Of a Feather: The next generation on the ground

By BILL CHAISSON
By Bill Chaisson

This past week my dog flushed a brood of ruffed grouse (Bonasa umbellus) on a couple of mornings. The poults are easily 8 inches long now and fairly adept fliers. They scatter in all directions so that the dog (a bird dog, but not trained to hunt) doesn’t know who to chase, and everyone gets away safely. The female waits until all the poults have flown off and then exits herself, in a direction not chosen by any of her young. Although it is just a game to the dog, it is about survival for the grouse. Next time the pursuer could be our local fox, who means business.

Ruffed grouse lay nine to 14 eggs, which they incubate for about 24 days. The chicks are precocial; they hatch covered with down (with a triangular patch of black feathers around the ears) and are ready to leave the nest and forage on their own almost immediately. This adaptation is common among non-passerine ground-nesting birds.

We have been listening to the male grouse drum in the woods for weeks now. We heard him as recently as two weeks ago. The male has nothing to do with the female after he mates with her, so he must be hoping for a second round with another mate. Or perhaps his hormone levels just remain high.

The length of incubation in birds roughly correlates with size. An adult ruffed grouse is 17 inches long and weighs over a pound. At the other end of the scale, an ovenbird (Seiurus aurocapillus), another ground-nesting forest bird, is 6 inches long and weighs 0.68 ounces (18 grams). This warbler has an incubation period of only 11 to 14 days.

Although ground-nesters, ovenbirds, like all songbirds, have altricial young; they are born blind, naked, and helpless, and after hatching must stay in the nest for 7 to 10 days, where they are fed by their parents. In other words, in spite of the size difference, it takes about the same length of time for a little ovenbird to leave the nest as it does a big ruffed grouse.

The field sparrow (Spizella pusilla), the subject of last week’s column, is not quite 6 inches long and weighs about 0.45 ounces (12.5 grams). That is, slightly smaller and lighter than the ovenbird. They build their first nest of the year on the ground and then tend to build a subsequent one up a foot or two. Their incubation period is 10 to 17 days, slightly longer than the ovenbird, and the young are altricial. They remain in the nest only 5 to 8 days, slightly less time than the warbler.

The worm-eating warbler (Helmitheros vermivora), another ground-nester, is smaller (5.25 inches) and lighter (0.46 oz. or 13 g) than the ovenbird. It incubates for 11-17 days and young stay in the nest for 9-11 days, longer than the ovenbird.

The savannah sparrow (Passerculus sandwichensis) is 5.5 inches and 0.7 oz. It incubates only 12-13 days and stays in the nest 8-13 days, overall, about the same as the longer but lighter field sparrow, but the latter has a shorter nestling period.

The worm-eating warbler has a long incubation period, in spite of being small and light. These warblers are forest birds while these sparrows are grassland dwellers, but their young take about the same time to leave the nest. In other words, the developmental trajectory of birds is affected by more than just size and weight.

Ground-nesting birds use varying strategies to ensure survival of their young. The ruffed grouse have a lot of young. A northern bird, it has just one brood per year. The bobwhite (Colinus virginianus), another gallinaceous bird albeit a quail, has a more southerly distribution. It may have up to three broods per year and may lay as many as 28 eggs. Although the bobwhite is smaller—less than 10 inches long and only 6 ounces—its incubation period is 24 days, like the grouse’s. Furthermore, although the quail are precocial, the chicks rely on the parents to stay warm and to help them forage.

The killdeer (Charadrius vociferus), a plover, nests on the ground and lays only 4-6 eggs but, like the quail, may have up to three broods each season. More sophisticated than the grouse, they scrape out several depressions before choosing one to complete a nest. This behavior is thought to be an attempt to confuse potential predators.

This inland shorebird is 10 or 11 inches long but weighs only a bit over 3 ounces. While the grouse is a sedentary bird that doesn’t stray far from its home range through the year, killdeers migrate as far as the Caribbean and northern South America each winter; they had better be light.

Killdeer incubate their eggs for 22 to 28 days, comparable to the grouse, in spite of their smaller size and lesser weight. Their young are precocial, like those of the grouse and quail. Covered with down at hatching, they are ready to leave the nest as soon as they dry.

Unlike the grouse, the killdeer male and female birds cooperate in the rearing of the next generation. This species is a dramatic practitioner of the “broken-wing” display. If the female is approached during incubation, she will flush into the air briefly and come down to the ground some distance away, dragging her wing and crying out piteously, her beak open and her body convulsed with panting. Meanwhile the male bird will fly in circles at a short distance “adding his protests and denunciations,” as George Gladden put it in “Birds of America” (1917).

This is an instance of independent evolution of the same behavior by unrelated species. In a 1989 paper in “Bird Observer,” William E. Davis, Jr. documented “distraction displays” in all the birds I’ve mentioned plus many more, although not field or savannah sparrows.

Bill Chaisson has been a birdwatcher from the age of 10. He is a former managing editor of the Eagle Times and now lives and works in the town of Wilmot. Contact him at [email protected].