In 1931, when A. W. Schorger published The Birds of Dane County, Wisconsin, the American redstart was one of the most common nesting birds in the county. It was, he wrote, "second only to yellow warblers in point of numbers," and he added that "every woodland contains at least one or more nesting pairs."
Yet today in southern Wisconsin (Madison is the seat of Dane County), the gaudy orange-and-black redstart is a very uncommon nesting species, rarely seen during the breeding season.
Habitat loss is the first cause that comes to mind to explain such a noticeable decline in numbers. The woodlands of Schorger's day are now subdivisions and shopping centers, unsuitable places for redstart nests. But it's not only that redstarts are absent from parking lots and suburban yards. They are also missing from most of the remaining woodlands in southern Wisconsin.
A wildlife ecologist at the University of Wisconsin named Stanley Temple has been studying, with various collaborators, the relationship beween habitat and bird populations in the woodlands of southern Wisconsin--as well as in prairie remnants in western Minnesota--by surveying nesting species.
He has discovered that many species of woodland birds are area-sensitive, that the size of a woodland is as important to these species as the kinds, and sizes, of trees and shrubs that grow in it.
Temple divided his woodland study areas into three size classes: 0-10 hectares, 11-100 hectares, and more than 100 hectares. A hectare is a metric unit of area, and as loyal Americans all we know about the metric system is the size of a two-liter soft-drink bottle, so I should explain that a hectare is equal to 2.47 acres. Of course, as city dwellers, we really don't have a very clear sense of what an acre is either. To put things in terms we all can understand, a standard Chicago city block, an eighth of a mile on a side, equals exactly 10 acres or roughly four hectares.
He then plotted the presence of songbirds and woodpeckers in the various size classes. In general terms, he found that a tenfold increase in woodland size--say from 10 to 100 hectares--nearly doubled the number of nesting species. And he also discovered a remarkable consistency in the species that lived in woodlots of various sizes.
For example, the really small woodlots--10 hectares or less--would almost certainly support downy woodpeckers, flickers, great crested flycatchers, house wrens, white-breasted nuthatches, robins, indigo buntings, and cowbirds.
The middle-sized woodlands-- 10 to 100 hectares--would add scarlet tanagers, yellow-throated vireos, and wood thrushes to the list. Only in the largest size class, woodlands covering a third of a square mile or more, would one be likely to find pileated woodpeckers; Acadian flycatchers; blue-gray gnatcatchers; chestnut-sided, cerulean, and hooded warblers; and, of course, American redstarts.
We can easily hazard a guess as to why pileated woodpeckers would live only in large woodlands. They are big birds, the size of crows, and presumably need a large amount of space to maintain themselves. But the rest of the big woods species are tiny songbirds, the biggest of them no larger than a house sparrow. They only need a couple of acres as a breeding territory; why does that small territory need to be surrounded by hundreds of acres of additional habitat?
To answer that, we need to look at something called the edge effect. William Beecher, former director of the Chicago Academy of Sciences, is generally credited with documenting the existence of an edge effect. Studying bird populations in the Chicago area, he discovered that birds tended to concentrate along the borders between different kinds of habitats. He found more nesting pairs where a woodland bordered a grassland, for example, than in the heart of either the woodland or the grassland.
Beecher's work had an appreciative audience in the world of wildlife management, because many game species--especially the white-tailed deer, North America's preeminent big-game animal--are essentially edge species. So wildlife managers could go into woodlands with chain saws and deliberately create openings, edges, to benefit the species they were concerned with.
But edges create problems for animals too. Denser populations mean more competition, both between and within individual species. More animals mean more predators to eat eggs and nestlings. And edges also mean more brown-headed cowbirds.
Brown-headed cowbirds are brood parasites. The females lay their eggs in the nests of other species, leaving their young to be raised by foster parents. Cowbirds are an edge species, living on the borders between grasslands and woodlands. The fragmentation of so much of our eastern forests over the past couple of centuries has produced thousands of miles of new edges and an enormous population increase and range expansion for the cowbird.
Other edge-bird species have developed defenses against cowbirds. Robins and catbirds, for example, can detect the alien egg in their clutch and remove it from the nest. Yellow warblers will often build a roof over a parasitized nest and build a new nest on top of it. Birds of the forest interior have fewer defenses, because they didn't have cowbirds to worry about until quite recently.
Edges are so important that Dr. Temple was able to chart their effects in woodlands of differing shapes. He was able to demonstrate that the critical ecological measurement in a woodland was the core area, the amount of space at least 100 meters from the edge. A 300-acre woodland that is more or less square will support many area-sensitive birds. A narrow strip of woods of the same overall size but with little or no core area will not.
An added problem for many of the forest-interior birds is that they are long-distance migrants, species that spend the winter in the tropics and come north only for a brief breeding season. Typically, birds like the redstart nest only once each year. If a raccoon eats their eggs or a cowbird parasitizes their nest, they have struck out for the year. Short-distance migrants--robins for example--may nest three or four times a summer. They can lose two or three clutches and still reproduce successfully.
Dr. Temple has also done some research on prairie remnants in western Minnesota that calls into question the applicability of the edge effect to bird populations. Working with one of his students, he did an intensive survey of the nests of five species of prairie birds: bobolinks, western meadowlarks, and clay-colored, savanna, and grasshopper sparrows.
The survey showed that the birds tended to nest in greater numbers near prairie-woodland edges. However, the edge nests were much less successful than open prairie nests. For these five species, the nesting site that was most likely to produce healthy fledglings was at least 45 meters from the nearest tree line.
Dr. Temple thinks that edges produce more insects, and that this food source, combined with the singing perches trees provide, may "seduce" the birds into nesting near the trees.
The conservation implications of Dr. Temple's work can be simply stated. When it comes to designing nature preserves, the bigger the better. Temple works with the Nature Conservancy in Wisconsin, and he told me of a change in their philosophy produced by research such as his. "They used to buy postage-stamp-sized bits of prairie because they were looking for pristine places. But these tiny places won't accommodate birds and mammals." Now the conservancy is working to create big holdings, using the pristine bits as nuclei and restoring the land around them.
The Illinois Nature Conservancy has undertaken a similar project at its Nashusa Grasslands Preserve near Dixon. In the Chicago area, Dr. Robert Betz pioneered large-scale prairie restoration at Fermilab, creating a natural environment hospitable enough to draw the endangered loggerhead shrike there this summer.
The Cook County Forest Preserve District owns many large blocks of uniform habitat, and this year the district began a joint project with the Nature Conservancy to restore prairie and savanna to 600 acres of the Poplar Creek Preserve. These restorations tend to look a little raggedy for the first few years, but for wildlife, Dr. Temple told me, "a lot of degraded habitat is better than a little bit of perfect habitat."