Kopecky, J., Reinhardt, V. 1991. Comparing the effectiveness of PVC swings versus PVC perches as environmental enrichment objects for caged female rhesus macaques. Laboratory Primate Newsletter 30(2), 5-6.

Single-housed subjects' preference for perches was probably related to the fact that perches, unlike swings, are fixed structures permitting continuous relaxed postures rather than short-term balancing. Moreover, perches, unlike swings, permit the animals to sit right in front of the cage with optimal visual control of the environment outside of the cage.