Fitness components associated with laying date in the Cliff Swallow

We report how clutch size, annual reproductive success, rind annual surviva l of breeders and offspring vary with laying date in Cliff Swallows (Petroc helidon pyrrhonota) in southwestern Nebraska, from 1982-1993. Clutch size d eclined significantly across the season for both parasite-free (fumigated) and naturally infested nests. Larger clutches of 4 and 5 eggs were initiate d significantly earlier in warm and dry seasons, laying dates for smaller c lutches showed no relationship with weather. Incubation period declined sig nificantly with laying date and was shorter in the absence of ectoparasites . Fledging success declined significantly with laying date in nonfumigated nests, primarily through greater loss of nestlings to parasitic swallow bug s (Oeciacus vicarius). First-year survival probabilities of offspring varie d significantly with date among nonfumigated nests but not among fumigated nests; earlier-hatched young survived best in some years, and in other year s intermediate- or late-hatched young had highest survival. Number of young locally recruited was highest for early nesters in 5 of 7 years with inter mediate nesters doing best in the remaining years. Annual survival of femal e breeders increased with laying date, suggesting a benefit to late nesting and 3 potential life history trade-off between reproduction and adult surv ival. Nestling body mass did not vary with laying date, suggesting no decli ne in food availability over the summer. Female body mass Frier to and duri ng laying declined significantly with laying date but juring brood rearing increased significantly with laying date. Female body mass had no significa nt effect on clutch size independent of laying date. The primary environmen tal factor causing the seasonal decline in reproductive success is ectopara sitism. A seasonal decline in clutch size may be a strategic adjustment to reduce ectoparasitism by lowering the parasite carrying capacity per nest. Greater apparent survival of late-nesting females may prevent directional s election for earlier bleeding times in Cliff Swallows.