Seasonal declines in reproductive success of the common tern Sterna hirundo : timing or parental quality?

Reproductive success declines over the course of the breeding season in many bird species. Two categories of hypothesis have been evoked to explain this decline. The “timing” hypothesis suggests that seasonal declines in breeding success are attributable to the date of laying. The “parental quality” hypothesis suggests that seasonal declines result from the fact that young, inexperienced, or low quality birds breed later in the season. To evaluate the relative importance of timing and parental quality, egg exchanges and removals were used to manipulate hatching dates of common terns Sterna hirundo . Indices of quality, attendance, provisioning rates, and reproductive success of birds in three experimental groups (delayed hatch pairs, advanced hatch pairs, and pairs induced to relay) were compared to those of date‐matched controls. Pairs that hatched chicks early raised more chicks than pairs hatching chicks late in the season, regardless of initial laying date. This suggests that hatching chicks early is advantageous in itself. Our results, however, also support the parental quality hypothesis. There was a significant negative relationship between natural laying date and fledging success, independent of hatching date. Differences in chick growth and survival suggest that higher quality adults may be able to compensate for the disadvantages of late hatching dates and achieve similar reproductive success to that of pairs hatching chicks early. We found that pairs hatching chicks late in the season were subject to more incidents of kleptoparasitism than those hatching chicks early. This may be a proximate factor contributing to seasonal declines in reproductive success for common terns, although such a mechanism would not be likely in non‐colonial species. Failure to control for egg quality may have biased the results of some prior egg exchange experiments. Additionally, altered cost of incubation may be an unavoidable confounding factor in studies designed to manipulate timing of breeding.