Abstract
Skull pneumatization was examined in relation to actual age for 37 known-age Hazel Grouse Bonasa bonasia raised in an aviary. Although pneumatization advanced with increasing age, unpneumatized areas (UPA) were present on the cranial roof even in old adult birds. Five indices indicating the size of UPA, i. e. 1) length of UPA, 2) width of UPA, 3) length of UPA × width of UPA, 4) length of UPA/total length of skull, and 5) width of UPA/greatest width of skull, were examined. Of the five indices, the fifth index was a reliable criterion for separating first-year birds of 4-8 months of age from older birds of_??_15 months of age; the index value was 0.158 to 0.306 for the former and 0.063 to 0.145 for the latter. The value for separation is 0.15. This criterion was adopted to determine the age of Hazel Grouse killed during hunting seasons from 1 October to 31 January in Hokkaido, Japan. Of 49 birds examined, first-year and older birds accounted for 77 and 23% respectively.
