Absence of egg rejection in an Asian population of house sparrow (Passer domesticus), a conspecific brood parasite in Europe

Abstract

The house sparrow (Passer domesticus) is well known for conspecific brood parasitism in Europe, and there it is capable of recognizing and rejecting non-mimetic foreign eggs and even mimetic conspecific eggs. Here, we tested the egg rejection of an Asian population of house sparrows by using model eggs, sparrow eggs painted red and sparrow eggs with added brown spots, of which their mimicry to sparrow eggs were quantified by granularity analysis based on avian vision. Surprisingly, house sparrows in Asia did not show any egg rejection. We suggest that the evolutionary equilibrium theory, which was first proposed to explain the absence of egg rejection in some hosts of interspecific brood parasites, may explain the deficiency of egg rejection in house sparrows, as a result of trade-offs between the high rejection costs and the costs of raising conspecific foreign chicks.