Brood parasitic European starlings do not lay high-quality eggs

Abstract

Chicks of obligate brood parasites employ a variety of morphological and behavioral strategies to outcompete nest mates. Elevated competitiveness is favored by natural selection because parasitic chicks are not related to their host parents or nest mates. When chicks of conspecific brood parasites (CBPs) are unrelated to their hosts, they and their parents would also benefit from traits that enhance competitiveness. However, these traits must be inducible tactics in CBPs, since conspecific brood parasitism (CBP) is facultative. Such tactics could be induced by resources passed to offspring through the egg. Thus, females engaging in CBP should allocate to their eggs resources that will enhance offspring competitiveness. We tested this prediction in a population of European starlings (Sturnus vulgaris) breeding in southern Sweden. Previous research showed that almost all CBPs in this population are floater females that have yet to breed in the current season. We identified putative brood parasitic eggs through monitoring egg laying and verified parasitism using protein fingerprinting. We then determined whether parasitic eggs were larger, larger-yolked, or had higher concentrations of yolk testosterone or androstenedione than control eggs. The 14 brood parasitic eggs laid in active nests (those with clutches of at least two eggs that were eventually incubated) did not differ from controls in any of these characteristics. Ten dumped eggs, laid in nonactive nest-boxes or on the ground, were smaller and smaller-yolked than control eggs but did not differ in yolk androgen concentrations. The failure of our prediction could be the result of high costs of investing in eggs, lack of competition-based benefits for chicks, or physiological constraints on egg manipulation.