Wild Side: Young birds face threats from all quarters

A chickadee builds a nest.
Photo by Chuck Tague, wqed.org

A chickadee builds a nest.

With the avian nesting season in full swing, the Island vibrates with fledglings squawking for food, adult birds scurrying to bring prey back to nests, and seemingly constant defensive actions, as chickadee parents chase marauding blue jays away from their nest site, blue jays chase grackles, and everybody gangs up on the crows.

The process of raising young is a stressful one for birds, in terms both of anxiety and energy expenditure. And it’s a critical time: parent birds have just a few weeks to bring their offspring to independence, fending off countless risks in the process.

The vast majority of birds, especially songbirds, make at least some sort of nest, even if it’s just a shallow scrape in the ground. Clearly nesting behavior evolved to give birds a greater chance of fledging their young. But a nest also creates risks for parents and nestlings alike. And a nest represents opportunity for an astonishingly wide array of other wildlife.

To start with, building a nest represents a liability for adult birds, who may spend many hours and make hundreds of trips gathering and assembling nesting material. This time and energy could be spent in other ways, like finding food for themselves, and so the process of nesting represents a dramatic sacrifice on the parts of parent birds. Females pay an additional price: they may expel the equivalent of half their body weight in the form of eggs, and their reproductive systems divert a large quantity of protein from sustaining the parent to creating the young.

Of course, everybody else is also desperate for protein at this season, and one bird’s eggs are another bird’s breakfast. Nearly any kind of bird will steal eggs, or prey on the nestlings of another species, if an opportunity presents itself, and for some birds, notably crows, eating the offspring of other species is the primary feeding strategy in early summer. And it isn’t just birds that cash in on this rich, relatively easy food source: some species of snakes prey heavily on the contents of bird nests, and mammalian predators, including skunks, raccoons, and cats, also can’t resist an egg or a tender nestling. The majority of the eggs of most songbirds end up in the bellies of some other species.

If they escape being devoured all at once, nestlings face a gamut of more subtle diners. Confined to the restricted space of a nest and quite helpless for the first week or so of their lives, baby birds are a prime target for parasites. Literally hundreds of species of mites, ticks, insect larvae, and biting flies have been documented feeding on nestlings. Some of these are at worst minor annoyances, but other parasites, if they become numerous, can kill a nestling outright or else weaken it to the point that it grows susceptible to death from another cause.

Finally, one can’t overlook the ways in which bird nests function as habitat for small animals that have limited interest in the birds themselves. For example, so-called Dermestid beetles, members of a family that specializes in eating skin and related animal products, are often found hiding out within the complex structure of bird nests. They find ample provisions in the form of food scraps or even skin cells sloughed off of baby birds.

With such a hive of would-be diners hovering about, it’s no wonder that songbirds have evolved an arsenal of methods for protecting their eggs and young. One key measure is keeping the nest tidy: the young of many species excrete bodily waste in the form of a neat, enclosed package, called a fecal sac, that an adult bird can easily remove from the nest.

Against predators, nearly any bird will counter-attack, giving a loud distress call and diving at the head of any would-be nest robber. Often other birds, even members of other species, will join in, united against a common threat. Determined defense is often successful — even crows will back off to avoid a pummeling — though nests remain vulnerable because the parents can’t spend all of their time manning the barricades.

Against other threats, a range of more subtle defenses have evolved. Some birds, such as the great crested flycatcher, incorporate shed snake skins into their nest sites. A recent study has shown the flycatcher nests with snakeskin fledge more young, on average, than nests without, and it appears that some predators, assuming that presence of snake skin means presence of a snake, seek a less risky dining option. Many birds incorporate aromatic herbs into their nests, and the chemicals in those plant fragments discourage invertebrate parasites from visiting. And the white-breasted nuthatch is said to smear pine pitch around the opening of the cavity it nests in, which intercepts any crawling insects or mites before they enter.

Clearly the primary purpose of a nest is to contain the young of the birds that built it. But nests also represent habitat for scavengers and a food source for a host of predators and parasites. More than just a home, a nest represents a temporary resource, functioning as a node for the flow of nutrients through the local ecosystem.