Cooper Island, Alaska, Aug. 2, 2009 — My focus on the birds breeding (or trying to breed) on Cooper Island runs the risk of making it seem like the island and surrounding waters are important to a relatively limited avifauna. In reality, the island is on one of the major migratory pathways for birds breeding on the tundra of Alaska’s North Slope and the western Canadian Arctic. In July and August, after breeding is complete, large numbers of waterfowl, shorebirds and seabirds move to the near shore Beaufort Sea and then westward to Point Barrow before heading south through the Chukchi Sea to wintering grounds, for many south of the equator.
Some of these species — like the strings of eiders that are regularly seen from the island — pass by without stopping to feed or rest in the near shore. For a number of species, however, Point Barrow and the barrier islands to the east (including Cooper Island) are important feeding areas as they prepare to undertake major migrations. Adults that just completed breeding need to restore their fat while just fledged young need to both build up reserves and develop foraging skills. Oceanographic conditions in the waters north of Point Barrow and Cooper Island increase the abundance and availability of marine invertebrates in this region, including the crustaceans commonly referred to as “krill”. Krill are an important food for bowhead whales, who sieve it with their baleen plates. The abundance of krill in this region is one of the reasons Barrow is a whaling community and also why there are so many whale bones on Cooper Island. It is also the reason why I now can’t walk down the beach without having hundreds of gulls circling overhead.
While in past years I have regularly seen flocks of birds feeding on krill just offshore or on the beach, this year the feeding flocks have been exceptionally large and persistent. Starting about three weeks ago, flocks of Sabine’s gulls, Arctic terns, glaucous gulls and black-legged kittiwakes began to feed around the island, with groups of many hundreds of birds roosting on the beach in compact flocks between feeding bouts. They were later joined by phalaropes, sandpipers, jaegers and some waterfowl that walked the beaches feeding on krill washed up by the waves. The number of birds, and the noise they generate, has been truly impressive, with at least five thousand birds wheeling over and near the island as they finish feeding on one swarm of krill and move on to another. The efficiency of the flocks feeding on the beached krill is amazing as they clear thousands of the half-inch long invertebrates within a few hours.
Growth rates of guillemot chicks have been high for the past two weeks, and arctic cod has been the primary prey — surprising findings given the distance to the pack ice and my past observations of how ice retreat affects cod availability and chick growth. It turns out that the cod, and the guillemots that feed on them, are benefiting from the abundance of krill. Examination of fish dropped in the colony by parents has found cod stomachs full of krill. Additionally, parent birds are feeding on krill directly as evidenced by the splashes of pink feces around nests. The krill are benefitting parent guillemots directly, by providing them with prey and indirectly, by increasing the densities of fish for them to feed their young.
The next few weeks are critical for the guillemot nestlings as they approach the weight of adults and have large energy needs to grow, maintain weight and push out the flight feathers that will allow them to fly out to sea at 35 days of age. Should krill become less abundant as this summer progresses it will be interesting to see how fish composition and chick growth is affected. While I don’t have any way of measuring krill abundance near the island, I do have a few thousand gulls and terns that will be letting me know if there are still krill in the area.
It is observations like these that keep me coming back to Cooper Island year after year. If as ice retreats annual krill densities increase, Arctic cod or even some other fish species, could maintain high densities near the island and allow successful breeding in the absence of ice. This field season could be a preview of how the guillemot colony might persist through summers with no late summer ice — and an important reminder that one needs to remember that species will be adapting to — and not just be impacted by — climate change.Share This Post