Supernormal Delay in Hatching, Embryo Cold Tolerance and Egg Fostering in the Black Guillemot Cepphus grylle

We observed an apparent suspension of embryonic growth in the Black Guillemot Cepphus grylle, a cavity-nesting alcid typically having a two-egg clutch and incubation period of approximately 28 d. Limited suspension of embryonic growth is common for species with a multi-egg clutch, but our observations are apparently the first to document an extended ( greater than or equal to 14 d) suspension prior to the onset of incubation in the wild. We found no previous record of the fostering and successful incubation of the egg of the previous owners by birds usurping a nest cavity. Observations were made on Cooper Island, in northern Alaska, where man-made structures on a sand and gravel substrate provide nest cavities. At the time of clutch completion a previously non-breeding pair displaced the original nest owners and pushed the two eggs to the edge of the nest depression. Approximately 7 d later the usurping pair moved the displaced eggs back into the nest depression. They added their own egg to the clutch 14 d later and began incubating all three eggs. The initial egg in the original clutch pipped after 28 d of incubation (45 d after deposition), but died while pipping. The other hatched after 33 d of incubation (47 d after deposition); indicating embryonic growth had been suspended for greater than or equal to 14 days. During the period of growth suspension, daily temperatures averaged 2.9 degree C (range 0.1-7.4 degree C) with daily minima < 0 degree C on 12 of the 18 days, demonstrating an extended cold tolerance in the species.