When I bought my first Peterson and my first pair of binoculars, I approached birding rather gingerly. I mainly concentrated on the easy stuff: kingbirds, and great blue herons and wood ducks, birds of such striking particularity that it was almost impossible to mistake them for anything else. When I paged through the illustrations in the field guide, I used to hurry past the tougher groups, the shorebirds, gulls, hawks, sparrows, and wood warblers. I didn't even want to think about how hard it would be to learn them all.
In those days, it was even harder than it is now, because we didn't have the lavishly illustrated field guides that have been published in the 80s. Peterson's first edition, published in 1934, offered only four pages in color. The edition I had was more generously endowed than that, but Peterson was still obliged to crowd 15 species of wood warblers onto a single four- by seven-inch page.
The yellow-rumped warbler was my first breakthrough, the first species I could separate from the tiny, crowded pictures and actually identify. I saw my first one about the middle of April, if memory serves. I was spending a weekend on some land that friends of mine owned in southwestern Wisconsin. Out for a walk one morning, I heard a sharp chipping noise from somewhere up in the treetops. The leaves were not out yet, so it didn't take me long to find the source of the call.
It was a small bird--about sparrow size--moving actively through the branches. Sometimes it flew up and hung in the air for an instant on fluttering wings. I decided that the time had come to identify a warbler, so I pulled my field guide from my pocket and started looking--consulting the book and studying the bird alternately until I settled on the correct species.
It was a gorgeous bird, a spring male in the brightness of its breeding plumage. Its crown was lemon yellow; a black mask covered its eyes. In front of each wing was another patch of yellow. Its breast and sides were streaked with black against a white background, and another patch of yellow marked its rump. The colors and patterns all matched the picture, and the description of the bird's call note was "a loud check," which pretty much described the noise I was hearing it make. I had identified a wood warbler. The scales were falling from my eyes. Of course, I didn't identify it as a yellow-rumped warbler, because in those days it was known as a myrtle warbler. The name change came a few years later.
The yellow-rumped warbler is a good place to start getting to know the whole tribe of wood warblers. It is easily the most common and widespread North American species of the family, and it is also with us for more of the year than any other. The usual pattern for the family is to head for the tropics in September. A few species winter in Florida or along the Gulf Coast, but only the yellow-rumped warbler regularly winters as far north as Illinois.
A bird that nests successfully from Alaska to Massachusetts and winters from Illinois to Guatemala figures to be a species with a highly adaptable nature. Consider the food habits of the yellow-rumped warbler. Wood warblers are generally rather stenophagous, as the biologists say. That is, they have relatively narrow food preferences. They eat small insects and arachnids, and almost nothing else. This narrow preference makes it impossible for them to live anywhere that is too cold for insects.
Yellow-rumped warblers are euryphagous. They eat insects in summer. But when the insects disappear in fall, the birds switch to berries and seeds--and get along quite well. That old common name myrtle warbler refers to their fondness for the berries of the wax myrtle, and they are also quite fond of the fruit of the closely related bayberry. Neither of these plants grows around Chicago, so in this area the birds switch to the berries of juniper, red cedar, dogwood, viburnum, honeysuckle, and poison ivy. If no fruit is around, they will switch to grass seed or sunflower seed or goldenrod seed. They come to bird feeders for seeds or bread crumbs. They search Florida orange groves for fruit that has fallen from the trees and broken open. They have even been seen eating doughnuts.
They are also tolerant of a variety of environments. They prefer nesting in conifers, but they will happily nest at the edges of dense woods or in scattered trees in an open field. They nest in the lower branches of tall trees or the upper branches of small trees, and in some places in Alaska they even nest on the ground.
The name change, from myrtle to yellow-rumped warbler, was decreed by the American Ornithologists Union in the early 70s. Before then the name myrtle warbler was applied to the eastern and northern birds, and the name Audubon's warbler was applied to a form that nested in the western mountains from southern Canada to northern Mexico. The only obvious difference between the two is that the myrtle has a white throat and the Audubon's has a yellow throat. After field studies had shown that the two types interbred freely at the border between their ranges, the AOU decided to lump them into one species. The new name actually revives an old name that goes back to Audubon's time.
The western form is as flexible as the eastern. The birds nest in the mountains as high as 10,000 feet. Some of them go south for the winter, but many make a very short migration flight down into the nearest warm valley.
In the Midwest, yellow-rumped warblers breed from northern Wisconsin and upper Michigan north to Hudson Bay. The earliest migrants begin leaving the nesting ground in August, and we start seeing them in large numbers in September. The flocks continue passing through in October and early November, long after other wood warblers have vanished for the winter.
We have a few strange man-made environments where several warbler species can be found in winter The North Shore Channel is one of these. The water, which is fresh from the sewage treatment plant, stays open all winter, supporting an insect population that feeds the warblers. Outside of anomalous places like the North Shore Channel, any warbler you see between now and next April will be a yellow-rump.
In spring, flocks of returning yellow-rumps will be the first warblers of the season. My first bird was easy to see because it had arrived before the leaves opened, so it couldn't hide in the treetops. The earliest arrivals are the males, which can be identified by the brightness and crisp definition of the plumage. The females, which show a duller and slightly blurred version of the male plumage, start arriving a couple of weeks after the first males. This males-first situation is common to many species. The males get to the nesting ground and set to work defining their territories. The females arrive later and select the male--or the territory--that pleases them most.
These birds usually build their nests on level limbs some distance out from the trunks of nesting trees. They seem to be falling victim to the cowbird with increasing frequency. This brood parasite lays its eggs in the nests of other species. Birds such as robins and yellow warblers, which have a long history of coexistence with this parasite, have evolved means of fighting it. But yellow-rumped warblers are birds of northern woodlands and seldom encountered cowbirds before our alteration of the landscape created a huge expansion of the parasite's range.
Yellow-rumped warblers are also threatened by the wholesale destruction of the forests of Mexico, and Central America, just as all Neotropical species are. However, the yellow-rump has more defenses against that destruction than many other species. Its enormous winter range helps a lot. The ornithologist Alexander Skutch tells of seeing a flock in New York City in January while he was waiting for a ship to take him to Guatemala. When he got off the ship in the hot, sultry tropics, he saw yet more yellow-rumped warblers.
Their general flexibility helps too. In the Yucatan, I've seen them deep in the forest, and I've also seen them happily gleaning insects from the leaves of decorative palms planted along the streets of Cancun. If we do not abate our assault on the natural world, the bird that was my first warbler may someday earn the unwanted distinction of being the world's last warbler.