REPEATABILITY OF CLUTCH SIZE IN HOUSE WRENS

Theories of clutch-size determination have focused primarily on first broods or on seasonal trends in clutch size. High repeatability between the sizes of first and second clutches could be expected because of individual genetic, vigor, and environmental differences. We examined the repeatability of clutch size in individual House Wrens (Troglodytes aedon) both within and across years using data collected between 1926-1937 by Kendeigh, Baldwin, and others. Contrary to expectation, repeatability of clutch size in first and second clutches within a year was low (r(I) = 0.10) and only about one-third that of first or second clutches among different years (r(I) = 0.27 or 0.30, respectively). Despite modest repeatability in clutch size, no significant correlation existed between number of fledglings in successful first broods and clutch size of second broods (r = 0.006). Clutch-size repeatability was associated with female identity and not nest box site, suggesting that differences in individual clutch size did not result from occupation of differentially suitable breeding territories. Estimates of heritability of clutch size were limited due to small numbers of fledglings that returned to natal study sites to breed; however, the estimate of heritability of clutch size was greater for mother-son combinations (h2 = 0.48) than for mother-daughter combinations (h2 = 0.21).