Seasonal Variation in Reproductive Output of House Sparrows: The Determination of Clutch Size

Abstract

Seasonal changes in reproductive output of house sparrows, Passer domesticus, were studied at 2 localities: Lawrence, Kansas, USA and Calgary, Alberta, Canada. In both localities the most productive clutch (6 eggs) was larger than the modal clutch (5 eggs). Most variation in clutch size could be attributed to within—individual seasonal changes. Laying of 6—egg clutches was generally restricted to the middle of the breeding season. Clutch size changes were not consistently synchronized with trends in predictable environmental variables such as photoperiod and temperature. Because seasonal trends in clutch size and fledging success were poorly correlated, there is no evidence that seasonal changes in clutch size represent adaptations to maximize the number of adequately nourished offspring. Although clutch size is ultimately determined by contribution to future generations, it is probably proximately controlled by resources at the time of oogenesis.