Abstract
Humans are transforming ecosystems by providing predictable anthropogenic food subsidies (PAFS), such as at landfills, now used by multiple animal taxa. In breeding animals, parental landfill use can influence offspring development, yet few studies have quantified parental landfill use and linked it to parental care and offspring development, behaviour or survival, limiting our understanding of the population-level consequences of PAFS use. Between 2021-2024, we GPS-tracked 56 adult and 79 juvenile Yellow-legged gulls (Larus michahellis) across five Mediterranean colonies. We also analysed the content of 373 pellets and the growth of 257 nestlings until fledging. Our aim here was to understand whether (i) PAFS use, particularly landfills, by breeding adults differed among colonies; (ii) landfill use affected parental foraging trips and plastic consumption; (iii) parental landfill use influenced chick growth and condition; and (iv) this affected post-fledging movements and survival of juveniles. We found that adult landfill use varied among colonies, independently of the distance to the nearest landfills. Higher landfill use resulted in fewer and shorter foraging trips during chick rearing and increased consumption of anthropogenic materials. Chicks from high-use colonies developed and fledged in poorer body condition, which delayed the onset (but not the distance) of dispersal movements, without affecting post-fledging survival. Our results reveal a potential trade-off in PAFS use, which may act as a time-saving foraging strategy for parents while compromising offspring body condition. This demonstrates how anthropogenic subsidies can shape the behaviour of breeding animals and generate carry-over effects on offspring development and early-life behaviour.