Timing of optimal reproduction can be affected by the presence of multiple broods, with multi-brooded species breeding earlier (and later) than the optimal timing of breeding as compared with single-brooded species that only need to optimize the timing of a single brood. Approximately two-thirds of barn swallows Hirundo rustica produce 2 broods per year, and I tested whether the constraints on timing of reproduction were affected by climate change because climatic amelioration would allow both an earlier start and a later termination of reproduction. The duration of the interval between first and second clutches and the variance in the duration increased during 1971–2005 when temperature during spring, but not summer, increased rapidly. Interclutch interval was shorter when mean date of breeding was late and also 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 long interval produced more fledglings than pairs with a short interval. Pairs with first broods with strong mean T-cell–mediated immune responses took shorter time to start their second clutch, whereas mean body mass or tarsus length of first broods were not significantly related to interclutch interval. Interclutch interval increased with the size of a secondary sexual character, the length of the outermost tail feathers of adult male barn swallows, but not with tail length of females, or with size of several other phenotypic characters in either sex. These findings are consistent with the hypothesis that the duration of the interclutch interval is determined by a combination of environmental conditions, reproductive effort, and sexual selection.