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.

Egg Colour Covaries with Female Expression of a Male Ornament in the Spotless Starling (Sturnus unicolor)

“…We assumed that a nest had been parasitized when two new eggs were found in a nest on a given day (as starlings lay one egg per day) or when a single egg differed markedly in appearance from the other eggs in a clutch. This criteria used for detecting parasitic eggs has been employed in another study in the European starling (Pilz et al. 2005).…”

Conspecific Brood Parasitism in Birds: A Life-History Perspective

“…Removing host eggs would seem to be the easiest way to reduce competition, but it has only been documented in one species, the European starling (Lombardo et al 1989). Starlings have also been studied to see if parasites fortify their eggs with testosterone to enhance chick competitiveness, but there was no evidence for this tactic (Pilz et al 2005).…”

The Starling Mating System as an Outcome of the Sexual Conflict

“…These females occupy restricted home-ranges in our study area (Sandell and Diemer, 1998), attempt to breed the current breeding season (Sandell and Diemer, 1998; see also Saitou, 2001), but are constrained from doing so by already mated females . They engage in an alternative reproductive strategy by parasitizing other females nests (Sandell and Diemer, 1998; see also Evans, 1988;Saitou, 2001), but the success of this strategy may be low because residents remove parasite eggs (Pinxten et al, 1991b) and parasitized clutches have lower success (Pinxten et al, 1991a) which is not offset by a higher competitiveness of parasitic offspring (Pilz et al, 2004). Thus, floater females seem to be doing the ''best of a bad job''.…”