Explanatory model showing possible relationships between date of snowmelt, diet quality and reproductive success in Pyrenean ptarmigan.
Average and both extremes of interannual snowmelt variation are shown. Snowmelt determines the beginning of growth and N content in the plants fed on by ptarmigan, which in turn regulates the N content of hen diets and protein increases during reproduction period. Previous data (Novoa et al. 2008) show that interannual variability of laying date (ILDV) are significantly less than interannual variability of snowmelt date (ISDV). Thus, in years of late snowmelt hens have less time to obtain good-quality food, resulting in poor body condition for laying and incubation, and diminishing breeding success. In years of early snowmelt, there is no loss of protein gain for ptarmigan hens, because of the asymmetric size of N content curve in plants and the appearance of new N peaks of major summer foods for ptarmigan (forbs and insects).