Ultimate regulation of fecundity in species with precocial young: declining marginal value of offspring with increasing brood size does not explain maximal clutch size in Black Brent geese

Lack 18:125–128 (1967) proposed that clutch size in precocial species was regulated by nutrients available to females during breeding. Drent and Daan 68:225–252 (1980) proposed the individual optimization hypothesis, whereby individual state determines the optimal combination of breeding date and clutch size. Neither hypothesis accounts for variation in nutrients among females at the end of egg laying, strong right truncations in clutch size distributions, or the fact that many species with precocial young are determinate layers. One solution is that there is a maximum clutch size, above which the number of fledged young declines. We manipulated brood size in Black Brent geese to decouple brood size from maternal quality and produce broods larger than the natural maximum. We recaptured marked goslings to assess variation in prefledging survival as a function of brood size and we estimated relative prefledging survival of goslings using a Bayesian hierarchical approach. We considered effects of natural clutch size, brood size and their interaction on probability that we captured goslings at about 4 weeks of age. Prefledging survival declined with increasing brood size (β^ = −0.53; 95% CI −0.91 to −0.16), while laid clutch size had little influence on prefledging survival (β^ = −0.04; 95% CI −0.42 to 0.32). Despite declining per capita survival with increasing brood size, the most productive brood size was six goslings, which is greater than the typical maximum clutch size of five. Thus, reduced survival in large broods, by itself, is not the sole mechanism that limits maximum clutch size. We suggest elsewhere that incubation limitation and lower residual reproductive value for females tending larger broods may be other mechanisms limiting maximal clutch size in brent.