Cooper Island, Alaska, July 21, 2010 — In addition to documenting the timing and success of the Black Guillemots breeding on Cooper Island I always make an assessment of the other avifauna attempting to raise their young here. The changes that have occurred in some of those populations have been almost as striking as the findings for Black Guillemots. Brant, a coastal goose, formed a major colony in the past two decades while Arctic Terns went from being the most abundant species on the island three decades ago to having just a few scattered pairs today.
An Arctic Tern takes to the skies.An Arctic Tern takes to the skies.
With the guillemots finishing up egg-laying last week, I was able to spend some time assessing the other bird populations and was reminded of the advantages of studying a cavity-nesting species, especially one breeding almost exclusively in cavities I created. I can walk the circuit of the approximately 200 structures with cavities that have been utilized by guillemots in the past while also checking the few pallets and rubble piles that have cavities but for some reason have never attracted guillemots. Having done that over the last month, I now can say with almost complete certainty that there are 146 pairs of guillemots breeding on the island in 2010. This is well below the over 200 pairs breeding here in the late 1980s, but about the same size found in the past five years.
Determining or even estimating the numbers of species not breeding in cavities is much less straightforward. The waterfowl, Arctic Terns and Sabine’s Gulls that lay their eggs in relatively exposed locations are much harder to census, in much the same way that censusing homeless humans is harder than censusing people who live in houses or apartments. And since most of the surface-nesting (vs. cavity-nesting) birds attempt to have their nests avoid detection by predators, with nest placement, egg and chick coloration and behavior all maximizing concealment, it is similar to censusing homeless populations trying to avoid detection. While requiring more effort than sampling the guillemots, searching for tern and gull nests in driftwood lines does provide more sport, and also the thrill of discovery that makes Easter egg hunts engaging — a similarity increased by the tern chicks looking remarkably like a buff-colored version of fluffy Easter chicks.
I began my assessment of the island last Friday by walking the driftwood lines at the west end of the island and found that the small tern colony there is now even smaller (< 5 pairs) and that chick hatching was just beginning, indicating like most things on the island this summer, terns were having a late breeding season. The decline of the Cooper Island Arctic Tern population from 75 pairs in the late 1970s to the current approximately 10 pairs is dramatic, but, since terns show less fidelity to breeding sites than most seabirds, it iis not that surprising nor something that can be easily linked to environmental changes near the island.
While terns frequently reveal the location of their nests by increasing the intensity of their calls and attacks (passing swipes at the intruder’s head) as one approaches, the opposite is true for Long-tailed Ducks (formerly Oldsquaw), which breed in the same driftwood lines. They have a completely different strategy — the female remains motionless until one is directly over the nest, when she then explodes into the air. Observing a nest with an incubating female requires systematic scanning of likely driftwood aggregations — and then realizing that one of the piles of driftwood is staring back at you.
Long-tailed Ducks (formerly Oldsquaw)
As I was weighing one of the tern chicks I felt a few drops of rain and was reminded that one of the big benefits for cavity-nesting species on Cooper Island is that they have a place to get out of the wind, rain and cold. Surface-nesting species have to not only provide warmth to the newly hatched chick (that will not be able to regulate its body temperature for a few days), but also act to prevent rain from soaking the nestling’s down during the period immediately following hatching. Concern for any chicks or eggs I might expose to the rain had me immediately return to the cabin, thinking I could resume censusing later in the day. I had no idea that those first few drops would be the start of the largest precipitation event I have seen on the island. Heavy rain fell for the rest of Friday (totaling 0.6 inches) and though there was a break on Saturday, another rain storm started on Sunday. By the time the second storm ended early Monday afternoon, over an inch of rain had fallen in a 24-hour period. To understand the magnitude of these storms (nearly 2 inches of rain in four days), one needs to remember that the Barrow area is in a “polar desert” with annual precipitation estimates of less than ten inches. Accurate estimates of annual precipitation are confounded by having much of the winter snow “fall” horizontally during wind events which precludes use of typical snowfall gauges.
The best way to appreciate a cabin with a temperature of 48 degrees F. is to go out into a rain storm with winds of almost 20 mph and temperatures hovering just above freezing.
During both storms, as I listened for hours to rain striking the roof of my cabin, I was reminded why I relate to the Black Guillemots. Like me, the attending parents and their eggs and chicks avoid the elements by being inside a box. While a cabin with an interior temperature of 48 degrees F. may seem like a chilly place to spend a summer afternoon, my Davis weather station let me know that it was considerably warmer than the 23 degree F. wind chill temperature outside. It did make me concerned for the just hatched tern chicks, however. Their survival during the storms was dependent on the attentiveness of the brooding parents, but knowing how dedicated tern parents can be, based on the hundreds of times they have hit me in the head, I have little doubt the chicks survived.