Life history theory predicts that increased investment in current offspring decreases future fecundity or survival. Avian parental investment decisions have been studied either via brood size manipulation or direct manipulation of parental energetic costs (also known as handicapping). However, we have limited experimental data on the potential interactive effects of these manipulations on parent behavior. Additionally, we know little about how these manipulations affect spatial foraging behavior away from the nest. We simultaneously manipulated brood size and parental costs (via added weight in the form of a GPS tag) in wild female barn swallows (Hirundo rustica). We measured multiple aspects of parent behavior at and away from the nest while controlling for measures of weather conditions. We found no significant interactive effects of manipulated brood size and parental costs. Both sexes increased their visitation rate with brood size, but nestlings in enlarged broods grew significantly less post-brood size manipulation than those in reduced broods. Foraging range area was highly variable among GPS-tagged females but was unaffected by brood size. As such, increased visitation rate in response to brood size may be more energetically costly for far-ranging females. GPS-tagged females did not alter their visitation rate relative to un-tagged birds, but their mates had higher visitation rates. This suggests that GPS tagging may affect some unmeasured aspect of female behavior, such as prey delivery. Our findings indicate that investigation of foraging tactics alongside visitation rate is critical to understanding parental investment and the benefits and costs of reproduction.
By simultaneously manipulating both brood size and parental costs and examining multiple metrics of parental care, including foraging behavior, we provide new insights into parental investment decisions during nestling rearing in a short-lived aerial insectivore. Contrary to our predictions, we did not find interactive effects of brood size manipulation and GPS tagging on parent behavior or nestling growth while controlling for measures of weather conditions, indicating that brood size and parental costs may have solely additive effects on parental care in our study system. Our results also suggest that investigation of foraging tactics alongside visitation rate is critical to understanding parental investment and the benefits and costs of reproduction. Barn swallow females with experimentally enlarged broods increased their visitation rate but did not adjust their foraging range area. Spatial foraging behavior was highly individually variable, which suggests that adjustments in visitation rate may be more costly for some individuals (those with larger foraging ranges) than others and indicates that visitation rate alone cannot fully capture parental investment. Additionally, GPS tagging did not affect female visitation rate, but males with tagged mates nonetheless had higher visitation rates. This finding suggests that GPS tagging may affect some unmeasured aspect of parental care or foraging behavior, such as prey delivery. Further work on of the effects of brood size manipulation and parental costs on multiple aspects of parental care and foraging behavior would be of great value
September 23, 2022
PUBLICATION AVAILABLE AT: https://link.springer.com/article/10.1007/s00265-022-03244-z#Sec11