Low nest illumination reduces egg rejection in Asian tits ( Parus cinereus )

The effect of nest illumination intensity on egg recognition remains largely unexplored. To address this gap, we assessed egg recognition in the Asian tit (Parus cinereus), a cavity-nesting species with strong egg recognition ability, across varying nest-brightness levels. The results showed that egg-rejection rates were significantly lower in shaded nests (11.8%, N = 34) than in unshaded ones (54.8%, N = 31), indicating that nest-cavity brightness significantly influences host egg-rejection behaviour. Egg-rejection rates of blue model eggs were lower in shaded nest boxes (15.8%, N = 19) than in unshaded controls (40.0%, N = 20). Likewise, egg-rejection rates of white model eggs were lower in shaded nest boxes (6.7%, N = 15) relative to unshaded controls (81.8%, N = 11). The colour of the model eggs appeared to have an interactive effect with nest box shading, with white model eggs experiencing a steeper reduction in egg-rejection rate in shaded conditions, although this difference was not statistically significant. These findings suggest that the relative pattern of rejection rates for differently coloured eggs differed between light conditions, suggesting that light conditions inside the nest might be an important environmental factor influencing host responses to egg colour variation.