Clutch variation and egg rejection in three hosts of the pallid cuckoo, Cuculus pallidus

In theory, hosts of avian brood parasites would benefit by modifying their egg appearance in two ways to help identify mimetic foreign eggs: (i) by laying clutches that are more uniform in appearance and (ii) by laying clutches that differ from those of other females in the population. Support for these theories is inconsistent, and few studies have used objective measures of clutch variation. Here we used reflectance spectrophotometry to quantify within-clutch and between-clutch variation of three host species of an Australian brood parasite, the pallid cuckoo (Cuculus pallidus). We used egg-swapping experiments in which subjects were presented with either a conspecific egg or a heterospecific egg to compare the egg rejection responses of a frequently parasitised host, the white-plumed honeyeater (Lichenostomus penicillatus), with two less frequently parasitised hosts, dusky woodswallows (Artamus cyanopterus) and willie wagtails (Rhipidura leucophrys). As predicted, rejection rate increased as contrast between foreign egg and host clutch increased. Further, the major host showed greater between-clutch variation than the occasional hosts, and also rejected more similar-looking eggs. Contrary to predictions however, within-clutch variation was not lower in the major host, nor was it important in predicting the rejection rate of foreign eggs by the three host species.