Stable isotope ratios in alpine rock ptarmigan and black grouse sampled along a precipitation gradient

Abstract

Rock ptarmigan (Lagopus muta) and black grouse (Tetrao tetrix) are two closely related alpine bird species that form relict populations in the European Alps. Besides manifold anthropogenic influences in this region, global climate change is forecast to lead to significant changes in temperatures and precipitation. We here analysed stable isotope ratios (δ13C and δ15N) of feathers of both bird species and their potential dietary plants across a longitudinal precipitation gradient in south-east Switzerland. Plant δ13C was higher at higher altitudes and in drier areas (coinciding with higher longitudes) while plant δ15N did not differ geographically. Black grouse δ13C reflected the longitudinal pattern in precipitation and plant δ13C, and there was no indication for a change in dietary composition with precipitation (i.e. no significant changes in δ15N). In contrast, rock ptarmigan δ13C was independent of precipitation and plant δ13C values and showed a significant increase in δ15N towards drier areas, suggesting a potential dietary shift. In rock ptarmigan, we furthermore investigated intraspecific differences with age, between males and females and among years, and did not find any biologically meaningful intraspecific differences. Interspecifically, rock ptarmigan feathers had significantly higher δ13C and lower δ15N values than black grouse, reflecting a dietary segregation between both species. This may partly be due to the higher altitudinal distribution of rock ptarmigan in combination with an altitudinal gradient in plant δ13C. In addition, however, species also segregated in δ15N, most likely caused by a higher proportion of invertebrate diet in black grouse.