Intraseasonal Reproductive Costs for the House Sparrow (Passer domesticus)

Abstract

Abstract House Sparrows (Passer domesticus) near Calgary, Alberta begin breeding in early spring and continue through to late summer. High productivity from previous broods is negatively correlated with fledgling production from second and third broods. Although fat reserves may limit the ability of females to raise young, there is no concomitant drop in clutch size or in the probability of renesting. Pairs that fledge many young in a year space fledgling production evenly over the breeding season but are most productive in mid-season. The interval between fledging and the initiation of the next clutch increases with the number fledged. This delay, an indication of the physiological strain involved in rearing young, is greater for later broods and for females nesting in trees. Measures of reproductive effort (clutch size, number fledged, length of the nestling period) vary seasonally but give no indication of peaking for last broods. Thus, reproductive effort is not adjusted to parallel changes in the probability of surviving to the next breeding attempt.