LAUSR

Food availability is a major characteristic of habitat quality, linking habitats with demographic parameters such as reproductive performance. Parent birds adjust their food provisioning to both habitat-specific food characteristics and the demands of their young. However, because habitat quality and the brood's food intake are often correlated, the underlying mechanisms of adjustments in parental provisioning remain entangled. How the relationship between habitat quality and parental provisioning behaviour affects the quantity of food available to nestlings and the resulting nestling growth and survival is therefore still incompletely established. We experimentally increased the food intake of little owl, Athene noctua, nestlings in two habitat types differing in food availability and used unsupplemented broods as controls. The food supplementation experiment allowed us to disentangle the effect of habitat type from the effect of the nestlings' food intake on parental provisioning behaviour. Camera traps recording a series of 10 consecutive images for each parental visit allowed us to quantify visiting rates and diet composition by applying a hierarchical multinomial model explicitly accounting for the observation process. Food supplementation caused parents to switch to smaller food items and to increase visiting rates, resulting in similar biomass brought to nestlings in supplemented and unsupplemented broods. Irrespective of the food supplementation, parents in low-quality habitats delivered 63% of the biomass delivered by those in high-quality habitats. Accordingly, we found an increase in nestling survival rates in response to food supplementation in low-quality habitats, but not in high-quality habitats. Our results show that habitat quality affects the biomass of prey delivered to the brood, whereas the nutritional state of the brood affects prey selection or foraging modes of parents. Reproductive output directly reflected habitat quality in terms of food availability, identifying food as the main factor underlying differential reproduction within and between populations.