Sherwin, C. M. 1995. Environmental enrichment for laying hens - Spherical objects in the feed trough. Animal Welfare 4, 41-51.

Brightly coloured spherical objects [balls 4-9 cm in diameter] placed in the feed trough are considered to be a promising method of successful environmental enrichment [simulating ground-litter] for caged laying hens. There was no indication of habituation.