Space use and agonistic behaviour in relation to sex composition in large flocks of laying hens

Some authors have found indications of subgroup formation when domestic fowl are forced to live together in large flocks, while others have not. In this study experiments were carried out to test the hypothesis that hens in large flocks have home ranges in parts of the pen and that they form subgroups. We also studied if this is influenced by males. In a tiered aviary system (density averaged 16 hens/m(2) of floor area) eight flocks of 568+/-59 ISA Brown laying hybrids were kept in pens. Half of the pens contained 1 male per on average 24 females (mixed flocks). At peak production (36-53 weeks of age) four females roosting closely together for about 14 days and four females roosting far apart from each other were taken out from each flock and put together in separate groups in small pens. Their agonistic behaviour was studied for 2 days before they were put back. This was repeated with new birds, resulting in 16 small sample groups being studied. At 70 weeks, three groups of 10 females per flock roosting closely together in different parts of the pen were dyed with different colours and their locations were observed for 2 nights and 2 days.The incidence of aggressive pecks during day 1 among birds that had been roosting close to each other tended to be lower (P=0.05) than among birds that had been roosting far apart. This effect was not significant among birds from all-female flocks, but among birds from mixed flocks (P<0.05). However, this indicates a recognition of roosting partners and possibly also a rebound effect of the males' reduction of female aggressiveness towards strangers. Irrespective of sex composition in the flocks, birds marked while roosting at the ends of the pens were significantly more often observed within these areas than in other areas of the pen during daytime and came back to the same roosting sites at night (P<0.05-P<0.001). This was not the case for birds from the middle of the pens, where the distribution in the pen in most cases did not differ from random. These results show that laying hens in large groups are rather constant in their use of space, which indicate the presence of home ranges. However, environmental features that facilitate localisation may be important. In summary, we think that these findings indicate the existence of subgroup formation.