Essay: Winter Birds, Part 1
One very cold but sunny Christmas day back in the early 1980s, I visited the park for a few hours of birding and to hopefully work off a few holiday calories. After a period of walking, I stumbled across an impressive display of bird numbers, enough to make me forget the cold: hundreds of noisy robins feeding on red holly berries; yellow-rumped warblers devouring poison ivy berries; dozens of kinglets scouring bare limbs and dried clumps of leaves for any arthropods they could find; chickadees and titmice searching for seeds and nuts; white-throated sparrows flushing from thickets and cane patches, winter wrens skulking around large logs and upturned root mounds, and woodpeckers excavating dead wood everywhere. Although the cold weather (I don’t think it got above freezing that day) contributed to the heavy bird activity, it is not all that uncommon to see abundant birdlife at Congaree in winter.
Unlike the nesting season, when birds are more evenly spread out in fixed territories, winter birds in Congaree and other eastern forests have a more clumped distribution, with what appears to be unoccupied space in-between. You may go for a long spell of walking in winter woods and never see the first bird, when all of a sudden the forest is full of them. Some of this sociability is due to a widely observed bird behavior known as the mixed-species foraging flock. In eastern forests these mixed winter flocks may consist of chickadees, titmice, downy woodpeckers, brown creepers, ruby-crowned and golden-crowned kinglets, nuthatches and sometimes a warbler or two like the black-and-white or pine. Ornithologists believe these mixed flocks serve two main purposes, one being better predator defense through security in numbers, the other, improved foraging opportunities.
Biologists have considered the breeding period as the raison d’etre of any animal’s existence, and ornithologists have traditionally focused their studies on the breeding season when nesting is underway and bird song is at a maximum. But more and more, biologists have come to realize that winter is a crucial period for bird (as well as other wildlife) survival and population regulation. In 1948 the National Audubon Society initiated the first Winter Bird Population Study (WBPS) as a companion to their Breeding Bird Census (BBC) study initiated a decade earlier. Like the BBC in summer, the WBPS is designed to increase our understanding, at a nationwide level, of winter bird distribution and abundance in various habitats. And like many avian field studies, the WBPS relies heavily on knowledgeable amateur ornithologists who have the time to visit an area eight to ten times over a several-month period, a process that can involve more than forty or fifty hours of effort. Unlike the BBC, which is a true census, the WBPS is a survey count since winter birds are much more mobile and transitory than summer birds. As I spent more and more time investigating Congaree’s winter bird population, I decided to initiate a formal Winter Bird Population Study on the same twenty-acre plot where I had earlier conducted my Breeding Bird Census. My first survey was done in the winter of 1986-87, followed up by another twelve years later in 1998-99. I also operated a bird-banding station on the same plot during the winter of 1991-92.
During the first survey I recorded a total of 28 bird species and found an average density of 166 birds per visit on the twenty-acre plot. The standard measure of comparison used for the WBPS is birds per square kilometer which in this case extrapolated to 2,062 birds (or 796 birds per square mile) per visit. This is one of the highest winter bird densities ever recorded in an Eastern forest. This very high density was related to several factors, chief among them being the large amount of “soft mast” available, notably the fruits of American holly, sugarberry, and poison ivy. The abundance of these fruits helped explain why three species were the dominant birds in the survey: American robin was by far the most abundant, accounting for a third of the total number (I sometimes think, with tongue in cheek, that every robin in Eastern North America spends the winter at Congaree), followed by the diminutive ruby-crowned kinglet (only half as abundant as the robin), and with the yellow-rumped warbler bringing up third place. These three species accounted for 63% of the total individuals found on the plot.
The fruits of holly and sugarberry are eagerly sought after by robins while the much smaller berries of poison ivy are attractive to kinglets, yellow-rumped warblers, and nearly thirty other species of birds that overwinter in the Congaree.
Other species recorded also rely heavily on mast: hermit thrushes, cardinals, cedar waxwings, goldfinches, white-throated sparrows, titmice, chickadees, and woodpeckers. In addition to feeding on poison ivy berries, several woodpecker species also feed on “hard mast” in the form of acorns from five different oak species found on the plot.
In addition to mast, a variety of animal foods, chiefly in the form of insects, spiders, and other invertebrates, are available to Congaree’s winter birds. Although they have to work harder to find it in winter, ruby-crowned and golden-crowned kinglets, Carolina wrens, winter wrens, brown creepers, Eastern phoebes, white-breasted nuthatches, and black-and-white warblers just to name a few, seem to be able to find enough animal protein to sustain themselves in the park, even on the coldest day. And on those days when animal protein is scarce, some of these species turn to berries and other plant material to supplement their diet.
Other factors that may help explain Congaree’s high winter bird numbers are abundant shelter and cover, especially in the form of hollow tree cavities and snags, some of the densest anywhere in the country, and the widespread availability of water for drinking (berry consumption makes birds thirsty).