This cold wet spring might be slowing down the northward migration of the wood warblers. They usually arrive coincident with bud break on the trees, but I heard no singing or calling when I went for a walk in the woods last weekend.
The decline in the number of wood warblers is a popular, if sad, topic among bird watchers and bird scientists. The initial blame was put on the Latin American countries where most species spend the winter. Like orioles and tanagers, wood warblers are thought of as Neotropical species that have expanded north into the temperate zone and return to their tropical homeland in the winter.
Tropical forests have been and still are being cut down on a large scale. A prominent example of this problem is the cerulean warbler, which winters in Colombia. Large tracts of forest in Colombia have been felled in order to plant coffee, which is more productive per plant with full sun. The advent of “shade-grown coffee” is in part due to the conservation efforts to preserve wintering populations of the cerulean and other warblers. While the yields are lower in the shade, the coffee is reputedly better and the plants live far longer, so it wasn’t just for the birds; the farmers benefit in the long run too.
The wood warblers constitute a large, diverse family, the Parulidae. There were used by evolutionary biologist Robert MacArthur as a an excellent example of a group that divides up habitat in order increase species diversity, which is called resource partitioning. Macarthurs looked at five species — Cape May, yellow-rumped (myrtle and Audubon’s), black-throated green, Blackburnian, and bay-breasted — that live in the spruce forests of Maine. According to a summation at the Stanford University website, the Cape May, for instance, stayed mostly toward the outside on the top, the Bay-breasted fed mostly around the middle interior, while the myrtle moved from part to part more than either of the other two. The Blackburnian and black-throated green forage in the same area as the Cape May, but range further down the tree and the black-throated green feeds more often by hovering in front of the branches. All of the warblers are insect eaters, but their feeding strategies will lead them to different insect species or different instars (developmental stages) of the same insect. The result is that five species of warbler can occupy, feed, and raise young in the same spruce forest.
MacArthur noted that the yellow-rumped warbler had the broadest feeding habits and ranged over the largest part of the tree. In other words it had the broadest niche, the most generalized ecology. Not coincidentally, it is also the most common of the five species, or at least the most commonly seen. While most warblers are insectivores, the yellow-rumped switches to eating fruit and seeds in the fall and winter. According to allaboutbirds.org, it is uniquely suited to eating bayberries and the fruit of the wax myrtle, which are abundant in coastal regions. This willingness and ability to switch diets allows the yellow-rumped warbler to winter further north than any other warbler species. It is found along the coast north to southern New England in the winters; I have counted them during the Christmas Bird Count on Martha’s Vineyard. They will even come to feeders in the winter and eat sunflower seeds, suet, and peanut butter.
Most warbler species are known only to birdwatchers because you have to go looking for them in places other than your backyard. Furthermore, they tend to be tiny and fast-moving, flitting in and out among the leaves in their ceaseless quest for a meal. Several species live high in the forest canopy, so that you are constantly leaning far backward and scanning the branches for movement. This leads to a form of pain unique to birdwatchers called “warbler neck.”
Two species, the yellowthroat and the yellow warbler, depart from this generality. The yellowthroat (not to be confused with the very different and rare yellow-throated warbler) lives almost anywhere that is a little wet and marshy with some shrubs sticking up above the forbs (perennial non-grass vegetation). But like the yellow-rumped warbler, the yellowthroat ranges more widely, readily living in “dry upland pine forests, palmetto thickets, drainage ditches, hedgerows, orchards, fields, burned-over oak forests, shrub-covered hillsides, river edges, and disturbed sites,” according to allaboutbirds.org. So ubiquitous is the yellowthroat that it was one of the first New World species to be cataloged by Linnaeus in 1766, based on a specimen collected in Maryland.
Unlike the yellow-rumped warbler, the yellowthroat sticks to eating insects. It therefore withdraws from much of its North American breeding range (it ranges up into Newfoundland in the east and the Yukon in the west), but is able to be a permanent resident in the coastal southeastern states and on the California coast.
The yellow warbler is even more wide-ranging, reaching Labrador and Alaska. Like the yellowthroat it prefers wet areas, but it tends to stay up in the trees and the tops of shrubs, while the yellowthroat skulks in the lower vegetation. The yellow warbler like “disturbed or regrowing habitats” where the trees are not large. This is why the average person is more likely to see this species, as suburban and small town landscapes are filled with areas that have been cleared for development and then let go. Even in lots that are developed, the edges may be neglected and grow up into suitable yellow warbler habitat.
All three of these warbler species — yellow-rumped, yellowthroat, and yellow — are among the most abundant, but their numbers are all declining. Yellow warbler numbers have decreased by 25 percent since 1966, the yellowthroat by 38 percent, and the yellow-rumped remaining relatively stable, but declining slightly. The logging of tropical forests is not the only culprit here. During migration these birds frequently collide with radio towers, tall buildings, but because they are insectivores, they are also part of food chain that makes them vulnerable to poisoning by insecticides and also going hungry because their prey has been decimated.
The two wetland species, the yellowthroat and the yellow, have suffered as a result of habitat loss, the widespread draining of wetlands. Environmental law permits a developer to fill in a wetland as long as they recreate a wetland of equal size. Unfortunately, a shopping mall may cover a large and varied tract of marsh and swamp and be legally replaced with a lake or a pond surrounded by a monoculture of cattails or phragmites, which simply does not support the same number of warblers.
Well, I just walked my dog outside the office here in downtown Claremont and what did I here welling up from the overgrown shores of the Sugar River? The sweet-sweet-sweet-ti-ti-to-soo song of the yellow warbler.