MOULT IN THE SHAG PHALACROCORAX ARISTOTELIS, AND THE ONTOGENY OF THE "STAFFELMAUSER"

There are three main types of plumage in the Shag, the juvenal which is brown, the post-juvenal which is dark brown and the nuptial which is black-green. The replacement of these plumages and the ontogeny of the Staffelmauser was studied in a sample of 566 Shags mainly from northeast England. The replacement of the juvenal primaries starts at the age of eight months with the loss of the innermost primary and continues outwards at a rate of about one primary per 17 days. Another cycle starts when the first reaches the 8th primary but both cycles soon pause for the winter. Successive cycles are established annually in August or September in this way, so that cycles which take longer than one year to complete result eventually in a Staffelmauser in which each feather is replaced once annually. Most individual Shags in a breeding population will retain feathers from two or three cycles, but a large sample of breeding adults of the same age will retain the feathers of four cycles. The winter pause is considered an adaptation to the increased probability of adverse conditions at this time. The system described for the ontogeny of the Staffelmauser is applicable to those other sea-birds which have been studied in detail, and is probably advantageous since it combines slow moult during the difficult early years with a highly efficient moult during the breeding years, especially where moult and breeding occur together.