds) and 12 large groups (30 birds; 432 birds in total) and observed the effect of this short term introduction on the subsequent incidence of aggressive behaviour. Half of the introduced animals originated from small groups and half from large ones. Members of the small groups initiated more fights (0.9/0.1) and delivered more aggressive pecks (20.3/5.0) to the introduced bird than members of the large groups. Also, birds originating from small groups initiated more fights (1.4/0.6) and delivered more aggressive pecks (7.4/3.2) than birds from large groups. Group members showed virtually no aggression towards reintroduced members of their own group. We demonstrate that groups of turkey toms react differently to
* Corresponding author. Tel.: +41 31 915 35 19; fax: +41 31 915 35 14. E-mail address: [email protected] (T. Buchwalder). 0168-1591/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2004.11.020
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an introduced conspecific depending on their group size and whether the introduced bird is a group member or not. # 2005 Elsevier B.V. All rights reserved. Keywords: Turkeys; Aggression; Injurious pecking; Familiarity; Group size
1. Introduction Wild turkeys have a highly competitive social system in which fights and other aggressive interactions between males are common (Buchholz, 1997). Half a year after hatching, young males of a brood flock will break away as a sibling group. They generally reject external males and form a closed unit for life. A very strict hierarchy is established by the rank fights both within and between sibling groups (Healy, 1992; Buchholz, 1997). Fights between wild turkeys are usually brief and hardly ever result in serious injuries (Healy, 1992). At the lekking sites, where the sibling groups perform vocal and visual displays during the breeding season, the dominant male of the dominant group does nearly all of the mating. As turkey males form groups with their brothers or half-brothers, males that are subdominant may still gain an evolutionary advantage by boosting the mating success of their dominant relative (Sherman, 1999) by supporting the group’s fight for a dominant position on the lek. Like wild turkeys, domestic turkeys also show agonistic behaviour and perform aggressive head pecking (Moinard et al., 2001). But with broad-breasted turkeys fattened under commercial conditions, aggressive interactions and injuries due to head pecking often become so severe that death occurs or culling is necessary (Hester et al., 1987; Classen et al., 1994; Sherwin and Kelland, 1998; Moinard et al., 2001). This aggressive behaviour is of considerable animal welfare and economic concern. In an attempt to lower its negative effects, turkey industry birds are frequently beaktrimmed and reared at low light intensities. With regard to animal welfare, neither measure is a satisfying solution to the problem of injuries caused by aggressive interactions. Individuals of different species, both mammals and birds, show increased aggression when mixed with unfamiliar conspecifics (Adams, 1981, 1991; Chase, 1985; Schaffner and French, 1997; Wiley et al., 1999; Estevez et al., 2002). Lindberg and Nicol (1996) reported that upon introduction into a group, laying hens behaved differently depending on the group being familiar or not. Domestic hens were highly aggressive to one another when first put together in small groups, but gradually became familiar and formed stable dominance hierarchies in which aggression levels were much lower. This suggests that members of a small stable group of laying hens can distinguish between group members and non-group members. In a recent study, we showed that domesticated broad-breasted fattening turkeys also distinguish between group members and non-group members (Buchwalder and Huber-Eicher, 2003). They antagonised unfamiliar birds markedly more than familiar ones, at least in groups of four. With respect to animal welfare and economic problems caused by the aggressive behaviour of domestic turkeys in commercial husbandry, the question is raised to which maximum group size turkeys are
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still able to identify their group members. Douglis (1948) found that laying hens could recognise and accordingly react to at least 27 other individual hens. No data are available for domestic turkeys. However, wild turkeys in groups of 2–20 were observed fighting as a group and rejecting non-group members (Watts and Stockes, 1971; Williams, 1981). In this work, we examined the effect of introducing individual birds into small or large test groups of turkey toms on the subsequent incidence of aggressive interactions. The group size was 6 or 30 birds, respectively. Under natural conditions, chicks reared together are very likely to be close kin. Familiar conspecifics are, therefore, relatives. Inclusive fitness theory (Hamilton, 1970) predicts that turkeys should distinguish between familiar and unfamiliar conspecifics and that aggression should be directed towards unfamiliar rather than towards familiar birds, i.e. close relatives. In addition, it is plausible that the more birds a group counts, the more difficult it will be for an individual to distinguish between familiar and unfamiliar ones. Consequently, we predicted that the reaction to an introduced bird should be more aggressive in small groups than in large groups.
2. Methods 2.1. Subjects and housing The birds used in this experiment were turkey toms of the breeding line B.U.T. 9 from British United Turkeys (BUT). The experiment was carried out in two consecutive runs, each lasting 9 weeks. For each run, 216 six-week-old, not beak-trimmed male poults were bought from a commercial breeder. They were assigned randomly to six groups of 6 (small groups) and six groups of 30 (large groups) animals and housed in pens of 2 m 3 m (1 bird per m2) and 3 m 4 m (2.5 birds per m2) for small and large groups, respectively. As some animals had to be separated (seven birds) or died (nine birds) because of aggressive interaction, leg breaks, wing breaks or heart-malfunction during rearing, group sizes at testing time were slightly different. Large groups consisted of 26–30 members (1 26, 1 27, 1 28, 6 29, 3 30) and small groups consisted of 5–6 members (1 5, 11 6). Pens were arranged on each side of two corridors in a poultry house and separated by plywood walls. The sides facing the corridors were made of wire mesh and reinforced with plywood up to one meter from the ground to prevent visual contact between the groups. Pens were littered with wood shavings and long-cut straw and contained a suspended bell-drinker (diameter, 40 cm) and feeder (diameter, 50 cm), a bale of straw and a perch (200 and 300 cm in small and large pens, respectively, height, 60 cm). The average daily temperature was 16 8C (minimum 11 8C, maximum 20 8C). Artificial light was provided with a light intensity of approximately 30 lx at the height of the animals (mean of five measurements: to the left, right, front, rear and upwards). Lights were on from 6 a.m. to 8 p.m. with a 15-min twilight phase at the beginning and end of each day. The animals were kept beyond the duration of the experiment until the end of the fattening period (week 16) and then slaughtered at a conventional slaughterhouse.
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2.2. Procedures Three tests were performed on each of the 12 large (30 birds) and the 12 small (6 birds) test groups when the turkeys were 12 weeks old: (i) introduction of a bird from a large group, (ii) introduction of a bird from a small group and (iii) reintroduction of a group member into its own group to check the effect of the introduction procedure per se. From each group, we picked two birds to be introduced into other groups and one bird to be reintroduced. Per test there was one group used and one animal introduced or reintroduced, this gives a total of seven and six birds for introduction- or reintroduction-tests, respectively, in small groups and 31 and 30 birds for introduction- or reintroduction-tests, respectively, in large groups per test. Birds were selected randomly with the restriction that their body weight did not differ more than 10% from the mean body weight of the group tested. Per day, the above experiments (i)– (iii) were performed in random order on one small and one large group. In the morning of the testing day, all the animals were weighed and a bar was sprayed across the tail feathers with black hair spray. The birds to be introduced received an additional black bar of the tail feather’s tips in order to stay identifiable among the birds of the tested group. In a pilot study, we had ascertained that the additional colouring was ignored by the birds and particularly that it did not attract pecking. In this pilot study, we saw that outside the test-phase, there was no aggression observed (0 fights, 0 pecks and 0 leaps while observing 2 birds per pen during 30 min in 16 different pens). The bird to be introduced or reintroduced was taken out of its home pen, kept in a covered box (55 cm 55 cm 60 cm) in the corridor for 10 min and then introduced into the group to be tested according to the following protocol: the observer carried the bird on her arms to the centre of the pen and placed it carefully on the floor. She then walked slowly to the rear wall and back to the entrance of the pen, focussing the introduced bird in order to start immediately with observations when leaving the pen. All the social interactions between the introduced bird and test group members were recorded during the following 30 min. After the test, the introduced bird was brought back to its group. 2.3. Behavioural observations Observations were made by one observer sitting on a chair in the corridor in front of the pen using the ‘observer’ program (Noldus Information Technology, NL). ‘‘All occurrences’’ sampling (Altmann, 1974) was used to record aggressive interactions between residents and introduced or reintroduced birds, distinguishing the initiator of the interaction. Aggressive interactions were subdivided into ‘fights’ (number and duration), ‘pecks’ (number) and ‘leaps’ (number). Before fighting, the initiating turkey tom provokes the opponent by standing in front of him, raising the head and giving ‘fighting purrs’ (Healy, 1992). ‘Fights’ actually start when the opponent is responsive and the two males face each other with the raised heads both giving ‘fighting purrs’. During the fight, opponents try to attack each other by pecking or leaping. ‘Fights’ end when one bird turns away from the opponent. Aggressive ‘pecks’ are defined as pecks to the head, neck or snood of the opponent; ‘leaps’ are defined as leaping at the opponent’s breast, extending the feet forward with the toes spread. ‘Pecks’ and ‘leaps’ were recorded independently of the fighting context. 70% of all the aggressive pecks and 32% of all the leaps were observed in a non-fighting context. In addition, tests were videotaped to allow verification later on
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when necessary. The occurrence of injuries larger than 4 mm2 was taken as a criterion to immediately stop a test. Using this criterion, four tests had to be aborted after 13, 14, 17 and 21 min. However, the results were included in the analysis. The results of the aborted tests might slightly underestimate the level of aggression in these groups because of the reduced observation time. 2.4. Statistical analysis The effects of run (first, second) and pen (1–12) were examined by analysis of variance (Siegel and Castellan, 1988). Since no effects were detected, data from the first and second run were merged and treated as one dataset for the statistical analysis. The mean number of aggressive interactions between test group members and introduced bird per test and the mean duration of fights between test group members and introduced bird per test were analysed using a repeated-measures analysis of variance (Zar, 1998). All tests are twosided. As the data of the three different behaviours (fights, pecks, leaps) might be interrelated, we applied Bonferroni Correction and set alpha level to 1.7%.
3. Results Members of the small groups initiated significantly more fights with the introduced bird and delivered more aggressive pecks to it than members of the large groups did. The
Table 1 Behavioural reactions of residents to an introduced bird (a and b) and of the introduced bird to the residents (c) in small and large groups of turkey toms (analysis of variance with repeated measures, N = 24) F1
p-Value
17.19 0.04 13.59 0.14
<0.001 0.85 0.001 0.71
(b) Reaction from the residents to the introduced bird, subject to its origin 0.3 (0.8) 0.7 (0.9) Fightsa Fightsb 63 (72.1) 37 (21.7) Pecksa 13.5 (13.0) 11.8 (15.1) 2.1 (2.8) 1.7 (3.3) Leapsa
2.20 0.43 0.43 0.22
0.15 0.56 0.52 0.64
(c) Reaction from the introduced bird to the residents 1.4 (1.4) 0.6 (1.0) Fightsa 52 (26.9) 36 (12.8) Fightsb Pecksa 7.4 (6.3) 3.2 (5.8) Leapsa 1.8 (3.0) 1.3 (2.9)
9.19 0.00 7.03 0.33
0.006 1.0 0.01 0.57
Small groups
Large groups
(a) Reaction from the residents to the introduced bird (S.D.) Fightsa 0.9 (1.0) 0.1 (0.4) Fightsb 47 (46.9) 30 (–) 20.3 (15.6) 5.0 (5.7) Pecksa Leapsa 2.0 (2.8) 1.7 (3.3) Originating from small groups
a b
Mean number per 30 min. Mean duration in second.
Originating from large groups
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duration of these fights and the number of leaps to the breast of the introduced bird were not significantly different in small and large groups (Table 1a). The reaction of the group to the introduced bird was not significantly affected by the introduced birds origin, i.e. whether it came from a small or large group (Table 1b). However, there were some differences in the response of the introduced birds to the test situation. Introduced birds originating from small groups initiated significantly more fights and delivered more aggressive pecks than birds from large groups. No significant differences were found in the duration of fights and in number of leaps delivered (Table 1c). The introduction procedure had no effect on aggression as group members showed virtually no aggression towards reintroduced members of the own group (fights: 0; pecks: 0.2; leaps: 0).
4. Discussion Our results show that groups of turkey toms, reared together from early age, react differently to an introduced conspecific depending on the group size. Members of small groups (6 animals) initiated more fights and delivered more aggressive pecks against an introduced unfamiliar bird than members of large groups (30 animals). Accordingly, introduced birds originating from small groups showed more aggressive behaviour against members of the test group than birds originating from large groups. The fact that birds of small groups antagonised unfamiliar birds, confirms earlier findings with small groups (Buchwalder and Huber-Eicher, 2003). According to the observations, wild turkeys show aggressive behaviour towards unfamiliar conspecifics in order to drive them out of the group (Watts and Stockes, 1971; Williams, 1981). In the case of wild turkey toms, group members are related conspecifics while non-group members are not and they are, therefore, not tolerated in the group for competitive reasons (Sherman, 1999). The same should also hold for larger groups. In our study, however, large groups of 30 birds show significantly less aggression towards an introduced unfamiliar bird. This is what we would expect under the assumption that with an increase of group size, it gets more difficult for an individual to distinguish between familiar and unfamiliar birds. Thirty birds seem to be above the group size limit, which allows recognition of familiar birds. The fact that there is apparently a limiting group size, which allows distinction of conspecifics, leads us to the conclusion that there is not a uniform group characteristic which serves for distinction. Instead, distinction of conspecifics must be based on individual recognition. The observed aggression in small groups may be interpreted in terms of evolutionary ecology but it may be argued that due to domestication, turkeys do not form such exclusive groups anymore. Considering this, the shown aggression of small groups towards introduced unfamiliar birds may be interpreted as an attempt to integrate a ‘newcomer’ into the hierarchy. There is a good evidence that wild turkeys and small groups of domesticated turkeys form stable hierarchies (Healy, 1992; Buchholz, 1997). Under commercial conditions, domesticated turkeys live in much larger groups than their ancestors did and the question arises as to what extent the birds are able to switch to a strategy more suited for large groups. According to the considerations mentioned above, the high level of aggression in commercial flocks of turkeys may be interpreted as an unsuccessful attempt
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to establish dominance hierarchies with each of the many individuals or the inability of adopting a strategy that reduces aggressive encounters in large groups and which is not based on individual recognition. In our study, we found high levels of aggression in small groups and low ones in large groups upon introduction of unfamiliar birds. This may prima facie seem contradictory to the fact that we find high overall levels of aggression in large commercial groups (Hester et al., 1987; Classen et al., 1994; Sherwin and Kelland, 1998; Moinard et al., 2001) and low ones in small groups (Engelmann, 1969; Healy, 1992). It is important to note that in this study we were looking at the immediate reaction of the group to the introduced bird. In terms of Pagel and Dawkins (1997), we were looking at ‘establishment fights’ in which an initial high level of aggression results in a low overall level in the long run and vice versa. To summarise, our results show that 12-week-old broad-breasted turkey toms in small groups, in which stable dominance hierarchies are established (Healy, 1992; Buchholz, 1997), distinguish between resident group members and introduced bird and react aggressive towards an introduced non-group member. In contrast, turkey toms in large groups react not at all or significantly less aggressive to an introduced non-group member, suggesting, that turkeys in large groups hardly are able of distinguishing between resident group members and introduced bird. This inability of individual recognition of group members in large groups of turkey toms makes it difficult to establish stable dominance hierarchies and might be an important reason for the excessive aggression in large commercial flocks of fattening turkey toms.
Acknowledgements This work was supported by grants of the Federal Veterinary Office. We thank our colleagues at the Centre for Proper Housing: Poultry and Rabbits and the staff of the Swiss Poultry Husbandry School for their co-operation. Finally, we thank two anonym referees for their helpful comments on the manuscript. The experiment reported here is part of a dissertation at the Faculty of Natural Sciences of the University of Berne, Switzerland.
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