Sometime in late March or early April, the first male field sparrow (Spizella pusilla) of the season sings his long down-slurring whistle that ends in an accelerating trill. Soon he is joined by other returning males, and our 37-acre First Field rings with their lovely songs. Even in a dawn chorus of residents and migrants—dark-eyed juncos, American tree sparrows, song sparrows, white-throated sparrows, blue-headed vireos, eastern towhees, and black-capped chickadees—the song of field sparrows stands out.
Not only do field sparrows start singing early, but they continue until at least the middle of August, only taking short breaks after pairing up with mates. Once the females begin incubating their eggs, males resume singing probably because such singing indicates their territorial holdings.
Their familiar song is referred to as their “simple song” by ornithologists, but there is nothing simple about it until you hear another quite different song as I did when walking across the field one morning. I was puzzled by it, but finally I spotted a singing field sparrow. He was singing an entirely different song, his “complex” or “dawn” song, which turned out to be the reverse of his “simple” song. It began with a trill of short notes and was followed by down-slurring whistles. Apparently, this is his more aggressive song sung at dawn or when interacting with other males over territorial rights.
Although field sparrows over-winter in some areas of Pennsylvania, notably the Coastal Plain, Piedmont, and river valleys in the Ridge and Valley, here on our mountain in the west central part of the state, we see our first field sparrow at our feeders in mid-to-late March. His white eye-ring and bright pink bill and legs distinguish this rusty, brown-backed bird from other sparrow species.
Both our First and Far fields host these birds because they have the shrubby, old field habitat field sparrows prefer. According to a long term study (1987 to present) of field sparrow habitat in Lackawanna County by Dr. Michael Carey, field sparrows favor fields that are open but have scattered shrub and wood vegetation, in Carey’s case, honeysuckle, dogwood, Rosa and Viburnum species, and in our case black locust saplings, blackberry, a few autumn olives, and a scattering of multiflora rose and Japanese barberry along its edges.
Furthermore, field sparrows will occupy an unmowed hayfield the first year after it has been mowed and has a cover of grasses and other forbs mainly goldenrod. Over the next ten years, their numbers will increase as shrubs and other small woody vegetation cover 30% of a field and then they will slowly decrease until the field has been unmown for 30 years. At that point, it no longer will suit field sparrows because it will be overgrown with trees and shrubs.
In summary, Carey found that fields eight to 11 years old after mowing produced the most field sparrow nests. As the fields neared 25 years of age, field sparrow territories were held only for a short time either by a male that failed to attract a mate or by a couple that lost a nest and left in the middle of the season.
Although our First Field has not been mown for more than 20 years, woody vegetation has not taken over because my husband, Bruce, has been hand-pruning the black locust saplings. Thus, it has remained 37 acres of mostly grasses and other forbs, especially goldenrod and asters.
Carey also found that many male field sparrows are homebodies. Even though their field habitat is no longer optimal, older birds that have nested in a particular field return each spring and defend their territory even, in one case, while it was being mown. But their offspring leave their natal home and search for their own optimal breeding area.
The best field sparrow habitat, though, according to Pennsylvania’s Second Breeding Bird Atlas survey, is reclaimed surface mine areas in the Pittsburgh Low Plateau Section, which had the highest number of breeding field sparrows. Altogether, the atlas survey found 210,000 singing males. While that sounds like a lot of field sparrows, their population size has declined 3% a year since Breeding Bird Surveys began in 1966. For that reason, we manage our First Field as field sparrow habitat and look forward to their return each spring.
Ten to 20 days after males arrive, the females, which look like the males, appear. Most likely they are not the same females the males mated with in previous years, but most will remain faithful to one another during the breeding season. They pair within a couple days and begin mating while the female chooses a nest site and builds a nest constructed of grasses and tucked in grass clumps or at the base of shrubs. It takes her five to eight days with the help of her mate who closely accompanies her and occasionally offers her nesting material.
Shortly after the nest is constructed, she lays two to five white or pale green spotted eggs and begins incubating after the last one is laid, although she may delay several days if the weather is cold. Most eggs are laid between May 3 and July 27 in Pennsylvania, but later nestings never have more than two to three eggs. They are also built higher in shrubs or saplings as the season progresses.
It takes 11 to 12 days of incubating before the chicks hatch, usually all on the same day. Born naked and helpless, their eyes open at four days. By seven to eight days of age they can leave their nest but remain in the low vegetation until they are 13 to 14 days old when they can fly short distances, but their parents continue to feed them. At last, they are off on their own when they are 26 to 34 days of age.
Throughout this period, the parents feed them a diet of almost exclusively arthropods. In Pennsylvania Carey reported that of 1,853 food items he examined, 80% were butterfly and moth caterpillars, 10% adult flies and bees, 6% katydids, 3% adult moths, and 1% spiders.
Of course, both eggs and nestlings are often eaten by predators, most commonly black rat snakes, but Carey also found instances of eastern garter snake predation. Possible mammalian predators include chipmunks, red and gray foxes, weasels, skunks, mink, raccoons, and opossums. Blue jays, house wrens, and American crows are proven bird predators on field sparrows.
After a loss to predators, a female begins laying eggs in a new nest she has quickly constructed a mere five days later. When the first nest is successful, she begins laying eggs in a new nest six to 20 days after the young fledge. Still, only about 40% of nests fledge young.
Once on their own, the immature birds flock together and may select their breeding habitat before their autumn migration. They also begin to expand their food sources from wholly insect to grass and forb seeds which sustain them through the winter months.
In addition, they are learning their hauntingly beautiful songs from those males they hear on their natal grounds. They perfect their songs on their wintering grounds in January and February and modify them the following spring to match those of nearby neighbors. Then, once again, I enjoy a choir of field sparrows sing as I walk the Butterfly Loop in First Field.