Wild Side : Christmas Bird Count reflects Island bird behavior
Photo by Susan Safford
The 51st Vineyard Christmas Bird Count (CBC) took place on Sunday, January 2, with about two dozen Islanders and guests participating. Our local count is an element of a continent-wide network of more than 2,000 counts coordinated by the National Audubon Society. Collectively, these annual surveys mark a traditional high point of a birder's year and provide one of the most powerful methods for monitoring bird distribution and populations.
Variables, notably local weather conditions and the number, skill, and level of effort by participants, limit the kinds of conclusions one can draw from any particular count. Sunday's Vineyard event was complicated by persistent fog, which limits visibility and suppresses bird activity. Also, many regular off-Island counters were unable to participate, having opted for the Nantucket count the preceding day. Between the weather and the thinned ranks, the 51st Vineyard count produced 117 species (several fewer than average) and 15,228 individual birds (among the lowest totals in recent memory).
Through the years and across the large number of counts conducted each year, however, such variations tend to even out, and statistical analysis of CBC data has emerged as a powerful tool for the study of birds on the continental scale. Even at the local level, CBC results cast sometimes surprising light on the world of birds and birders.
The half-century's worth of Vineyard CBCs has seen dramatic changes in Vineyard habitats, with abandoned pasture succeeding into woodland and woodland increasingly fragmented by residential development. Skunks and raccoons – potent predators of ground-nesting birds – have proliferated. On a larger scale, some ecological threats (like DDT) have abated, while others (like deforestation in northern woodlands) have accelerated. All these factors affect bird numbers and distribution. Meanwhile, birding has changed, with steadily growing numbers of participants, steadily increasing knowledge of bird identification and distribution, and steadily improving optics. Hints of all these changes appear in the count's database.
Most obvious, and perhaps most significant, is the appearance or disappearance of entire species from the count's results. Sometimes these changes reflect alterations in local ecology: for example, the northern bobwhite quail, once a regular on CBC checklists, has virtually dropped of our count's radar. Local birders put the blame mainly on two factors: the establishment of skunks on the Island, which dramatically increased predation of the eggs and nestlings of ground-nesting birds, and the gradual succession of shrubby, semi-open habitat preferred by quail into denser woodland.
More positively, the peregrine falcon has become a reliable bird on Vineyard CBCs, as this species has bounced back from decimation by insecticide poisoning (the prime culprit, DDT, was banned in the U.S. in 1972). Cooper's hawks have likewise rebounded; the species, once spotty here in winter, is now regular and fairly common, and indeed has resumed nesting on the Vineyard after decades of absence. Great horned owls, tufted titmice, red-bellied woodpeckers, and several other birds have either colonized the Island or increased markedly in abundance during the period covered by our CBC.
One may need to look harder to explain the increased regularity with which our count records some other birds. Black-legged kittiwake, for example, has grown far more regular. This unusual gull nests relatively undisturbed in the far north, and its numbers are believed to be healthy; it is distinctive enough to be readily identifiable if you see it well. But its habits tend to keep it far from shore, and most kittiwakes identified by shore-based observers are seen through a telescope at "artillery range." Improvements and optics and more widespread use of powerful spotting scopes probably explain much of the increasing regularity of this species on the Vineyard count.
The count's data may even hold hints of improving birder skills and knowledge, both of which have advanced steadily as more and better bird books have become available and as the experience of generations of birders and scientists has accumulated. I've always been troubled, for example, by two records from early in the Vineyard count's history of Swainson's thrush, now known to vacate the Northeast (indeed, the entire U.S.) quite thoroughly during fall migration. Moreover, the species is never numerous on the Vineyard, its migratory routing tending to keep it west of us for the most part.
Theoretically possible on a CBC? Sure. But twice? Swainson's thrushes are easily confused with hermit thrushes, which occur reliably here in early winter. At the risk of offending someone I may know (I have no idea who contributed those old records), I suspect that the CBC records of Swainson's thrush may reflect errors in identification. At the very least, modern knowledge of Swainson's thrush movements and hermit thrush variability would guarantee that a Swainson's thrush on a CBC today would receive very careful review, and would be carefully documented (since the observer would likely know the full significance of the report).
Neither birds nor birders persist unchanging over time. Species come and go, skills develop, old knowledge gives way to new. And repeated events like the Christmas Bird Count offer an enjoyable way to make these changes visible.