Great tits, Paras major, lay too many eggs: experimental evidence in mid-boreal habitats

This study shows that great tits lay too large clutches in mid-boreal habit ats. First, breeding success, measured with number of fledglings or proport ion of eggs that produce fledglings, in northern Finland (65 degreesN) is m uch poorer than in central and western Europe. Second, brood size manipulat ions (ca +/- 30% of the natural mean) revealed that reduced broods produced equal numbers of and larger-sized fledglings than control and enlarged bro ods, giving thus the best fitness value For reduced broods. Third, parents of enlarged broods could not adjust (i.e. increase) their feeding effort to the greater number of nestlings. Fourth, extra feeding (about 1/3 of the t heoretical maximal needs of the nestlings) during the nestling period resul ted in more numerous and larger-sized fledglings in comparison to control b roods. We suggest that the ultimate explanation For the too large clutches is gene flow from the southern population. which prevents local adaptations in the north. Consequently, the main reason for food limitation during the nestling period is that northern great tits apply "southern" decision rule s for timing of breeding, clutch size and foraging behaviour. Thus, they te nd to breed too early in comparison to the food abundance peak, lay too lar ge clutches in comparison to the level of resources and, perhaps, forage on a too narrow diet (75% caterpillars). Since the late broods that matched t he local food abundance peak did nor succeed better than the mismatched ear lier ones, the most crucial fault of northern great tits seems to be that t hey overestimate food abundance during peak demands and lay too large clutc hes. Another explanation for this could be that northern great tits have ad opted a brood reduction strategy. However, the long-tenn data reveal that y ears of high breeding success, which would maintain large clutches in the p opulation, are very rare in the north. Therefore, it is unlikely that a bro od reduction strategy per se could explain the phenomenon. Instead, it coul d work together with the gene flow against local adaptation for clutch adju stment.