Individual optimization of clutch size in great tits

Abstract

Individual birds within a population often lay clutches of very different sizes^1,2, even though those laying the larger clutches tend to produce more young which survive to enter the subsequent breeding population (recruits)³. Two hypotheses have been proposed to account for such differences in clutch size⁴. The individual optimization hypothesis proposes that parents lay that size of clutch from which they can maximize recruitment: adding or taking away young from their nests will result in lowered recruitment⁵. The trade-off hypothesis assumes a cost of reproduction; the rearing of offspring leads to the parents having a lowered future survival or fecundity^6,7. When given experimentally enlarged broods to rear parents should therefore show an increased mortality or reduced fecundity. We tested the predictions of these two hypotheses by manipulating the brood size of great tits Parus major over nine years, and following subsequent survival of offspring and parents and future breeding performance of parents. Parents differed in their ability to recruit offspring, this being reflected in the size of clutch that they laid. Parents did best, in terms of the number of offspring recruited, by rearing their own clutch size: adding or removing young did not increase recruitment rates. These results strongly support the individual optimization hypothesis. There was no evidence that parents raising enlarged broods suffered a higher mortality or decreased fecundity compared with those which raised their own natural or decreased broods. We are therefore unable to establish a cost of reproduction and consequently the predictions of the trade-off hypothesis are not supported.