SEASONAL DECLINES IN DURATION OF INCUBATION AND CHICK PERIODS OF COMMON MURRES AT BLUFF, ALASKA IN 1987-1991

The Common Murre (Uria aalge) produces a single-egg clutch, and the fe male may replace the egg if it is lost. Even at breeding colonies in h ighly seasonal environments, egg laying may span several weeks. For ex ample, the mean range of egg-laying dates was 41 days in a five-year p eriod at Bluff, a coastal colony in western Alaska. In each of those f ive years, incubation periods of eggs were negatively related to layin g date (i.e. incubation periods of eggs laid later in season were shor ter). This is the first documentation of a seasonal decline in the inc ubation period of this species, and one of the few for birds in genera l. Analyses of seasonal changes in weather patterns indicated that the seasonal shortening of the incubation period cannot be explained by c hanges in environmental conditions. The duration of the chick period a lso declined seasonally in all five years, similar to results reported elsewhere. Overall, duration of the period between egg laying and fle dging averaged about six days less for the latest breeding pairs than for the earliest breeding pairs. There is no evidence of a seasonal de cline in hatching or fledging success at this colony. Consequently, th ese results indicate that murres breeding later in the season at Bluff reduce the duration of both the incubation and chick periods well in advance of seasonal deterioration of environmental conditions. Chicks develop more rapidly than embryos in eggs, and chicks at sea likely gr ow and develop faster than chicks at the colony. As the time remaining for completion of development decreases through the summer, quicker h atching of eggs and sea-going of chicks will be advantageous. Formatio n of sea ice throughout the region in late fall likely is the primary time constraint selecting for accelerated developmental patterns of eg gs and chicks of late-breeding murres at Bluff. In highly seasonal env ironments, seasonal time constraints may influence most or all phases of avian breeding cycles.