Jones, R. B., Harvey, S., Hughes, B. O. et al. 1980. Growth and the plasma concentrations of growth hormone and prolactin in chicks: Effects of 'environmental enrichment', sex and strain. British Poultry Science 21(6), 457-462.

The enriched boxes were similar to the bare ones but also contained the following objects: two large red rubbing bungs, a golf ball, a bottle top, two coloured table tennis balls hanging from the cage lid, two lengths of string... The present results indicate that environmental enrichment has a growth-stimulating effect.