Interval between clutches, fitness, and climate change

Timing of optimal reproduction can be affected by the presenceof multiple broods, with multi-brooded species breeding earlier(and later) than the optimal timing of breeding as comparedwith single-brooded species that only need to optimize the timingof a single brood. Approximately two-thirds of barn swallowsHirundo rustica produce 2 broods per year, and I tested whetherthe constraints on timing of reproduction were affected by climatechange because climatic amelioration would allow both an earlierstart and a later termination of reproduction. The durationof the interval between first and second clutches and the variancein the duration increased during 1971–2005 when temperatureduring spring, but not summer, increased rapidly. Interclutchinterval was shorter when mean date of breeding was late andalso among late-breeding individuals during individual years.When clutch size and brood size of the first clutch were large,interval until the second brood increased. Pairs with a longinterval produced more fledglings than pairs with a short interval.Pairs with first broods with strong mean T-cell–mediatedimmune responses took shorter time to start their second clutch,whereas mean body mass or tarsus length of first broods werenot significantly related to interclutch interval. Interclutchinterval increased with the size of a secondary sexual character,the length of the outermost tail feathers of adult male barnswallows, but not with tail length of females, or with sizeof several other phenotypic characters in either sex. Thesefindings are consistent with the hypothesis that the durationof the interclutch interval is determined by a combination ofenvironmental conditions, reproductive effort, and sexual selection.