Shelters or ecological traps? Context-dependent effects of nestboxes on breeding success in a colonial raptor

Artificial breeding structures, such as nestboxes, can potentially influence the population size and conservation status of bird species relying on heavily human-modified environments such as agroecosystems and urban areas. However, the effectiveness of these interventions may vary, as artificial structures could attract individuals to suboptimal breeding sites, potentially reducing breeding success compared to natural nesting sites and resulting in ecological traps. European populations of the Lesser Kestrel Falco naumanni, a secondary cavity-nesting raptor, have greatly benefitted from extensive provisioning of artificial nestboxes in recent decades. However, the long-term conservation effectiveness of such efforts may be jeopardized because of recent changes in local climatic conditions, with nestboxes becoming unsuitable under climate warming as nest temperatures become too high, exposing developing embryos and nestlings to lethal hyperthermia. We assessed variation in nest temperature and breeding success between naturally available nest-cavities (natural nests) and nestboxes in two Italian Lesser Kestrel populations settled in different climatic zones (Po Plain, subcontinental; Matera, semi-arid). In both areas, natural nests were ~5°C cooler than nestboxes. In the Po Plain, nestling survival was substantially lower in natural nests (44%) than in nestboxes (96%), whereas the opposite was the case in Matera (92% vs 45%). In Matera, frequent heatwaves led to very high temperatures (>44°C) inside nestboxes, which in turn resulted in lethal hyperthermia and a substantial reduction in nestling survival, while this was not the case in natural nests. In contrast, nestbox temperatures in the Po Plain were always below critical values, and nestling predation was lower compared to natural nests. These findings highlight that the appropriateness of nestbox provisioning should be carefully evaluated on a case-by-case basis, and that considering thermal safety, climate resilience and predation risk is crucial when designing and building artificial nest structures to ensure their long-term conservation effectiveness.