Cooper Island, Alaska, Sept. 7, 2010 — The increasing distance between Cooper Island and the August pack ice has resulted in a range of problems for the black guillemots breeding on the island. The decreased access to their preferred prey of Arctic cod, which live under sea ice and in the cold waters adjacent to the ice, has caused decreased production of young. The northward range expansion of the horned puffin, partially in response to the retreat of summer pack ice, has greatly reduced guillemot nestling survival as puffins kill (but do not eat) the young of the guillemots during nest site prospecting. However, the most dramatic and, potentially, long lasting impact has been the death of guillemot chicks as polar bears scavenge on the island in August, since their preferred pack ice habitat has been greatly reduced.
Last year a bear circled around my cabin and made its way through the colony
turning over nest boxes.
In the past eight years as I have watched polar bears flip over, rip open and crush guillemot nest sites and eat nestlings, I have thought about what sort of structure might stand up to a large carnivore attempting to consume the chicks in nest cavities. There would be great benefits from a site that would protect chicks from bears, in that guillemot parents provisioning chicks could continue to serve as monitors of the availability of prey near the island. A bear-proof site would also allow some chicks to successfully fledge from Cooper Island in late August and have the potential of returning to the island as adults where they could join the breeding population and maintain, at least a minimal, guillemot colony in northern Alaska.
Bears on Cooper Island were rare until a few years ago.
Casual discussions last April about the possible designs for a bear-proof nest site motivated me to seriously consider a number of possibilities. Most of my ideas (and those provided by others) entailed a good deal of construction and materials that would be costly to get to the island or complicated ways of accessing nest contents. As I was readying my gear for the field season and took a number of my Pelican cases out of storage, so that I could ship fragile camera equipment to Alaska, I began to wonder if the cases could withstand mistreatment by the world’s largest terrestrial predator. I decided to find out by ordering 11 cases of various sizes and drilling a 3-inch hole in one end, allowing parents access to the interior. I also inserted a wooden baffle just inside the nest opening to create a more secure location for nest contents and to prevent chicks from falling out the opening when the sites are tipped.
Our Seattle Maine coon cat, Petey, a not-quite-so large terrestrial predator of another sort, inspects the results of the modified Pelican case.
When I arrived on the island this June I found a number of the wooden nest sites used last summer that had been destroyed by bears after my departure last August. Parent birds, showing the high site fidelity characteristic of the species and most seabirds, were sitting on and next to the sites that now lacked a nest cavity for breeding. I replaced a number of those damaged sites, and some others that have been deteriorating over the years, with Pelican cases to see if the birds found them attractive nesting locations.
A Black Guillemot sits in front of a destroyed nest site.
The response of the guillemots was immediate, but not really surprising given how readily guillemots have adopted any dark nesting cavity on the island over recent decades. Nine of the eleven Pelican cases had eggs laid in them with hatching success similar to adjacent nest sites. Most importantly the five nests that contained chicks when polar bears arrived on the island in mid-August all survived attempts by bears to get to the nestlings. No chicks died as a result of the bears’ flipping the nest sites, sticking their nose in the nest entrances or using their forepaws to try to crush the cases with their weight.
Black guillemots chicks make themselves at home in the Pelican cases.
The success of the Pelican Cases, both in their attractiveness to breeding guillemots and their ability to protect guillemot nestlings from polar bears, means that one of the major factors decreasing guillemot breeding success on the island can potentially be eliminated or minimized. By removing polar bear predation as a factor for some of the nest sites, parent guillemots feeding their protected chicks can continue to be monitors of prey availability in future years when fish populations will be responding to a change to warmer waters with less ice cover.
Bear standing on Pelican case.
After the 2008-2009 field seasons, when polar bears played a major role in decreasing the productivity of guillemots, it appeared that Cooper Island could not produce enough chicks to maintain the colony, assuming pack ice retreats from the island each August. Placement of more Pelican Cases in the nesting colony could extend the life of the colony by allowing some young to fledge and return as adults. Continued maintenance of a colony to monitor both natural and human impacts to the marine waters of northern Alaska will be important during a time when offshore oil and gas leasing and increasing ship traffic is expected. Guillemot nestlings would still be subject to the vagaries of prey decreases and puffin disturbance, but neither of those is as devastating to the colony as having a polar bear systematically move from nest to nest eating chicks. We will almost certainly put more Pelican Cases out next summer and will spend the next few months deciding how best to deploy them and examining design changes that could further enhance their use.
Next post will discuss the entire 2010 field season, which was unique in a number of ways.