Seasonal variation in reproductive success and post-nuptial moult of blue tits in southern Europe: an experimental study

Post-nuptial moult and reproductive success were studied in relation to tim ing of breeding in blue tits, Parus caeruleus, breeding in southern Europe. A group of experimentally delayed pairs was created by removing first clut ches, thereby inducing late repeat clutches. Reproductive success and post- nuptial moult of delayed pairs were compared with both control pairs that b red early and unmanipulated late-breeding pairs. Delayed pairs fledged fewe r young and with a lower body mass than control pairs. However, the number of fledged young and fledgling mass did not differ between delayed and late -breeding pairs. These results were more consistent with the date hypothesi s, and it is concluded that the timing of breeding and reproductive success may be causally related in the blue tit. This study reveals a harmful effe ct of relaying on female body mass at the end of the nestling period, There fore, females apparently pay the costs of relaying, since a reduction in bo dy mass during the nestling period may be accompanied by a lowered survival probability. Delayed and late-breeding males often began moulting while st ill feeding young, but neither control males nor females from the three stu dy groups did so. These results support the view that timing-related energy constraints on breeding may be important causes of a seasonal decline in r eproductive success at different latitudes.