Context-dependent conservation of the cavity-nesting European Roller.

To maximise the effectiveness of conservation interventions, it is crucial to have an understanding of how intra‐specific variation determines the relative importance of potential limiting factors. For bird populations, limiting factors include nest‐site availability and foraging resources, with the former often addressed through the provision of artificial nest‐boxes. However, the effectiveness of artificial nest‐boxes depends on the relative importance of nest‐site versus foraging resource limitation. Here, we investigate factors driving variation in breeding density, nest‐box occupation and productivity in two contrasting study populations of the European Roller Coracias garrulus, an obligate cavity‐nesting insectivorous bird. Breeding density was more than 4 times higher at the French study site than at the Latvian site, and there was a positive relationship between breeding density (at the 1‐km2 scale) and nest‐site availability in France, but between breeding density and foraging resource availability in Latvia. Similarly, the probability of a nest‐box being occupied increased with predicted foraging resource availability in Latvia, but not in France. We detected no positive effect of foraging resource availability on productivity at either site, with most variation in breeding success driven by temporal effects; a seasonal decline in France, and strong inter‐annual fluctuations in Latvia. Our results indicate that the factors limiting local breeding density can vary across a species’ range, resulting in different conservation priorities. Nest‐box provisioning is a sufficient short‐term conservation solution at our French study site, where foraging resources are typically abundant, but in Latvia the restoration of foraging habitat may be more important