Differences in the gonadal cycles of adult and yearling blue grouse

Gonadal cycles of adult and yearling female and male blue grouse (Dendragapus obscurus fuliginosus) collected from Vancouver Island, British Columbia, are described and compared. Reproductive organs of yearling females develop later, produce eggs at a lower oviduct weight, and regress faster to a lower oviduct weight than do those of adults. Yearling males, although most do not breed, go through the entire gonadal cycle and produce viable sperm. Testes of yearlings develop later, have a lower volume, and regress earlier than those of adults. Yearlings of both sexes appear less sexually mature than adults, but delays in or lack of breeding could be partly a result of social interaction with resident adults.