PREDATION ON EGGS AND NESTLINGS OF COMMON MURRES (URIA-AALGE) AT BLUFF, ALASKA

We studied predation on the eggs and nestlings of Common Murres at Blu ff, Alaska, in 1957, 1988, and 1991. Common Ravens and Glaucous Culls were the primary predators of eggs. Nestling losses were low and few c ould be attributed to predation. A raven usually took a murre egg afte r pulling the incubating adult from the egg and off the cliff; we neve r observed ravens taking abandoned eggs. The intensity of raven predat ion was highest on earliest-laid eggs. Virtually all raven attacks wer e on murres that had no active breeding neighbor; at the time of the a ttach. Non-incubating adult murres typically flushed as a raven approa ched, leaving sites where a raven could then land and attack an incuba ting adult. Raven predation accounted for up to about half of egg loss es on study plots. Raven predation acts evolutionarily as directional selection against early asynchronous egg-laying. However the intensity of this selection is reduced by high probability of relaying for eggs lost earliest in the season and the lack of a marked seasonal decline in breeding success of murres at this colony. Except for a few rare o ccurrences, Glaucous Gulls took only unattended or abandoned eggs, oft en following human disturbances or rock slides. Gulls preyed predomina tely on eggs and only rarely on nestlings. Most gull predation on eggs occurred early in the breeding season of murres, when human disturban ce was highest and murres seemed most likely to temporarily abandon eg gs. Gull predation and accidental dislodgment of eggs by flushing adul ts likely accounted for locally high losses of eggs on days when human hunters discharged firearms and shot adults on or near the cliffs.