Rare rejection and incomplete responses to experimental eggs in a colonial bird

Abstract

Egg recognition has been well studied in brood-parasitic systems, yet its occurrence and function remain poorly understood in colonial species that experience little or no parasitism. We examined egg recognition in the little egret (Egretta garzetta), a colonial waterbird in which brood parasitism appears to be rare. We experimentally introduced conspecific, white-breasted waterhen (Amaurornis phoenicurus), and quail (Coturnix japonica) eggs into egret nests (n = 141) and tested the effects of clutch size, breeding stage, and parental presence on egg recognition. Rejection occurred in only four nests (2.8%) and was restricted to quail eggs, which were substantially smaller and visually dissimilar to host eggs. All conspecific and waterhen eggs were accepted. Rejection involved shell damage consistent with pecking or grasping attempts, but no eggs were completely removed. Females that showed rejection did not differ in egg size from non-rejectors. The Bayesian-regularized logistic model estimates indicated an inverse relationship between egg size relative to the host and the probability of rejection, although credible intervals were wide due to the rarity of events. Several predictors, including clutch size, breeding stage, parental presence, and year, showed highly uncertain effects. Although rejection appeared more common in larger clutches, model estimates did not support a clutch-size effect, likely due to rare events and uneven sampling. Overall, the recognition of foreign eggs by little egrets was poor. In the absence of brood parasitism and low frequency of egg rejection, interactions with dissimilar eggs may be more consistent with generalized nest sanitation or responses to anomalous objects than evolved antiparasitic discrimination.