Predicting Clutch Size of the House Wren with the Murray-Nolan Equation

Abstract

Although there have been many hypotheses and much research concerning factors that influence clutch size, there have been few attempts to predict clutch size. Murray (1979) suggested that females in a population should lay as few eggs per clutch as required to replace themselves and therefore maximize their survivorship and lifetime reproduction. Murray and Nolan (1989) used this idea to develop an equation that predicts clutch size for birds based on female survivorship and other demographic data. When such data for Prairie Warblers (Dendroica discolor) and Florida Scrub Jays (Aphelocoma c. coerulescens) were used in this equation, close agreement was found between predicted and observed clutch sizes (Murray and Nolan 1989, Murray et al. 1989). To test the Murray-Nolan equation as a predictor of clutch size in House Wrens (Troglodytes aedon), I used demographic data extracted from the field notes of the Baldwin Bird Research Laboratory. Female House Wrens, unlike female Prairie Warblers and Florida Scrub Jays, often rear > 1 brood during a season, so this paper represents an extension of earlier tests of the Murray-Nolan equation to a predominantly double-brooded species. Between 1919 and 1940, S. P. Baldwin, S. C. Kendeigh, and others at the Baldwin Bird Research Laboratory, Gates Mills, Ohio, studied breeding and survivorship of House Wrens. Field notes of this research were made available to me from the library of the Bird Division, Museum of Zoology, University of Michigan. I compiled and analyzed the records from the notebooks for 1926-1937, the 12 yr of most active research. For each nest I recorded the band numbers of both male and female adults, the number of eggs laid, the number of eggs that hatched, the number of hatchlings that successfully left the nest (fledged), and the band numbers of those hatchlings that fledged. Each year, almost all breeding birds were captured at the box and banded or, if previously banded, had their band numbers recorded. Breeding birds not captured were usually those whose clutches failed shortly after laying began. Breeding females were assigned an age of 1 yr if they had not been caught previously at the study sites (see Kendeigh and Baldwin 1937) or if they had been banded as nestlings during the previous year. Only 69 of 4,188 (1.7%) females banded as nestlings returned to breed on the study site; 64 of the 69 (93%)