Brood Size Manipulation Experiments Revisited: Does Variation along the Slow-Fast Continuum Explain Variation in Responses?

Brood size manipulation (BSM) experiments have commonly been used to test survival – reproduction trade-offs, but observed responses vary across species and populations. This variation may reflect differences in life history strategies and how reproductive costs are divided between parents and offspring. In this study, we investigated whether variation in parental response to BSM across passerine birds is shaped by population and species position along the life history continuum (using clutch size as a proxy) and their foraging strategy, while controlling for manipulation magnitude and phylogeny. We found that in ground foraging and aerial foraging birds, clutch size predicted the difference in fledgling mass between experimentally enlarged broods and control broods. This supported life history predictions that slow-paced populations are more likely than fast-paced ones to shift the costs of reproduction onto their offspring. However, this pattern did not hold for other foraging strategies, suggesting that behavioural factors such as provisioning ability and behavioural plasticity also influence parental responses. Unexpectedly, magnitude of manipulation was not found to be significant, which could underly a difference in magnitude sensitivity between foraging strategies. However, several limitations such as correlations between variables and small sample size constrain broad generalisations of our results. Nevertheless, our results highlight the need to consider both behavioural and life history factors when evaluating parental care strategies. Future research should broaden taxonomic and geographic scopes and incorporate measures beyond clutch size, such as yearly fecundity and adult survival, to more accurately capture variation in parental care across species and populations.