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Abstract

According to life history theory, reproduction is costly, and increased parental effort can lead to reduced survival and fecundity. Animal parents are thus expected to balance trade-offs between investment in their current offspring and investment in their own survival and future reproductive attempts to maximize their lifetime fitness. In birds, brood size manipulation (BSM) experiments have been used to examine how avian parents balance these life history trade-offs. Previous research has investigated whether parents are willing to alter their visitation rate and the amount, type, or size of prey they bring to nestlings in response to changes in brood size, but our knowledge about how BSM affects the spatial foraging patterns of parents is limited. Together with a research team, I set out to understand how BSM affects parent foraging patterns and other metrics of parental effort. I altered the brood size of North American Barn Swallows (Hirundo rustica erythrogaster) at several breeding sites in Boulder County, Colorado. Using GPS tags and on-site observation, I tracked visitation rate, brood growth rate, and foraging distance from the nest before and after manipulation. I found that female visitation rate significantly increased by 105% following brood enlargement and non-significantly decreased following brood reduction. Brood growth rate significantly increased by 1.05 grams/hour following brood enlargement and non-significantly decreased following brood reduction. Foraging distance was unaffected by changes in brood size. My results suggest that while Barn Swallow parents are willing to scale some aspects of feeding effort to nestling number, foraging distance is similar regardless of brood size. These data contribute to a more complete understanding of how avian parents balance life history trade-offs when provisioning nestlings.