For the Birds Radio Program: Warblers and Edge
In June I got a thoughtful letter from a Hayward listener. He had heard a “For the Birds” segment about Nashville Warblers and writes, “You said that most warblers are interior species needing large, undisturbed blocks of [forest]… As a very amateur birder, I feel somewhat presumptuous disputing what all the experts say is true, but my observations suggest that… [this is] actually the opposite of the truth.”
This listener is right that few northern warblers require undisturbed closed canopy hardwood forest. The Black-throated Blue Warbler lives in mature maple-basswood forests, and the Ovenbird can be found in just about any woods, but most northern warbler species require at least a coniferous component. And many, perhaps all, warblers adjust to edges quite well. Although virtually all (except Yellow Warblers and Common Yellowthroats) stay within forested habitats, many warblers nest immediately adjacent to clearcuts and other disturbances. Warblers require smaller territories than most other birds and, as far as I can tell, select a nesting territory based on the immediate patch of land rather than the habitat surrounding it.
All a warbler wants is a little piece of land in a specific habitat. I find Nashvilles in aspen stands. Cape May and Bay-breasted Warblers thrive in spruce budworm outbreak areas. Blackburnians and Black-throated Greens prefer forests with both deciduous and coniferous trees. Magnolias require a large conifer component. Pine Warblers nest only in mature pines. Northern Parulas nest in usnea lichen from dying balsams. Redstarts require a dense understory. Each species has precise nesting and feeding niches that help keep it from competing with the others, which is why a small forest can harbor so many different warbler species.
Nevertheless, I keep insisting that warblers require large, undisturbed blocks of forest habitat. Is this inconsistent? No. Disturbance is bad for them because the ones that nest on edges are far, far more vulnerable to opportunistic omnivores and predators that gravitate to edges. We think of the Ruffed Grouse as an edge species, but it really spends the majority of its life in the interior of an aspen forest. Gordon Gullion, one of the world’s authorities on grouse, once said that edges are not where grouse live—edges are where they die.
Human hunters focus on edges, and that’s also where foxes, coyotes, wolves, jays, crows, and many hawks have success-small birds are simply more conspicuous on an edge than in the interior.
Even more insidiously for warblers, cowbirds parasitize nests ONLY near edges. When a cowbird mother tosses out one warbler egg before laying her own, she automatically reduces the warbler’s nesting success by 20-25%, and because the cowbird nestling demands so much food, in years of poor weather or limited food, the cowbird is often the only baby in a nest to survive. So between predation and parasitism, edges end up being warbler population sinks, with edge numbers augmented from the surplus produced in interior or more contiguous habitat.
I have spent a lot of time in and near the Boundary Waters, where no logging takes place. It is the richest warbler habitat I’ve ever seen, and the excess population produced there is probably the source of many of the warblers that we see in less pristine areas where cowbirds and predators take a higher toll than can be replaced. So managing forests to provide for birds while allowing enough logging to satisfy demand is extraordinarily complex. Warblers are functionally illiterate and inarticulate, so I have taken it upon myself to speak for them. I hope the people who truly love the north woods, both for its natural wonders and for the practical ways we humans use it, can figure out intelligent and generous ways to manage it for humans and warblers both.