For the Birds: Bobolinks

A suite of birds collectively called the grassland birds nest in New England. This group of birds includes the Upland Sandpiper, Savannah Sparrow, Vesper Sparrow, Grasshopper Sparrow, Eastern Meadowlark and Bobolink. Wildlife biologists are concerned about this group of birds because their populations are declining in New England, including our Maine populations.

Grassland habitats are rather limited in New England and have invited development into farms or towns because tree cutting is not required adapt the line for human uses. Fortunately, grassland birds are adaptable and often nest in hayfields or blueberry barrens. However, many hayfields in Maine are no longer maintained and are reverting to forest. We have much more forested land now in Maine than we did in 1900. Thus, the decline of their required habitat helps explain the decline of these grassland birds.

Today’s column is the first of three on the grassland birds of Maine. We’ll start with one of the most beloved birds of Maine, the Bobolink.

With their beautiful plumage (mostly black with white on the back and wings, and a yellow patch on the back of the head and nape), male Bobolinks are strikingly handsome birds. Some waggish authors have described them as having tuxedos on backwards.

The female is nondescript, brown on the uppersides with streaking and yellowish underneath. This plumage makes them nearly impossible to see in a grassy field.

The male’s song is equally distinctive: a long, bubbly series of metallic notes. A student of mine, an obvious Star Wars fan, said the song sounds like the robot R2D2! The Bobolink song has inspired poetry. The American poet, William Cullen Bryant, based his poem “Robert of Lincoln” on the song of the Bobolink.

The breeding range of Bobolinks extends across the northern tier of the United States from Maine to Washington as well as the southern portion of the Canadian provinces.

Bobolinks winter south of the equator in South America, The wintering habitat is the grasslands known as the pampas in southwestern Brazil and Argentina. Each year, a Bobolink makes a round-trip of 12,000 miles. We know of one banded female that lived to be nine years old. Presuming she made the trip every year, she traveled the equivalent of circling the globe at the equator four times.

Orientation and navigation have been fairly well studied for Bobolinks. Researchers have found that the primary cue for navigation in Bobolinks is the earth’s magnetic field.

Bobolinks arrive in central Maine by the second week of May. Males are conspicuous as they stake out territories and await the arrival of the females. When a female passes overhead, a male will perform an aerial song display to attract the female’s attention. If the female lands, the male will go through a series of courtship behaviors to court the female. These behaviors include a low circle flight on stiffly held wings and an abrupt drop to a low perch or the ground. When the male drops, he holds his wings in a shallow V (like the gliding silhouette of a Turkey Vulture) and gives a few buzzy notes, dangling his legs on the descent. When he touches the ground, he may hold his wings in the shallow V for several seconds.

A female is slow to choose a mate. A male may have to perform this elaborate courtship sequence five times a minute for a half-hour or more. A pair bond is formed when the female leads the male on a long chase, flying for up to six minutes.

Bobolinks are examples of polygynous birds; a male may have more than one female on his territory. The frequency of polygyny seems to be related to habitat quality. In high quality habitats where food is easier to find, a male frequently has two or more mates. In lower quality habitats, single females are typically found with the males. In Wisconsin, over half a population of males had at least two mates and one male had four mates. On the other hand in New York, fewer than 15% of the males had more than one mate.

Nests are always made on the ground in dense vegetation. The nest is usually located at the base of herbaceous, non-grassy vegetation like goldenrods or clover. The female chooses the nest location and builds the nest in one or two days.

A female typically lays five eggs although as many as seven can be laid by some females. Both the female and the male feed the young. Even a male with multiple mates participates in feeding although most assistance may be given to his primary mate.

[First published on June 14, 2008]