One of the very few advantages of aging is that as we do, we gain a larger perspective on events in the natural world. When I first began performing systematic surveys of Connecticut birds nearly 40 years ago, the Yellow-bellied Sapsucker was a decidedly uncommon and local species of the highest elevations of northwestern Connecticut. I found it in places like open swamps at the summit of Canaan Mt. Observers from a century previous reported its occurrence in much the same manner. It was primaily a boreal species that straggled south to enter the coolest locations of southern New England.
Then, something happened- something that made this northern bird expand dramatically at its southern range limit in opposition to the larger (and likely global warming-related) trend of southern species expanding north.
From its few residences in Connecticut in the early 1970s, it began expanding through the northwest hills. During the initial year of the Forest Bird Survey of Southern New England (2001), it also appeared in the northeast CT town of Stafford. Much as it had been in northwestern CT, it was associated with open swamps. Initial population estimates there were 126. Upon resurvey of the region in 2004, the number grew to an estimated 502. However, even this number paled in comparison to the 21,587 estimated for northwestern CT in 2007. Indeed, the sapsucker had become the commonest woodpecker of that region. Surveys of southwestern CT also showed a substantial population of 891. Furthermore, the species had expanded habitat use to virtually every kind of mature forest present.
So, what happened? What could have released this boreal species to expand into mature upland forests and inhabit them at high densities?
Within their principal range, sapusckers are characteristically birds of earlier successional, open deciduous forests, often those near water and with dead trees. This characterization matches well with the swamp habitats in which birds first appeared in both northeastern and northwestern CT, and it suggests that this type of environment represents core habitat. But what of this new, huge population now inhabiting mature upland forest? How did that fit?
One can erect a number of alternate hypotheses to investigate- ones that would make interesting thesis projects for enterprising students. One strong possibility is that southern New England forests have become increasingly interspersed with open swamps due to growth in beaver populations over the period of sapsucker expansion. The landscape as a whole may, therefore, now be much more attractive to the species even though much of the forest has mature trees not generally favored by sapsuckers.
Another possibility is evolution at the range limit. Although range limit populations tend to have reduced genetic variability, some key genetic change might have occurred that permitted our local population to expand into a new habitat. There are strong genetic arguments that can be made against this being very likely, but still, it's an intriguing thought.
Still a third hypothesis is that population pressure from the heart of the sapsuckers' range is driving peripheral populations into marginal habitats. This sort of thing has been documented for other bird species. In this case the possibility can be explored by examining long term population trends recorded by the Breeding Bird Survey. However, when one does this, population declines appear to have occurred across maritime Canada during the past 40 years, which argues against this hypothesis. Moreover, the sheer abundance of sapsuckers in northwest (and now northeast) CT forests suggests that the species is doing very nicely there.
So, does anyone else have any speculations about what's going on here? This is a prime example of the dynamic rather than static nature of natural systems.