Are Carrion Crows that congregate in spring roosts juveniles or adults?

Abstract

Carrion Crows Corvus corone often establish spring roosts even during the breeding season. To examine whether the individuals roosting in spring are juveniles with immature reproductive organs, we compared the body size, sexual organs, and plumage of 150 roosting and 35 breeding individuals collected in the Joetsu region, Niigata, Japan, between 20 April and 9 May 1992. In both sexes, roosting individuals had significantly lighter bodies, and shorter wings and tails than breeding individuals. Roosting males had significantly lighter and shorter testes, and a lower testes/body percentage than breeding males. All 15 breeding males and 17 of 70 roosting males (24.3%) had mature testes, while 53 of 70 roosting males (75.7%) had immature testes. Roosting females had significantly lighter ovaries and a lower ovary/body percentage than breeding females. All 20 breeding females and 19 of 80 roosting females (23.8%) had long, enlarged oviducts, while 61 roosting females (76.2%) had undeveloped oviducts. Based on plumage characteristics, we estimated that 112 of the 150 roosting crows (74.7%) were juveniles. The reproductive organs of 108 of the 112 juveniles (96.4%) and 6 of 38 adults (15.8%) were immature. These results suggest that the spring roost consisted mainly of juveniles with immature reproductive organs. Thirty-six roosting individuals had mature reproductive organs. We considered them to be either territorial adults that had attempted to breed near the roost but failed, or sexually mature non-territorial males.