Yellow-billed Chough

The adult of the nominate subspecies of the Alpine chough has glossy black plumage, a short yellow bill, dark brown irises, and red legs. It is slightly smaller than red-billed chough, at 37–39 centimetres (15–15 inches) length with a 12–14 cm (4.7–5.5 in) tail and a 75–85 cm (30–33 in) wingspan, but has a proportionally longer tail and shorter wings than its relative. It has a similar buoyant and easy flight. The sexes are identical in appearance although the male averages slightly larger than the female. The juvenile is duller than the adult with a dull yellow bill and brownish legs. The Alpine chough is unlikely to be confused with any other species; although the jackdaw and red-billed chough share its range, the jackdaw is smaller and has unglossed grey plumage, and the red-billed chough has a long red bill. The subspecies P. g. digitatus averages slightly larger than the nominate form, weighing 191–244 g (6.7–8.6 oz) against 188–252 g (6.6–8.9 oz) for P. g. graculus, and it has stronger feet. This is in accordance with Bergmann's rule, which predicts that the largest birds should be found higher elevations or in colder and more arid regions. The extremities of the body, the bill and tarsus, were longer in warmer areas, in line with Allen's rule. Temperature seemed to be the most important cause of body variation in the Alpine chough. The flight of the Alpine chough is swift and acrobatic with loose deep wing beats. Its high manoeuvrability is accomplished by fanning the tail, folding its wings, and soaring in the updraughts at cliff faces. Even in flight, it can be distinguished from the red-billed chough by its less rectangular wings, and longer, less square-ended tail. The rippling preep and whistled sweeeooo calls of the Alpine chough are quite different from the more typically crow-like chee-ow vocalisations of the jackdaw and the red-billed chough. It also has a rolling churr alarm call, and a variety of quiet warbles and squeaks given by resting or feeding birds. In a study of chough calls throughout the Palearctic region it was found that call frequencies in the Alpine chough showed an inverse relationship between body size and frequency, being higher-pitched in smaller-bodied populations.