Social discrimination and aggression by laying hens in large groups: from peck orders to social tolerance

Domestic fowl (Gallus gallus domesticus) naturally live in small groups, with a dominance hierarchy (pecking order) which is most likely based on establishment fights, followed by remembered assessment of status involving individual recognition. In larger groups, this system is thought to break-down, and hens may adapt by becoming less aggressive, or by restricting their movements to defined 'territories'. In the present study, we tested the ability of laying hens living in groups of 10 and 120 to discriminate flockmates from unfamiliar birds. Subjects were presented in test cages with vertical bars, positioned outside the home pens in one of two places. Sixteen subject hens from each group size were presented to their home group (familiar) and a different group of the same size (unfamiliar) on each of four test days. For half of the test days, the groups were swapped between pens to control for the effect of location. Regardless of the location, hens in small groups discriminated between familiar (F) and unfamiliar (U) subjects by showing more aggression towards unfamiliar hens (attempted fights: F = 18.8%, U = 56.3% of tests; all aggression: F = 60.9%, U = 85.9% of tests). In large groups, the overall level of aggression towards subjects was reduced in that attempted fights were rare (F = 0.0%, U = 4.7% of tests), and aggressive outcomes were no different in response to unfamiliar or familiar hens (F = 46.9%, U = 42.2% of tests). The absence of flockmate/stranger discrimination in these large groups makes it unlikely that these hens had a conventional pecking order based on individual recognition within the group. Regardless of the location of the test cage beside the pen, certain individual hens in large groups were more likely to approach it, showing no evidence that hens have limited 'territories' within the pen. In large groups (but not in small), hens that approached and were aggressive were both heavier and had larger combs than the subjects. Taken together, these findings are consistent with the idea that in large groups hens become less aggressive and may change their social system to one where dominance is determined through direct assessment and 'status signalling' rather than the remembered individual assessment of a small group pecking order. (C) 2003 Elsevier B.V. All rights reserved.