Nesting energetics of house wrens (Troglodytes aedon) in relation to maximal rates of energy flow

-We tested the hypothesis that brood size in House Wrens (Troglodytes aedon), which have a mean mass of 10.6 g, is confined by digestive limits to maximal rate of energy flow in parents feeding nestlings. Using doubly-labeled water (DLW), we measured field metabolic rate during feeding of nestlings (FMRr), tested whether it increased with brood size, and compared it with the wrens' near-maximal rate of intake, digestion, and assimilation measured in the laboratory. To expand the range of House Wrens' effective brood sizes (i.e. brood size/parent), we manipulated some brood sizes by removal of the male parent or by addition of extra chicks or eggs. The rate of nest visits by parent wrens was positively correlated with mass of chicks (P < 0.001). Various measures of brood size were regressed against FMR,. However, when brood size and overnight minimum air temperature were entered together in a stepwise multiple-regression model, only minimum air temperature was significantly related to FMRr (P = 0.013, r2 = 0.201). Mean FMR, (60.8 ? 2.0 kJ/day) was lower than the near-maximal rate of intake, digestion and assimilation measured in the laboratory (92.7 kJ/day). Therefore, we reject the hypothesis that brood size is limited by digestive constraints on maximum metabolic rate in House Wrens. Received 6 November 1991, accepted 29 October 1992. FACTORS THAT limit clutch size in birds can influence an individual's reproductive success and, thus, are important ecologically. Several hypotheses attempt to explain the limitation of clutch size (Stearns 1976:12, Murphy and Haukioja 1986:141). One involves an energetic constraint on brood size and is based upon the ideas that: (1) parental field metabolic rate during reproduction (FMRr) increases with increasing numbers of chicks (Drent and Daan 1980:242, Yom-Tov and Hilborn 1981); and (2) there is a physiological limit to the parent's FMR, that is approached in the field so that maximal metabolic rate sets a limit on brood size (Masman et al. 1989, Peterson et al. 1990). If maximal sustainable metabolic rate is to be a limiting factor on brood size, FMRr must increase with brood size. The relationship of FMRr to brood size or mass is not consistent among species. Parental FMRr increased with number of young in Savannah Sparrows (Passer sandwichensis; Williams 1987) and adult male House Martins (Delichon urbica; Hails and Bryant 1979), but has not been shown to be related to brood size in at least seven other species of birds (reviewed by Bryant 1988). We tested the hypothesis that FMRr will increase with brood mass (and, hence, limits to energy flow could potentially limit brood size). The second part of the hypothesis is difficult to evaluate, in part because no upper benchmark (i.e. maximum metabolic rate) has been described. In fact, determinants of maximal rate of energy flow are unknown, although maximal rate of energy flow has been proposed to limit reproductive effort (Masman et al. 1989) and various other ecological parameters (Root 1988). Maximal rate of intake, digestion and assimilation may represent maximal energy flow (Kirkwood 1983, Weiner 1992), but it is possible that factors limiting energy expenditure (e.g. mobilization of nutrients, metabolism by tissues) might determine maximal rate of energy flow (Karasov 1986, Peterson et al. 1990). Some investigators have proposed an upper benchmark of three to five times SMR (Karasov 1990:415), or three to six times fasting metabolic rate (Kirkwood 1983), but there have been few studies designed to experimentally force birds to their maximal rates. Two allometric analyses suggested that the FMR of at least some birds may approach these proposed maximal rates. Karasov (1990:415) found that estimates of maximal rate of energy intake averaged 36% higher than measures of FMRr in breeding birds. Masman et al. (1989) showed with data from 30 species that maximum sustainable rates of energy flow may be more limiting to FMRr in small