1. Effect of perches at different positions in conventional cages for laying hens of two diffent, strains;Abrahamsson;Acta Agric. Scand.,1993
2. Feather pecking and cannibalism in a caged layer flock;Allen;Br. Poult. Sci.,1975
3. The effects of environmental enrichment on cage-cleaning aggression in male laboratory mice;Ambrose;B&K Sci. Now,1997
4. Allowing captive primates to forage;Anderson,1984