THE FLEDGING OF THE GUILLEMOT URIA AALGE WITH NOTES ON THE RAZORBILL ALCA TORDA

Summary The chicks of the cliff nesting auks go to sea when only one‐third the weight of the adults and before the flight‐feathers are fully grown. Sea‐going occurs in the evening (except in the high Arctic) and the advantages and disadvantages of this timing are discussed. In the Guillemot, the behaviour which occurs on the ledges before the first‐flight consists of ritualized intention movements, serving to synchronize the behaviour of parent and chick so that when the latter jumps from the ledge the parent follows it. The “water‐call” is made by all chicks when they arrive on the water (many of them make it on the ledges just before flighting). It serves to direct the adult to the chick. A parent and chick swimming out to sea may receive mock attacks from adults unaccompanied by chicks on the water below the cliffs. These birds direct their activities mainly to chicks whose parents have not joined them on the water, but do not adopt them and they ultimately perish. High winds and rough seas depress fledging activity. Low temperatures and rain seem to have no comparable effect. The number of chicks whose parents do not join them on the water is proportionately higher on rough, windy evenings. The fledging of the Razorbill follows a very similar course to that of the Guillemot. Predation, mainly by gulls, of auk chicks at the time of fledging may be high, though fewer chicks perish as a result of predation alone than as a result of desertion by their parents.