Supplementary materials for: «Food supplementation reduces early-stage nest attendance under adverse weather conditions in red kites Milvus milvus"

Abstract Parental care strategies have evolved to ensure optimal offspring rearing under varying environmental conditions. Nest attendance is a fundamental behaviour providing protection against nest predators and adverse weather, but it is traded against time spent foraging. Investigating environmental factors acting on this trade-off is crucial for understanding the ecological mechanisms underlying population-level reproductive output. We therefore experimentally increased the food available to breeding pairs of a long-lived raptor, the red kite Milvus milvus, and monitored parental nest attendance in food-supplemented (N = 37) and non-supplemented (N = 58) broods in two breeding seasons. As food supplementation has recently been shown to increase nestling body condition, we hypothesized that supplementary food would allow parents to reduce their investment in foraging and instead increase nest attendance. Nest attendance was highest early in the nestling phase and increased with rainfall in control nests. However, contrary to what expected under our hypothesis, nest attendance did not increase in food-supplemented pairs, neither in dry nor in rainy weather conditions. This suggests that the extra time gained through supplementary feeding is not invested in additional nest attendance but is likely allocated to other parental or self-maintenance activities, such as predator vigilance, self-foraging, or territory defence. Our study emphasizes the cost of nest attendance, particularly early in the nestling-rearing phase and suggests that regulating these costs is a key mechanism by which environmental conditions shape the trade-off between parental self-maintenance and current reproduction.