Wild Side : Fledglings get a move on
In a hole in a maple branch overhanging Wing Road in Oak Bluffs, a pair of downy woodpeckers have been packing insects into two or three youngsters in a hole in a branch. Each time an adult arrives at the nest, carrying a grub or bug, the youngsters clamor for attention, their gaping beaks and begging cries filling the narrow cavity. The opening of the nest cavity faces down, which probably works well for keeping the nest dry and making it inaccessible to predators. But the baby woodpeckers, who are growing fast and getting mighty crowded in that hole, will soon bail out of the nest ten feet over the roofs of passing cars.
That's a drama I'm not sure I want to see the end of. But it's typical: it's fledging season on the Vineyard, and all around, baby birds are leaving the relatively secure spot they've lived in for all two weeks of their lives to confront a bewildering world. The process is noisy, hectic, and above all perilous; it is also absolutely critical. Young birds have just a few short weeks to learn how to avoid danger and find food and shelter. And it is during the days right after they leave the nest that the challenge is greatest.
The early development of a bird is a period of astonishing growth. Typically, a songbird egg hatches about two weeks after it is laid, and the nestling leaves the nest about two weeks after that. In that short time, a fertilized embryo grows to nearly full adult size, acquiring a beak, feathers, and all the trappings of birdness. Parent birds drive this process with nearly constant feeding — insects are favored because of their high protein content — and a nestling's life consists largely of competing with its siblings for parental care.
Leaving the nest initiates the next critical period in the life of a young bird. It's not that life in the nest is risk-free; indeed, predation of eggs and nestlings is probably the greatest source of mortality for songbirds. But the chicks that survive to leave the nest face a complex world armed with nothing but instinct and the ability to imitate the behavior of their parents; they must learn quickly how to feed themselves, and how to stay safe in a hostile environment.
The term "fledging," with its suggestion of taking flight, is a bit misleading. For some birds, like the osprey, leaving the nest truly is the same thing as taking your first flight: There's no other way down off that pole. But many songbirds nest on or very near the ground, and their young can simply step out onto terra firma, developing their flying ability over a couple of days of progressively more ambitious exercise. And even some arboreal nestlings, like those of the Baltimore oriole, scramble around the outside and surroundings of their nest, developing their perching ability before they try aviating.
In general, fledging is a noisy process. The young birds, hungry and disoriented, hide in dense foliage but call nearly incessantly for their first few days outside the nest. The parents, agitated and working hard to care for the offspring, call back. The noises made by baby birds often differ from those made by adults of the same species. But sometimes I can hear a hint of adult vocalizations in the calls of a fledgling.
The downy woodpeckers over Wing Road beg in single notes much like the noises adults string together into their whinny of a song. Black-capped chickadee fledglings, which are amazingly noisy considering their small size, sound much like calling adults, only wheezier. A robin fresh from the nest makes a remarkably annoying squeal of a call, not unlike some squeaky notes that adults give.
The instinct to call incessantly definitely carries risks for a fledgling. On the one hand, the sound of a fledgling's calls keeps the adults agitated and attentive, and allows them to know the fledgling's location at all times. But endless squawking or cheeping also reveals your location to everybody else, including cats, skunks, raccoons, and marauding crows and jays. There's a delicate balance here, between risking separation or neglect from the parents and calling in the cause of your own demise, and it is luck as much as instinct that determines who survives.
From here on in, the ruckus should decrease. Some pairs of adults that lost their young to predation or other causes will try again, producing, with luck, a late-season brood. And some species routinely try to crank out a second crop, laying more eggs after their first set of young are independent. But generally, the season's fledglings (I mean the ones that haven't ended up as a crow's lunch) are growing used to the world and developing the skills they need to survive. Their calling steadily decreases as both they and their parents grow less agitated and proceed more smoothly about the process of finding food. By summer's end, the lucky survivors are ready to face migration or the rigors of winter, still remote but already approaching.