Theoretical models are presented of the effects of space, facilities and group size on the behaviour of chickens at high stocking densities, with relevance for all animals. The appropriateness of each model is supported by published data, although such data are scant for some important variables. Freedom of movement is analysed by taking the area of a hen as 475 cm2 and finding the number of free bird spaces left at different space allowances. This provides support for current recommendations of a maximum of seven laying hens per m2 on deep litter, but suggests that a maximum for broilers of 34 kg/m2 unacceptably restricts freedom of movement. In cages, freedom of movement increases with space allowance per hen, and, for a given space allowance, with cage and group size. Nesting behaviour is analysed for synchrony, which decreases with group size. Perching and feeding are often synchronous and the space needed for these is determined by body width. Recommendations are derived for hens in furnished cages. The main part of the cage should be as large as possible; an absolute minimum of 600 cm2 per bird is suggested, but 675 cm2 per bird is probably the minimum practical. Perch and feeder space should be provided at 14 cm or more per bird, with a possible derogation for light hybrids to 12 cm. The number of nest spaces needed varies with the number of birds, with nest spaces being 300 cm2 each. These recommendations sum to a minimum of 800 cm2 per bird for groups of eight or more, 850 cm2 for groups of four to seven, and 900 cm2 for groups of three or fewer, plus litter area. Crowding is primarily caused by limited space allowance, but for a given space allowance it is worse in small enclosures and groups.