For bobolinks, savannah sparrows, and other birds that make their nests in hayfields, a delayed harvest can spell the difference between life and death.
Vermont farmers still grow hay, of course, including 227,000 acres in the Lake Champlain Valley's picturesque dairy country. However, many of those fields have become "ecological traps" (Strong's term). Meadows that once were mowed with horses in mid-summer after the young birds had fledged are now cut early and often by farmers trying to maximize their hay harvest. First nests and any attempts to re-nest are obliterated, yet bobolinks try to use those deadly fields year after year.
I know that fate awaits my favorite bobolink fields, and it would not be a pleasant thing for a bird person to witness. "There are lots of ways to die in a hayfield," Strong said. "The nests are crushed, chopped, and eventually baled and fed to cows with the rest of the forage. Predators--gulls and crows by day, mammals at night--will eat any eggs or nestlings that might survive the mowing. The saddest thing," he continued, "is seeing the adult birds hopping up and down the windrows, trying to find nestlings buried under the cut hay. The parents may hear the young birds begging for food, but they can't get to them." Indeed, bobolink nestling mortality in mowed hayfields was estimated at 94 percent in a Cornell University study, including a heavy toll of young bobolinks that had already fledged. In contrast, Strong reports that on unmowed hayfields near Lake Champlain, where predation can be high, 46 percent of bobolink nests successfully fledged young.
Bobolinks are amazing migrants. Their round-trip to and from wintering grounds on South America's pampas--a vast swath of grasslands, marshes, and rice and sorghum fields that extends from Bolivia to northern Argentina--covers some 12,000 miles. That's one of the longest songbird migrations in the Western Hemisphere. They reach New England in mid-May, when grasses, clovers, and other herbs have grown tall enough to provide them with cover. But there's no time for weary travelers to rest. The emergent hayfields soon bristle with activity, which the noted ornithologist Olin Sewall Pettingill (1907-2001), who studied bobolinks in Michigan over many summers, vividly recounted for the January 1983 issue of Audubon.
"The males in their striking livery of black-and-white showed up first, brimming with vitality," Pettingill related. "Each at once claimed a segment of a field by repeatedly rising and circling over it, wings down-bent and quivering, while voicing a rollicking medley of tinkles and bubbles. Then each one settled down on a plant stalk as if to pinpoint his territorial claim, and there spread his wings and lifted his buffy nape and white shoulder feathers in display.
"The females, yellowish-brown with dark streaks that simulated dead grass stems, arrived two or three days later," Pettingill continued. "There was a frenzy of sound and action, females beseeching the attention of males with buzzy calls and taking to the air with them in close pursuit. Then they would circle back and drop into the grass, where mating was consummated. In the ensuing two weeks the females built their nests--simple cups of grasses on the ground, well shaded and obscured--and laid clutches of five to six eggs." By then it was early June. The chicks, Pettingill documented, hatched in 12 or 13 days, became strong fliers by mid-July, and by the first of August both juveniles and adults had left the natal field. They would soon embark on a prolonged fall migration in which they would not reach their final destination until January.
That's a nine-week-long breeding season when the mowing machines must somehow be kept at bay.
Shelburne Farms is an environmental education center on the Lake Champlain shore south of Burlington, Vermont. It is also a fabulous place to do field work, with spectacular views of New York's Adirondack Mountains across the blue waters. Once a railroad mogul's luxurious estate, Shelburne is still a working farm, with rolling hayfields that support a herd of 125 Brown Swiss cows whose milk is made into cheddar on the premises. The bobolink season was two weeks along when I met Strong and Perlut at one of those fields, where a careful search for nests was under way.
The first hay harvest on this 45-acre field, for example, would be delayed until August 1. That's a tactic widely used in grassland bird conservation efforts, and it would be perfect for Vermont's nesting bobolinks and savannah sparrows. But not so good for the state's dairy cows. As the biologists explained, hay that's cut late loses much of its crude protein content, though it can still be used for beef, horses and sheep. Few dairy farmers, they pointed out, would be willing to delay that important first cut on one of their fields to benefit grassland birds.
Strong and Perlut had a better plan that they believed would appeal to dairy farmers: harvesting the hay early, by June 2, and then waiting 65 days before the second cut. While some egg clutches would be lost in the first mowing, the birds will soon return to nest. Strong and I walked another research field, recently mowed and spread with manure, where the scientist pointed out a savannah sparrow's nest that had been crushed by the machinery. In a week or so the grass would be tall enough to provide nesting habitat, and in fact we startled a male bobolink, which burst into song, hovering over our heads like a small black helicopter. Presumably he had already staked out his territory and was waiting for one or more mates to arrive. (Bobolinks tend to be polygamous, and males may have four females under their wings at the same time.)