How animals perceive group size: Aggression among hens in partitioned pens

Abstract

The effects of group size and stocking density on the behaviour and welfare of captive animals are variable, and depend on other factors such as environmental complexity. An important effect of such factors will be their influence on how the animals themselves perceive group size and stocking density. This was investigated in a study of mediumweight hybrid laying hens. Four groups of 40 birds were placed in littered floor pens of 5.7 m 2 which were sub-divided by partitions 600 mm high into four equal sections; birds readily moved onto or over the partitions. The most common number of birds on the floor in a section was from four to seven and frequency of aggression was lowest over the same range (approximately 5 aggressive acts per bird h ~ ). Average frequency of aggression was higher when either more (up to 16) or fewer birds were in a section (up to 12 aggressive acts per bird h ~ ). However, this was strongly affected by the behaviour of certain aggressive individuals which responded to the occurrence of a large number of birds in their section by driving all the others away. The most important aspect of group size or stocking density perceived by such individuals was proximity of others, with a threshold effect causing increased aggression. Sub-group size and local stocking density are important aspects of such perception and should be taken into account in the design of animal housing. 0168-1591/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0168-1591 (93)00475-G