Cooper Island, Alaska, Aug. 18, 2010 — The transitions in the summer season on Cooper Island tend to be step changes rather than gradual trends for both me and the Black Guillemots I am studying. This reflects, in part, the rapid shifts that occur in the Arctic when it goes from “winter” to “summer” in a matter of days as the snow melts from the island and then, about a month later, when the nearshore waters go from ice-covered to open water with one good windstorm. The breeding season for the Black Guillemots, which has three distinct periods, is marked by similar rapid transitions.
One of the lucky chicks as I explain below.
The “pre-egg” period after the birds arrive in early June is characterized by birds reforming or forming pair bonds, competition for nest-sites and mates and the female producing the two eggs that are the typical clutch size for the species. This is followed by the “incubation” period, from mid-June to late July, both parents take turns attending the egg, which needs to be incubated almost constantly, but during which each parent is still free to forage for prey for about 12 hours a day.
Black Guillemot eggs
The hatching of the chicks is the event that most drastically changes the season, since once the chicks hatch they must be fed enough prey over the next five weeks to increase their weight from 35 grams to 350 grams. It is during this “nestling” period, which extends until early September, that the abundance and distribution of prey near the island is most important to the breeding guillemots. For 35 days, parent birds need to find suitable prey as close as possible to the island, to minimize their energy expenditure, while also selecting the highest quality prey (in terms of energy content) to return to their young. Since I track the daily changes in chick weight and prey type it is also an extremely busy time for me.
It is during this period of chick rearing that most major environmental changes have taken place over recent decades, and that is reflected in the high annual variability in nestling survival and the near complete nesting failures like the one that occurred last year. In the 1970s and 1980s when sea ice was typically a few hundred meters offshore (or at least no further than the horizon) parent birds had short commutes to an abundant and high energy food source of Arctic cod, their preferred prey. By contrast this year, as in the majority of summers of the past decade, ice retreat from the island was extreme — last reports were that the main pack ice was at least one hundred miles to the north. This creates a major problem for Black Guillemots, one of the few pack ice obligates in the Arctic.
An adult black guillemot now needs to fly longer distances to find food for its young due to the retreating pack ice.
In past years the absence of ice within the parent’s foraging distance meant birds had to turn to lower quality prey such as sculpin, which do not have the high energy content of cod and are also frequently rejected by chicks, who can choke on their large spiny head. Surprisingly, this year parent birds were able to keep finding Arctic cod after the major ice retreat (indicating the water temperature north of the island is not changing rapidly) but were clearly finding fewer of them. When parent guillemots are unable to find sufficient prey to raise two young, the older (‘alpha”) chick in the nest monopolizes any prey returned to the nest and the younger (‘beta”) chick loses weight and starves if prey abundance does not change. Although many alpha and “singleton” chicks are doing quite well, so far this year beta chicks have died in approximately 75 percent of the nests that had two chicks hatch. Those that remain are much thinner than their “alpha” siblings.
The chicks find the spiny head of the sculpin hard to take.
Watching the nesting failures during the nestling period provides one of the more dramatic examples of the changes that are occurring in the Arctic. When ice was close to the island, as it was for most of the first 25 years of the study (1975-1990), 75 percent of all chicks I was weighing would survive to leave the island and about a third of them would return in three to four years to breed. There was a good chance I would see any given nestling as an adult in a few years and then be able to follow its breeding history over a decade or more. Now, with most chicks failing to survive the nestling period, daily nest checks result in my guessing how many more days a given chick might live and, if things are as bad as last year, any chicks will survive to leave the island.
The August-September nestling period on Cooper Island has always been one with more fog and clouds than earlier in the season, as both decreasing ice cover and increasing air temperature increases the moisture in the air. While the bright sunlight and 24 hours of daylight in June and early July can elevate one’s emotional state, the gray skies and decreasing daylight later in the season have the opposite effect. This change of mood is increased by returning to the cabin every day with a collection of dead chicks found during nest checks. In periods such as this it is good to be able to waken early in the morning and have one’s optimism renewed by an inspiring sunrise like the one I saw at 4 a.m. earlier this month.
This sunset helped to raise my spirits.
The fate of the chicks remaining on the island will play out in the next two weeks and depends much on the distribution and abundance or guillemot prey and predators during that time. My next post will discuss how we hope that an experimental nest site may allow guillemots to avoid the sort of polar bear predation that accounted for approximately 50 percent of the nesting mortality last year.