Essay: Peterson’s Challenge

For much of June and July, 2015, I made a detour from Congaree and visited Canada’s magnificent martime provinces. I will pick back up with Congaree Journal on July 27th but in the interim will post a couple of essays that I hope you will find interesting. Here is one I’m calling “Peterson’s Challenge.”

In March of 1937 Roger Tory Peterson, destined to become the most famous ornithological figure of the twentieth century, and Lester Walsh, an official with the National Audubon Society, made a ten-day float trip down nearly the entire length of the Santee River. They were searching for some of the last ivory-billed woodpeckers left on earth, ivory-bills that had recently been discovered in the depths of the lower Santee River Swamp. Although they failed to find the great woodpecker on this trip (Peterson did manage to see it five years later in the famous Singer Tract in northeastern Louisiana), both men came away mightily impressed with the extensive old-growth hardwood forests of the Santee bottoms.

Peterson’s experiences in these two large southern river swamps, both faced with imminent destruction (by 1942 much of Santee’s old-growth forest was under thirty-five feet of water from the Santee-Cooper Hydroelectric Project, and all of the Singer Tract was cut over by the end of World War II), led him to urge that “someone should start taking bird censuses in an original river-swamp habitat before it is all gone.”  Peterson suspected that bird populations would be greater in original forest than mature second-growth because the “trees of virgin growth were so large and widely spaced that they formed their own edge.”

Peterson’s appeal fell on deaf ears, not due to a lack of interest, but because in the ensuing decades after World War II nearly all of the South’s old-growth bottomland forests was logged, except for a few parcels here and there. Congaree remained the largest, but was largely unknown except to a few local hunters and fishermen. Fortunately, much of it was saved in a last-minute effort by conservationists and protected in 1976 by the National Park Service. In 1980, two years after being hired as a wildlife biologist by the South Carolina Department of Natural Resources, I was fortunate in being able to take Peterson up on his challenge of forty years ago. I initiated what is known as a Breeding Bird Census (or BBC, a program of the National Audubon Society) on a representative twenty-acre plot of Congaree’s “original river-swamp habitat.”

My Congaree bird census plot in 1980 shows a very dense canopy with little understory growth.

I conducted the first year’s census in the late spring and early summer of 1980, followed up by three more censuses in 1981, 1982, and 1984. For the four censuses I recorded a cumulative total of twenty-four bird species, fifty percent of which were Neotropical migrants, those birds that nest in North America and overwinter in the tropics. These species also comprised 65% of the total breeding territories.

It was probably no surprise that I found by far the most abundant breeding bird at Congaree to be the diminutive Northern parula warbler, one of the most characteristic birds of southern bottomlands. For the four censuses I found, on average, nearly twenty parula territories per twenty acres, an incredible density of one pair per acre. In fact it appeared at times that the parulas were using one large old-growth hardwood tree as their territory. These large trees are usually draped in Spanish moss, a key ingredient for the warbler, which nests and forages in clumps of moss. And as it turns out, the density of these large hardwoods was about one per acre.

The diminutive Norther parula warbler, by far Congaree’s most abundant breeding bird.

Coming in at a distant second with an average of six territories was the ubiquitous red-eyed vireo, which Peterson considered as the most common bird of Eastern deciduous forests.

Tied for third, with three to four territories per twenty acres, were Northern cardinal, Carolina wren, Acadian flycatcher, and Eastern tufted titmouse. Three species, yellow-billed cuckoo, Carolina chickadee, and yellow-throated warbler, had an average of two to three territories, while eight species averaged between one and two territories per twenty acres, they being pileated woodpecker, hairy woodpecker, downy woodpecker, red-bellied woodpecker, white-breasted nuthatch, great crested flycatcher, white-eyed vireo, and prothonotary warbler.

Seven species, characterized as having large territories or being sparsely distributed, were found to have only partial territories on the plot: wood duck, barred owl, red-shouldered hawk, blue gray gnatcatcher, yellow-throated vireo, and hooded warbler.

The total density of nesting birds, an important figure for comparative purposes, was, as expected, high, ranging from 728 to 932 pairs (or territories) per square kilometer (250 acres) with a four-year average of 800 pairs per 250 acres, or an average of 3.2 nesting bird pairs per acre.

It was significant that my old-growth Congaree bird plot did not have the “edge” conditions – “early-successional” habitats of thickets and scrubby patches of cover – associated with large, widely-spaced trees, as postulated by Peterson. This was borne out by the fact that a characteristic edge (or thicket) species of bottomlands, the white-eyed vireo, had only one to two territories per twenty acres, and the hooded warbler, another bird of dense cover, had only a partial territory.——-to be continued-———-