Cooper Island, Alaska, July 12, 2009 — Many bird species are thought to be “Arctic birds” because they migrate to the Arctic every spring, utilize the region’s resources for breeding, and then depart for more southern wintering areas in late summer and early fall, spending the majority of the year well south of the Arctic Circle. These species have a “summer romance” with the Arctic, enjoying the benefits of constant daylight and the abundant food resources provided by the region’s insect and fish populations. A much smaller number of bird species are truly “married” to the Arctic, remaining after breeding is done to cope with months of darkness, cold and ubiquitous snow and ice, apparently aware that in any committed relationship all cannot be 24 hours of sunlight.
Cooper Island is home to two species that represent extremes of these relationships with the Arctic. The Arctic tern, which used to be the most abundant species on the island, spends the non-breeding season in the extreme Southern Hemisphere in waters adjacent to the Antarctic. Its annual migration to and from the Arctic takes them many thousands of miles from Cooper Island –— and the southern location of their wintering areas allows them to experience more daylight annually than any other species.
On the other hand, the black guillemot subspecies that breeds in the western Arctic, including Cooper Island, is one of the few truly Arctic seabirds, wintering in the Arctic Ocean and into the Bering Sea only as far south as the pack ice extends in mid winter. Some individuals breeding on Cooper Island may well winter as little as 25 miles from the island in the ice pack north and west of Point Barrow, where winds and currents keep some areas of water open throughout the winter and guillemots are regularly seen by hunters on the winter sea ice. While the migratory strategies of the two species could not be more different, for two to three months every summer they both breed on Cooper Island and are dependent on marine prey adjacent to the island during incubation and while feeding nestlings. That both Arctic terns and black guillemots have suffered major decreases in population size and breeding success in recent years is an indication of how prey availability and abundance are changing in the marine waters of arctic Alaska.
The Arctic tern population on Cooper Island used to be the largest on any of the barrier islands in northern Alaska, with 150 breeding birds in small sub-colonies spread over the two-mile long island. This year there are only 20 breeding birds, restricted to one small patch of driftwood. The black guillemot population numbered slightly over 400 breeding birds in the late 1980s, with an additional 200 non-breeders. This year there are only 270 breeding birds with less than 50 non-breeders.
Because all guillemot young and most guillemot adults are banded we know more about the causes of their decline. Immigration of birds that fledged from other colonies, probably primarily Russian islands, played a major role in maintaining the Cooper Island colony. In recent years there have been far fewer immigrants. This appears to indicate that those colonies are also experiencing low productivity. With no immigration at current levels of breeding success, neither the terns nor guillemots can persist on Cooper Island.
One species of seabird breeding on Cooper Island, the horned puffin, has experienced an increase in numbers with 6-8 breeding birds in recent years. This doesn’t seem like much until one considers that the species was rare in Arctic Alaska until the 1970s and first bred in the region in 1986 when a pair occupied a nest box on Cooper Island —kicking out the guillemots that previously owned it. That this sub-Arctic species, that has no dependence on sea ice, is now breeding this far north is another sign of how warming of both the ocean and atmosphere is affecting the Cooper Island avifauna.
All of these species are now incubating eggs and I am anticipating the start of hatching in about a week. It is then, when parent birds have to continuously find fish for chicks, that the guillemots and terns experience troubles if prey abundance is low.