National Geographic : 1993 Jun
access to songbird nests for skulking cowbirds and abundant predators like the black rat snake. "We don't have large parks and preserves in Illi nois," says Robinson. "Our research provides a warning for other parts of the country. This is what could happen to your birds." What has happened to the birds in the Shawnee National Forest of southernmost Illinois is sobering. Robinson found that 90 percent of the wood thrush nests are commandeered by cowbirds, and the output of thrush fledglings has fallen far below the level needed to sustain the local population. "The wood thrush is remarkably defenseless," Robinson tells me with a measure of awe. "They have blue eggs. Cowbird eggs are white with brown speckles. Yet they can't recognize a cowbird egg in their nest and chuck it out. It's per fectly normal for a wood thrush to sit on a clutch of five or six cowbird eggs with none of her own left." There is more bad news. In the Shawnee research area, 80 percent of the nests of scarlet tanagers, summer tanagers, and other canopy-nesting species such as the yellow-throated warbler contain cowbird eggs or young. A notable exception is the eastern wood-pewee, which is aggres sive in defending its lofty nest. LTHOUGH the Shawnee spreads from the Mississippi River on the west to the Ohio River on the east, it hardly fits the pop ular image of a national forest as a blanket of green. What Robinson calls his "laboratory of fragmenta tion" consists of hundreds of small woodland tracts splintered by private inholdings-farms, pastures, pig feedlots, orchards, backyards, all places where cowbirds congregate to feed. "The area is utterly saturated with cowbirds," Robinson laments, "and there is no forest here that is large enough for songbirds to escape them." He describes his project as "brute force science-inelegant, labor intensive. I hire skilled bird-watchers to find every nest, and undergradu ates to check them every two days. And then I get graduate students to study the species of greatest interest, like the wood thrush." Brute-force science, as I learned one morning following two students, also means crashing through grasping brambles and tangles of poison ivy in sopping heat, with a sharp eye for Silence of the Songbirds bird species over three decades. Now the urban oasis is being crowded by a new housingdevel opment. "They've just scalped a whole hill side," sighs Briggs.