Egg Size as the Main Stimulus to Discriminatory Behavior of the Yellow-Browed Warbler (Phylloscopus inornatus, Phylloscopidae) under Brood Parasitism of the Oriental Cuckoo (Cuculus optatus, Cuculidae)

Experimental studies were carried out on the behavior of the host concerning egg discrimination in three geographic populations of the Yellow-browed Warbler with different levels of brood parasitism of the Oriental Cuckoo. The absence of a spotted pattern on the shell of foreign eggs was shown to fail to serve as a stimulus to recognition. The acceptance or rejection of a foreign egg is determined by the ratio of the parasite to the host egg breadth, the mean size of the eggs in the individual host clutch also being of importance. This suggests that the recognition of parasite eggs by the host is based on the tactile contact of the female with the uneven surface of the clutch. The mean breadth of the Oriental Cuckoo eggs introduced into Yellow-browed Warbler nests is significantly smaller than the value at which the probability of egg rejection is equal to the probability of acceptance. As it is also smaller than in other gentes, this shows the adaptation of the brood parasite to this particular host. The acceptance of a foreign egg, ejection of it from the nest, and clutch desertion represent three stages of the same reaction in response to variation in a quantitative stimulus.