The population size and breeding biology of the Rook (Corvus frugilegus) in northern Finland

Abstract

We studied the number and the breeding ecology of Rooks in the surroundings ofOulu, northern Finland (65°N, 25°30'E) during 1987-1992. The number of breeding pairs was about 500 in 10 colonies . Non-breeding birds formed 15-35% of the population . Breeding started in mid April. Average clutch size (3 .7) was only slightly smaller but the mean numberof young produced (1 .6 in successful nestings and 1 .1 in all nests) was considerably smaller than in Middle Europe. The most productive clutch size varied between 4 and 6 annually . Early breeders had larger clutches and produced slightly more young than late breeders . Nest site did not affect breeding success; however, Rooks breeding in the top sites of multiple nested trees bred earlier and had larger clutches than others . Normally, repeat nesting was rare . Annual variation was very high, both in clutch and brood sizes. Mean May temperature and duration of ground frost appeared to predict rather well the forthcoming breeding success. Low success in cold springs mayreflect reduced availability of earthworms (main food for the nestlings) due to the prolonged ground frost period . Nowadays, the Oulu population is at its largest, and has doubled during the last two decades. It seems that this increase can not be explained by the local reproduction, but has more obviously been based on immigration from the south and/or east . More information, especially ringing records, is needed to confirm this .