For the Birds Radio Program: Cape May Warbler

Original Air Date: May 31, 1995

Today Laura Erickson talks about one of the most common migrants this spring. 3:19

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This spring migration has been one of the slowest, poorest showings I’ve ever witnessed, but the one wonderfully bright spot in an otherwise drab season has been the incredible numbers of Cape May Warblers coursing through. Cape Mays appeared early in May, concentrating in sapsucker trees where they could get running sap since so few insects were available yet. In every single warbler flock I’ve seen this spring, from central Wisconsin through northern Minnesota, there’ s been at least one Cape May and often several brightening the leafless trees. They’ve even been smacking into some windows, though the females are pretty hard to recognize.

Male Cape May Warblers are pretty little things, deep lemon-yellow beneath with intense black streaks that almost form the tiger stripes for which they take their scientific name. They have orange cheeks that almost look artificial, as if a giggling little warbler found some rouge at a pajama party and put it on a Cape May. Like several of their relatives, Cape Mays have a yellow rump and white tail spots, but the bright yellow underside is more eye-catching. Females have these two marks, but prefer to travel incognito, so their underside is pale, their streaks dull, and they lack the orange cheeks. Even in the hand, female Cape Mays are tricky except for people who know how to penetrate their disguise.

During the breeding season, Cape May Warblers are strictly birds of the northern coniferous forest, and specialize on feeding on spruce budworm. Their numbers used to fluctuate wildly as spruce budworm destroyed huge tracts of forest–the Cape Mays would feast for some years, and then when the forest crashed, they’ d disappear for years until it regenerated and their tasty arboreal disease bearers returned once again.

On migration and in their tropical wintering grounds, Cape Mays eat a much more varied diet, including fruits as well as insects. In the Bahamas, Cape Mays get aggressive and territorial when they find a blooming century plant–this plant offers a rich supply of both insects and nectar, and Cape Mays are simply not good at sharing. They may spend 90% of their time fighting off other warblers and only 10% of their time actually enjoying their meals. Some ornithologists believe there is sometimes actually an energy loss when Cape Mays defend a plant too vigorously, but perhaps for a feisty little species, aggressiveness is its own reward. I watched one Cape May this spring doing frequent territorial battle over several days against a Yellow¬≠ bellied Sapsucker. This was actually counter-productive, since the sapsucker was necessary to keep sap flowing in fresh holes, but it was fun watching a quarter-ounce mite taking on a woodpecker five times its weight and sometimes actually winning. I wish they had as good success doing battle with picture windows.