Background Nestling discrimination and feeding habits during brooding are important factors affecting host selection of parasitic birds. Some host birds can avoid being parasitized by discriminating their nestlings or feeding food not suitable for parasitic nestlings. Thrushes are common medium-sized birds with widespread distribution and an open nesting habit, but they are rarely parasitized. It remains controversial whether this is due to feeding habits and/or nestling discrimination.

Methods In this study, we tested the nestling discrimination ability and feeding habits of Chestnut Thrushes (Turdus rubrocanus) which is distributed in China's multi-cuckoo parasitism system. Their nestling discriminability and feeding habits during brooding were studied by cross-fostering experiments and video recording to examine evolutionary restrictions on nestling discrimination and whether feeding habits are consistent with the growth of cuckoo nestlings.

Results Our results indicate that Chestnut Thrushes using earthworms as the main brooding food can feed and maintain cuckoo nestlings and show no nestling discrimination.

Conclusions The present study confirms that feeding habits cannot be regarded as the main factor affecting Chestnut Thrushes being rarely parasitized by cuckoos but suggests that egg rejection is likely to limit the evolution of nestling discrimination in thrushes.