Environmental predictors of breeding phenology differ with elevation in a common mountain bird species

Temperatures in mountain areas are increasing at a higher rate than the Northern Hemisphere land average, but how fauna may respond, in particular in terms of phenology, remains poorly understood. The aim of this study was to assess how elevation could modify the relationships between climate variability (air temperature, snow melt-out date), the timing of plant phenology and egg-laying date of the coal tit (Periparus ater). We collected 9 years (2011-2019) of data on egg-laying date, spring air temperature, snow melt-out date and larch budburst date at two elevations (~1300m and ~1900m asl) on a slope located in the Mont-Blanc Massif in the French Alps. We found that at low elevation, larch budburst date had a direct influence on egg-laying date, while at high altitude snow melt-out date was the limiting factor. At both elevations, air temperature had only an indirect effect on egg-laying date. Our results shed light on proximate drivers of breeding phenology responses to inter-annual ...