Bye, bye blackbirds?

By BILL CHAISSON
Of a Feather
It is time to start looking for the blackbirds. While most migrants begin appearing at this latitude in April and May, the blackbirds arrive… or pass through, in late February and March. Last year at this time I focused on the most common blackbird: the red-winged. The other abundant resident blackbird is the common grackle. We can save them for a later column when I can watch them enjoy the repeated apparent self-strangulation required for them to “sing.”

There are also two blackbirds that only pass through. The breeding grounds of the rusty blackbird are mostly in Canada, although they do extend down into northern New England and New York. The Brewer’s blackbird is a western species that occasionally passes through the east during migration and even more occasionally stays to breed.

In the way of many blackbirds (and icterids, in general) these species show pronounced sexual dimorphism. It is only the males that are black, while the females of both species are a drab brownish gray. During the spring migration the male Brewer’s and rusty are difficult to distinguish from one another. Both are glossy black birds with pale yellow eyes, but Sibley describes the rusty as shorter tailed, thinner billed, and shorter legged than the Brewer’s. Most of the female Brewer’s have dark eyes, which separates them from female rusty blackbirds. But some female Brewer’s may have yellow eyes.

Blackbirds molt once a year in the late summer, which is when the male rusty blackbird acquires the plumage that gives it its name. From August to January the birds are covered with scattered patches of rust like a North Country car. The rust is concentrated on the head and body, but does extend to the tips of some wing feathers and the top of the tail. During the winter the rusty edges wear off and the birds assume their black breeding plumage in time for spring migration. If you are looking at a mixed flock of blackbirds, the Brewer’s, in addition to being longer legged and longer tailed, will be glossier than the rusty.

The rusty blackbird population has been in steep decline for decades; since the mid 1960s its numbers have declined by almost 90%. Scientists are somewhat baffled as to the reasons. On the face of it the bird has an ecology that would seem to protect it. In both the breeding and winter seasons it is a bird of remote swamps, fens and beaver ponds. And it has a generalized diet of insects in the summer and seeds in the winter. However, the International Rusty Blackbird Working Group has identified some potential pressure points. About 80% of the population migrates to the southeastern United States, where the extent of wooded wetland habitat has been greatly diminished by development. In addition, the higher latitudes are more strongly affected by recent climate change; wet areas are now more prone to drying out in the summer, and seasonal cycles are increasingly erratic. Finally, for reasons that are not understood, the bodies of many of these birds have very high mercury levels.

The winter range of the Brewer’s blackbird extends eastward to the Florida panhandle, Georgia, and South Carolina. Its breeding range extends eastward to Michigan and the portion of Ontario north of Lake Huron. Unlike the rusty blackbird, it is common in towns and in open habitats across the West. Like most blackbirds it is often found nesting in loose colonies and gathers in large flocks after breeding is finished.

Like the rusty blackbird, Brewer’s blackbird numbers have declined since the mid ‘60s, but “only” by about 70%. They are viewed as agricultural pests and have been persistently subject to poisoning, shooting, and traps. Although they do feed on grain, they have a flexible diet and quickly change their focus during outbreaks of insect pests in agricultural fields.

The “Atlas of Breeding Birds in New Hampshire” shows rusty blackbirds nesting in scattered locations in Grafton, Carroll and Coös counties. The birds arrive in southern New Hampshire in late March and move into their breeding grounds soon after. The male’s song is a creaky “kush-a-lee” or “ksh-lay.” Unlike other blackbirds, it does not nest in colonies.

The atlas records a gradual increase in breeding in the state through the 20th century, with nests steadily further south, reaching below the White Mountains after the 1950s. However, in the second half of the century, numbers declined and they disappeared from some locations where they had formerly bred.

Brewer’s blackbirds spread eastward through the 20th Century, first nesting east of the Great Plains after 1914. Their range then expanded at an average of 11 miles per year for four decades. The allaboutbirds.org distribution map shows dots in southern Ontario indicating isolated breeding east of contiguous range. The map in Sibley’s guide shows many green dots all through the eastern states. Concurrently, the wintering range also expanded eastward.

Where the Brewer’s encountered common grackles, they displaced them from farmland and other rural settings, but the grackles prevent them from breeding in eastern suburban habitats. As grackles have spread westward, this competitive exclusion has been repeated in reverse, with grackles displacing Brewer’s blackbirds in western suburbs.

Bill Chaisson, who has been a birdwatcher since age 11, is a former editor of the Eagle Times. He now works for the Town of Wilmot and lives in Sutton.