Body Size and Condition, Timing of Breeding, and Aspects of Egg Production in Eastern Kingbirds

-Variation in timing of breeding in Eastern Kingbirds (Tyrannus tyrannus) was correlated only weakly with external morphological characters, but was correlated positively and significantly with estimates of body size based on measurements of skeletons and muscle weights. Small females apparently held a reproductive advantage in being able to mobilize resources for reproduction before large females. Egg weight was independent of all measures of female size, but was directly and significantly (P = 0.03) correlated with standard flight muscle weight, a relative index of body condition. Egg size was thus a function more of female body condition than size. On average, shell, yolk, and albumen comprised 5.6%, 21.9%, and 72.5%, respectively, of fresh egg weight. Dry albumen and dry shell increased proportionately with fresh egg weight, but dry yolk did not. Total protein, lipid, and energy of fresh eggs all increased proportionately with weight. Comparison of egg composition and standard flight muscle weight of laying females indicated moderate positive, although nonsignificant, correlations between body condition and measures of egg quality, especially lipid content. Based on postegg-laying body composition and nutrient requirements for the production of one egg, it appeared that most females probably could have laid an additional egg almost solely from body reserves. Clutch size thus appears to be independent of body condition. Received 20 June 1985, accepted 16 December 1985. RECENT studies on the evolution of reproductive patterns in birds have emphasized the proximate controls of reproduction, and the bases for phenotypic variation in reproductive traits (Ankney and MacInnes 1978, Drent and Daan 1980, Petrie 1983, Jarvinen and Vaisanen 1984, Murphy et al. 1984). Because intrapopulation variation in timing of breeding is often considerable, and because of the direct influence of size on energy metabolism (Lasiewski and Dawson 1967, Walsberg 1983), various authors have proposed a dependence of breeding date on body size. Small females presumably can divert a greater portion of daily food intake to reproduction, and are predicted to become reproductively active before large females (Wiley 1974, Downhower 1976). Numerous studies (see Davies and Lundberg 1985) support claims that timing of breeding is often energy limited in birds (Perrins 1970, Yom-Tov and Hilborn 1981). Early breeding is advantageous because 1 Present address: Department of Life Sciences, Indiana State University, Terre Haute, Indiana 47809 USA. it increases the probability of survival of fledged young (Campbell in Lack 1966; Nelson 1966; Perrins 1966, 1970, 1980; Nisbet and Drury 1972; Garnett 1981; Cooke et al. 1984; Newton and Marquiss 1984; Arcese and Smith 1985), raises the potential for rearing multiple broods or laying replacement clutches (Price 1984), and affects clutch size itself in species exhibiting a seasonal drop in clutch size. Examination of the relationship between body size and breeding date thus offers one means of testing for an energetic limit to the start of breeding. Eastern Kingbirds (Tyrannus tyrannus) annually raise a single brood of 3 or 4 young (Murphy 1983a). In all years clutch size declines seasonally, yet each season initial clutches of different females are laid as much as 5, but usually 4, weeks apart. Egg weight also exhibits wide variability. I have proposed that intrapopulation variation in kingbird egg weight reflects size differences among females and that kingbirds delay breeding to avoid weather-induced food shortages early in the breeding season (Murphy 1983a). Other observations, however, are consistent with the hypothesis that egg production is constrained by insufficient 465 The Auk 103: 465-476. July 1986 This content downloaded from 207.46.13.169 on Sat, 01 Oct 2016 06:24:13 UTC All use subject to http://about.jstor.org/terms 466 MICHAEL T. MURPHY [Auk, Vol. 103 resources. These include periods of up to 2 weeks between completion of nest construction and egg-laying, occasional suspension of laying during clutch production (Murphy 1983a), and a direct correlation between egg size and air temperature during the period of egg formation (Murphy 1985). In the present investigation I test the hypotheses that timing of breeding is energy limited in Eastern Kingbirds, and that egg size variation stems from size differences among females. I also examine the potential for an energy or nutrient limit to clutch size. Tests were made using information on body size and composition of female kingbirds with known reproductive histories, and through examination of egg composition and energy content.