Egg recognition and nestling discrimination in a cuckoo host

Abstract Most studies on host recognition of brood parasitism have focused on colour and pattern cues, with limited attention being paid to size differences. We examined the role of size in egg and nestling discrimination by the red-vented bulbul (Pycnonotus cafer), a common host of the Jacobin cuckoo (Clamator jacobinus). Experimentally, we introduced both small and large foreign eggs and nestlings into bulbul nests. Bulbuls accepted most of the host-sized eggs but consistently rejected large eggs. Nestling discrimination appeared to be influenced by skin colour, with mismatched nestlings being left unfed, although this finding was based on a small sample size and should be interpreted with caution. In a few instances, hosts rejected their own clutch if a foreign egg hatched first, indicating that the hatching order affects the host response. Our results reveal that in the red-vented bulbul, egg recognition is based on size cues, whereas nestling discrimination is based on skin colour. Moreover, rejection behaviour is significantly affected by the presence of parents in the nest. This study emphasizes context-dependent defence in the red-vented bulbul, whereby visual, temporal, and behavioural cues are processed across breeding stages.