Laura Erickson's For the Birds

One of the lovely but little known treasures of the north woods is the Golden-winged Warbler. This pretty but elusive little bird lives in aspen forests, but its voice is quiet, and it usually sticks to the treetops, so is one of the less-often seen of our woodland birds. It’s also one of the least-studied of all warblers, perhaps because it’s in that awkward middle ground–not threatened enough to merit a lot of research funding, but too hard to find to be an easy subject for researchers.

Last year, the Cornell Laboratory of Ornithology initiated a Golden-winged Warbler Atlas Project, trying to determine exactly where the species is found, and what its abundance is. I volunteered to help, and so I spent a few days early this summer driving around to likely spots to listen and watch for them. I could recognize suitable habitat a mile off, because Golden-winged Warblers are found in aspen forests, and this year those forests were defoliated by forest tent caterpillars. And, interestingly, in just about every block of defoliated woods I heard at least one singing Golden-winged Warbler.

This set me to thinking about these fascinating insects and the animals that have to live with them during army worm invasion years. The oldest Golden-winged Warbler known lived 7 years 11 months. So the vast majority of them only encounter an invasion cycle once in a lifetime. During the first year, it’s likely that Golden-wings don’ t have a clue how to eat these hairy critters. But during the course of that year and the following couple of years, they might develop techniques for eating the caterpillars. Whether or not they actually do eat them, the birds I was finding on my survey areas in Douglas and Bayfield Counties in Wisconsin were definitely common in the areas where the army worms were. It got me to thinking, and so I looked up my Breeding Bird Survey records for Golden-wings, and noticed that the only years I’ve recorded them at all were army worm years. So now I suddenly want to take some time to look at Breeding Bird Survey data from throughout the northeastern states and eastern Canada to see if Golden-winged Warbler numbers change with changes in caterpillar populations.

The connection between Golden-winged Warblers and army worms is only one mystery about the species that has yet to be unraveled. One of the biggest problems facing the species is actually from competition and hybridization with the closely related Blue­ winged Warbler. This species, limited to the southeastern US in the 1700s and 1800s, has been extending its range northward. Wherever it turns up, it hybridizes with Golden­ wings, though at a smaller rate than if the birds were actually the same species. But as Blue-winged Warblers move north, they somehow swamp out the Golden-wings, which are steadily disappearing. Up here where Blue-winged Warblers have not arrived, we have a good population of Golden-wings, but as our little planet heats up, the northward movements of Blue-wings may be speeding up, and one day we’ll have an army worm invasion with no Golden-winged Warblers to feast on them. And hardly anyone will notice. And that in itself is a very sad thing.