Timing of breeding and environmental factors as determinants of reproductive performance of tree swallows

Decreased reproductive success among birds breeding later in the season is a common pattern in temperate environments, although the underlying mechanisms remain unresolved. The quality hypothesis suggests that high-quality individuals can begin breeding earlier in the season than poorer quality parents, and are also able to invest more in reproduction. Alternatively, the date hypothesis suggests that reduced success among late birds is due to some correlate of date, such as decreased food abundance or offspring value. To test these hypotheses, I manipulated the date that tree swallows, Tachycineta bicolor (Vieillot, 1808), raised offspring by swapping clutches among nests so that birds were raising young either earlier or later than intended, and compared reproductive performance with control nests. My results showed that fledging success was related to the date hypothesis, with later birds being less successful in raising offspring than early breeding birds. Size and mass of offspring were not explained by either hypothesis, but rather by weather conditions experienced prior to measurements being taken. My results highlight the importance of events such as periods of inclement weather that can have significant impacts on offspring quality independently of both date of breeding and parental quality.