Fitness components associated with clutch size in cliff swallows

We report measures of annual reproductive success and survival probabilities of adult and offspring Cliff Swallows (Petrochelidon pyrrhonota) associated with clutches of different sizes in southwestern Nebraska from 1982 to 1997. Clutch size 4 was the most common (47.6% of nests). Mean clutch size varied among years, mostly through differences in the proportions of clutch sizes 3 and 4. Temperature and rainfall during egg laying were unrelated to mean annual clutch size. Incubation period declined slightly with clutch size, suggesting no incubation costs associated with larger clutches. Fledging success increased with clutch size. Ectoparasites lowered fledging success significantly in clutch sizes 3 and 4 but not in smaller or larger clutch sizes. First-year survival probability was unaffected by clutch or brood size when ectoparasites were removed, but under natural conditions survival probabilities varied among years and differed significantly among birds reared in brood sizes 1 to 2, 3 to 4, and 5. Birds tending clutch size 4 produced the highest number of young recruited as breeders in four of eight years; in the remaining years, birds tending clutch size 5 apparently did best. Fitness associated with clutch size 5 varied significantly with temperature during brood rearing, being lowest in warm years. Averaged over all years, the highest fitness was conferred at clutch size 5. Parental survival did not differ among clutch or brood sizes for birds under natural levels of ectoparasitism. Body mass of nestlings at 10 days did not vary among clutch sizes in nonfumigated nests and was significantly higher for nestlings in fumigated nests only for the intermediate clutch sizes (2 to 4). Classical life-history tradeoffs probably cannot account for the observed distribution of clutch sizes in Cliff Swallows. Climate and ectoparasite load vary enough annually and are unpredictable enough that Cliff Swallows apparently pursue the risk-averse strategy of laying smaller clutches on average than they can sometimes raise. Ectoparasitism accounted for much of the risk associated with producing clutch size 5, but Cliff Swallows did not adjust their clutch sizes in response to levers of ectoparasitism. Clutch size in aerial insectivores such as swallows may also reflect energetic constraints associated with egg production early in the breeding season when food is often reduced by cold weather.