Over most of its range the Black Guillemot is a nearshore seabird, occupying coastal waters during both the breeding and nonbreeding seasons, as do other members of the genus Cepphus. Pelagic or open ocean waters can offer abundant prey resources, but these options are often distant, patchy and unpredictable.
The nearshore typically offers seabirds a smaller but more reliable source prey base consisting of forage fish and benthic fauna from the ocean floor such as crustaceans or mussels.
The Arctic Ocean has extensive sea ice cover in the nearshore for the majority of the year; this presents a number of challenges to a nearshore species. Our work on the Cooper Island Black Guillemots has revealed a number of ways in which the species has met these challenges.
The current view from my cabin window illustrates one of the major problems guillemots face in the Arctic. Sea ice extends from the north beach of the island to the horizon and covers Elson Lagoon to the south. The only water available to the guillemots is a brackish pond in the center of the colony that provides no prey but is deep enough to provide sanctuary if the guillemots need to dive when pursued by an owl or falcon — regular visitors to the island.
While guillemots arrived on the island almost a month ago and egg laying is now complete, until recently the closest predictable open water where guillemots could find prey was approximately 20 miles away, off Point Barrow where winds and currents shift the sea ice creating an area of open water. This opening is called a lead. The Cooper Island guillemots stage there in April and May before coming to the island. (Editor’s note: Leads are important for wildlife, because they allow for access to oxygen in the case of seals and walruses and prey in the case of seabirds; you can read more from the National Snow and Ice Data Center here.)
This distance between the Cooper Island guillemots’ nesting colony and access to their prey resources during egg laying and incubation is in sharp contrast to what guillemots breeding in subarctic or temperate waters find at their breeding colonies. These birds occupy waters directly adjacent to colonies well before egg laying and foraging areas may even be within sight of nests. The birds breeding on Cooper Island (and likely all colonies of Mandt’s Black Guillemot Cepphus grylle mandti, the high Arctic subspecies of Black Guillemot) have responded to this spatial disconnect by having a well-defined periodicity in their daily colony attendance. Every day, the parent not incubating eggs and all nonbreeding individuals vacate the colony from approximately noon until midnight. The birds fly individually or in small groups to open water where they can feed for almost half the day before returning to the colony just as the “midnight sun” is at its lowest point in the sky.
MODIS image from July 9; snow and ice have blue/cyan color, while clouds will be lighter gray/white. Image Credit: David Douglass/USGS
While it seems individual birds could fly offshore to open water to feed anytime during the day, there are a number of possible reasons the observed colony-wide pattern of attendance and abandonment developed. For the half of the day when the guillemots are absent – from approximately noon to midnight – there is no evidence that Cooper Island supports a colony of Black Guillemots. It appears to be just a barren sandbar that happens to inexplicably have 200 scattered black plastic cases along with a small cabin surrounded by a bear fence. Falcons, Snowy Owls, and other predators moving along the barrier islands would have little reason to be attracted to this place.
The timing of the birds’ departure and return may be related to changes in air temperature and its effect on ice formation. On nights when the air temperature is below freezing (as it was last night), I have frequently observed the formation of new ice on the surface of the few spaces of open water in the sea ice directly adjacent to shore. This newly formed ice melts in the morning as air temperatures rise. Nocturnal formation of new ice in the waters adjacent to the pack ice reduces the amount of open water available for guillemots to dive for the prey.
This temporary daily reduction in foraging area could be expected to have been pronounced during the Last Glacial Maximum when air temperatures were lower and the ancestors of the Cooper Island guillemots occupied an Arctic refugium. The current pattern of colony attendance for the Cooper Island colony – foraging during the warmest part of the day and attending the breeding colony at night – could have evolved as a way of maximizing the amount of open water available for guillemots.
The large expanse of shorefast ice north of the island this year is persisting later than expected compared to recent years. While the nearshore ice may now be forcing the Cooper Island guillemots to fly further in search of prey, it could benefit the colony later this summer should ice remain in the nearshore close to the colony. In recent years a lack of sea ice when the guillemots are feeding young resulted in increased nestling mortality as higher sea surface temperatures reduced the availability of Arctic Cod, the guillemots’ preferred prey.
Should this year’s nearshore ice break up slowly over the next month, Arctic Cod could remain in the guillemots’ foraging range and allow increased chick growth and fledging success. The latter is urgently needed for the colony to reduce its current population decline. First eggs will be hatching in about two weeks and our daily weighing of nestlings and prey observations should demonstrate how much this year’s persistent sea ice has affected the guillemots’ nearshore environment.