Reduced extrapair paternity in response to experimental stimulation of earlier incubation onset in blue tits

Although the causes and consequences of extrapair paternity (EPP) have been studied extensively in birds, little is known about the regulation of extrapair copulation (EPC) behavior and how it may tie in with other aspects of avian reproduction. In birds, the presence of eggs stimulates incubation and, subsequently, the cessation of egg production. We propose that the same mechanism also regulates female motivation to engage in EPCs. To test this idea, we simulated the earlier onset of laying in blue tits (Cyanistes caeruleus), by adding model eggs to nests before natural laying commenced. Most females accepted these eggs, covering them with nest material in the natural way. As predicted, these broods hatched more asynchronously than control broods, revealing an earlier onset of incubation, and were less likely to contain extrapair offspring (EPO) suggesting that stimulation from eggs also inhibits motivation to seek EPCs. Egg stimulation is thought to cause cessation of laying a fixed number of days before clutch completion, after a certain hormonal threshold is exceeded. Similarly, a lower threshold may inhibit engagement in EPCs relative to clutch completion, explaining the proportional increase in EPO toward the extremes of clutch size, which we also observed, and would be consistent with a fertility or compatibility insurance function for EPCs. Our findings may represent the best experimental evidence for female-mediated effects on the timing of EPCs and suggest how EPP may be integrated within the regulatory mechanism of avian reproduction.