Fitness consequences of pre- and post-fledging timing decisions in a double-brooded passerine.

The fitness consequences of a delayed timing of breeding are expected to affect the temporal characteristics of the whole annual breeding system. One major problem in quantifying the fitness relevance of timing is that individual differences between pairs may cause the seasonal trend. Differentials in juvenile survival due to pre-fledging timing decisions often only appear after fledging of the chicks. Therefore, timing decisions in the post-fledging period, i.e., the duration of parental care, might additionally influence juvenile survival. We tested the effects of timing and parental competence on the post-fledging survival of second-brood juvenile Barn Swallows (Hirundo rustica L.) by swapping earlier and later hatching clutches and radio-tracking the juvenile subjects. The mark-recapture models controlled for the effects of duration of post-fledging care and food availability. There was an annually varying negative seasonal trend in offspring survival that was associated with environmental conditions. Directional selection for early breeding occurred in the two years with scarce autumnal food supply. Furthermore, we found strong selection for long post-fledging parental care. The duration of care neither declined seasonally, nor did longer care compensate for the seasonal decline of juvenile survival. Hence, the reproductive output three weeks after fledging was determined by two parental timing decisions: the timing of breeding and the timing of family breakup. We suggest that differential survival of second-brood fledglings in relation to these decisions is an important part of the selective mechanisms shaping the reproductive system of Barn Swallows.