Body weight and lengths of the wing, tail, tarsus, and beak were measured in 139 black grouse (BG) Lyrurus tetrix (Linnaeus, 1758) and 318 hazel grouse (HG) Tetrastes bonasia (Linnaeus, 1758) captured in the north of the Arkhangelsk Region (Russia) in 1990–2006. Almost all the estimates are within the average range of the parameters for Northern Europe (Norway – Komi Republic). Sexual dimorphism in BG was revealed for all traits except the wing length; dimorphism by tail length in Northern Europe appears to be growing from east to west. Size dimorphism in HG is inexplicit and, apparently, minimal in spring; females before and during egg-laying are heavier than males. In young BG, the dimensional growth of tarsi in males and females is completed in the 2nd-3rd months since birth, that of beak – in the 3rd-4th month, that of tail and wings – in the 4th-5th months. In HG, these processes are shorter: tarsi and beak – in the 2nd month, wings and tail – in the 3rd-4th months. Tail and beak lengths and body weight showed the greatest variation in both species, while wings and tarsi were the most stable parts of their bodies. Tarsus was also utterly independent of the other four traits, while wing length correlated with tail length and body weight (BG) or tail length only (HG). The age-related variation patterns for studied traits and correlations between them indicate the likelihood of migration processes, especially for BG. The arrival of first-year birds of both species in study area from August to May-June is hardly probable, but their emigration may take place. Part of the size variation in adult birds is apparently due to arrivals from outside.