GEOGRAPHIC AND SEASONAL PATTERNS OF CLUTCH-SIZE VARIATION IN HOUSE WRENS

Abstract

ABsTRAcr.-I report on geographic variation and seasonal decline in clutch size in House Wrens (Troglodytes aedon), a species that breeds throughout much of the New World. Nest records from British Columbia to Tierra del Fuego (n = 3,246 clutches) show that clutch size increases with latitude in both the Northern and Southern hemispheres. Seasonality of food resources, as measured by actual evapotranspiration (AE), does not explain much of the variation in clutch size, especially when controlling for latitude. This result shows that either seasonality is unimportant or that AE is a poor indicator of the food available to House Wrens. Seasonal decline in clutch size varies geographically such that in northern populations, clutch size declines much more rapidly during the breeding season than in low latitude populations. A model explaining this pattern, the offspring-survivorship hypothesis, predicts that the rate of seasonal decline in clutch size is related to the difference in survivorship between earlyand late-fledged young. Field study in Monteverde, Costa Rica, showed that clutch size did not change seasonally and that late-fledged young were at least as likely to survive to the following breeding season as early-fledged young. This pattern is in contrast to North Temperate areas where clutch size declines sharply during the breeding season and late-fledged young are much less likely to survive to the following year than early-fledged young. Received