Costs and benefits of maternal nest choice: Trade-offs between brood survival and thermal stress in bees.

Optimal nest site selection is crucial in animals whose offspring are completely dependent on the shelter of a nest. Parental decisions influencing nest thermal conditions are particularly important because temperature strongly influences juvenile activity, metabolism, growth, developmental rate, survival, and adult body size. In small ectotherms such as bees, maternal decisions to nest in sun-exposed or shady sites can lead to marked differences in thermal microenvironments inside nests. Small carpenter bees (Ceratina calcarata) strongly prefer to nest in sun but also prefer nesting substrates more frequently found in shade, suggesting that nest site selection is based on a trade-off between costs and benefits of warmer versus cooler nest sites. We investigated the consequences of sun and shade nesting for mothers and their offspring using a field experiment in which mothers and newly founded nests were placed in sunny or shady habitats. Maternal costs and benefits in each treatment were quantified by comparing maternal foraging effort, nest size, number of brood provisioned, and number and size of live offspring. These demographic measures allowed us to estimate fitness for mothers nesting in sun versus shade. For juvenile bees from sun and shade nests, we quantified two thermal traits, high-temperature tolerance (CT