We went into the field this week to speak with Carol Foss, Member of the International Rusty Blackbird Working Group and NH Coordinator of the Rusty Blackbird Spring Migration Blitz.
Rusty Blackbird populations have fallen over the last century: by between 80 and 90-percent. Last fall the working group decided to make careful study of the spring migration, and coordinated hundreds of volunteer scientists along the migration route to track the birds.
Rustys are difficult to pick out from a group of black birds, which may include Red-winged Blackbirds, Cowbirds, and Grackles, among others. Though possibly named for their rust-colored winter plumage, by migration time they are more a true black color. From a distance or in the right light the many black birds look very similar to each other. Rusty's giveaway is a bright yellow eye.
But armed with binoculars and our ears, we followed Foss into a flooded corn field on the the banks of the Merrimack River in Penacook. We started scouring the trees and corn stalk stubble with our binoculars, scanning for the tell-tale eye. There was a lot of bird chatter and a lot of perched dark shapes to sift through but we eventually heard the unmistakable call of the Rusty. It sounds like a tape being run backwards.
Foss and the working group are hoping that this year's "blitz" will help "figure out what caused the decline of Rusty populations; and figure out what conservation measures can be taken to help sustain existing populations and encourage them to survive and grow the population."
Experts don't fully understand what has been causing the population decline. Foss says, "There are many hypotheses but no smoking guns." In the Rusty's southern wintering grounds, several species of oaks were occupying soils that were ideal for farming. When those forests were cleared, it meant the acorns that were an important part of the bird's diet disappeared. In the 1970s a lot of DDT was used in Canada and northern New England (Rusty's breeding grounds) to check a major Spruce budworm outbreak.
There's only a narrow window of time to observe the Rustys in transit, they only stop over long enough to fuel up for the next leg of migration. In fact, by now most have landed in their breeding grounds and begun laying eggs. It's not easy to tease out the many factors that may have contributed to the decline, but Foss and her colleagues are hoping more data will lead to stronger hypotheses.