Could a Factor That Does Not Affect Egg Recognition Influence the Decision of Rejection?

Rejection of the parasitic egg is the most important defence of hosts against brood parasites. However, this response is variable among and within species, and egg discrimination is not always followed by egg rejection. Low risk of parasitism and high risk of rejection costs may lead to the acceptance of the parasitic egg even if it has been previously recognized. The main aim of this paper is to answer a relevant question: can a single egg trait provoke the acceptance of an experimental egg previously recognized as foreign? Increased egg mass should hamper the ejection of an egg that has been discriminated because ejection of a heavy egg may imply higher rejection costs for hosts. We have tested this prediction by experimentally parasitizing natural nests of Common Blackbirds (Turdus merula) with non-mimetic model eggs of different mass (heavy, normal-weight, and light) while controlling for potential confounding factors such as egg size and colour. Our results showed that blackbirds more frequently accepted heavy eggs, even when previously recognized. This differential acceptance may be related to insufficient motivation to assume the higher costs that the ejection of a heavy egg could impose.Financial support has been provided by the Consejería Economía, Innovación, Ciencia y Empleo, Junta de Andalucia (research project CVI-6653)

Ejection rate for each experimental treatment: light, normal-weight, and heavy eggs.

<p>Sample sizes for each treatment are shown at the top of each column.</p

Mean ± SE of female recognition touches for the first visit, touches per visit, and during incubation in the three non-mimetic model eggs (light, normal-weight, and heavy eggs) and natural blackbirds eggs.

<p>Mean ± SE of female recognition touches for the first visit, touches per visit, and during incubation in the three non-mimetic model eggs (light, normal-weight, and heavy eggs) and natural blackbirds eggs.</p

Photo of a blackbird nest containing three natural eggs parasitized with a normal-weight non-mimetic egg.

<p>Photo of a blackbird nest containing three natural eggs parasitized with a normal-weight non-mimetic egg.</p