Geographical variation in spotting patterns on Hazel Grouse Bonasa bonasia primary feathers: consequences for age determination

Abstract

The spotting pattern on the ninth primary feather (numbered from the proximal to the distal) has been reported to be a rather accurate indicator of age in Hazel Grouse Bonasa bonasia, separating first-year from older birds, in Finland and European Russia. However, the recommended criteria for separating age classes differed . We compared these studies with data from southern Poland and southcentral Sweden and found significant geographical differences in spotting patterns among adult grouse . The spotting pattern was most different between the population in Poland (B. b. rupestris) and the other three populations (all B. b. bonasia), and was a poor indicator of age there. In the Swedish population, the criteria recommended for Finnish birds were valid. We urge caution in using spotting patterns of primary feathers to determine age of Hazel Grouse in areas where the technique has not been verified.