Birds display a latitudinal gradient in clutch size. Ashmole's hypothesis explains this geographic pattern by differences in seasonality of resources resulting in different levels of winter mortality. Ashmole assumed that populations are strongly limited by resources during the non-breeding season. and the level of resources available in the breeding season relative to the non-breeding season determines clutch size. The main problem with Ashmole's hypothesis is that it does not take into consideration trade-offs of reproduction. Applying a simulation approach and concepts of life-history theory, we therefore re-evaluate the hypothesis. In particular, we analyse four alternative mechanisms that may generate a gradient in clutch size: (1) differences in levels of seasonality of resources causing winter mortality and no cost of reproduction (= Ashmole's hypothesis): (2) differences in levels of seasonality of resources and a cost of reproduction that may act on juveniles and/or adults (3) no differences in levels of seasonality of resources but there is a cost of reproduction: and (4) no differences in levels of seasonality of resources and no cost of reproduction. To model cost of reproduction, three general cost functions were assumed: linear, hyperbolic or exponential decrease in future survival of individuals for increasing clutch sizes. Whereas the mechanisms implemented in alternatives (1), (3) and (4) did not generate a gradient in clutch size, those given in alternative (2) were able to generate this pattern. This suggests that Ashmole identified seasonality of resources as one important mechanism for geographic variation in clutch size, but did not recognize cost of reproduction as a second mechanism. In particular. we observed the gradient in our simulations in two situations: (i) linear or hyperbolic cost of reproduction for offspring and no costs for parents, and (ii) linear or hyperbolic cost of reproduction for offspring together with any type of cost of reproduction for parents. In these situations, widely accepted differences and correlations among life-table variables of tropical and temperate avian species did hold. Our results are valid for a wide range of geographic clutch size variation that may be the result of adaptation to seasonal environments.
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