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The season's first tree swallows showed up the week after the official beginning of spring. The weather was very springlike, if we define springlike to mean "like Chicago at the end of March."

The whole family of swallows are aerial feeders. They capture flying insects right out of the air. With their tiny beaks and huge, gaping mouths, you can imagine them ingesting insects the way blue whales ingest plankton by just swimming around with their jaws open. What swallows actually do is much more complex than straining plankton out of seawater, but regardless of how they do it, late March in Chicago is not a good time and place for eaters of flying insects.

So the tree swallows turn to fruit and seeds. Tree swallows tend to favor wet areas, which is why the bird books mention the seeds of bulrushes, sedges, and smartweeds. An investigator who studied the stomach contents of 343 swallows taken during every month of the year over a wide range of the eastern U.S. (hows that for a job?) calculated that 80 percent of the tree swallow's food was animal matter, mostly insects, and 20 percent was plant material.

Since those figures are averages over the year, we can safely guess that totals for July would be almost 100 percent insects and totals for March and April would contain a lot more sedge seeds and bayberry fruit. The latter item was the most commonly found vegetable food in the study, which means the wide range from which the 343 swallows were taken may not have included Chicago; the berry cited, Myrica carolinensis, doesn't grow around here. In fact, we don't have any plants of the genus Myrica in the Chicago area. Does anyone know what Chicago tree swallows eat in March?

The first tree swallow I remember ever seeing was feeding young in a nest box on the front porch of a restaurant next to the ferry dock at Gill's Rock, Wisconsin. I was having lunch just on the other side of a plate-glass window, and as the bird poked its head through the nest hole its tail was only about four feet from my eyes.

I studied it carefully before paging through my brand-new copy of the Peterson field guide and discovering it was indeed a tree swallow. It is not a difficult bird to identify. Its underparts are snowy white. Its upperparts are iridescent blue. There is nothing else that looks anything like it.

The swallow shape is equally distinctive: bullet-headed; slender; wings long, narrow, and pointed. Unlike swifts, which appear to have no tails at all, swallows are variously tailed. The barn swallow wears the true forked swallowtail, with outer tail feathers an inch or more longer than the inner feathers. Others, like the tree swallow, have tails with only shallow notches at the center. The only real difficulty in identifying swallows comes from the fact that they rarely hold still.

The best time to see them holding still is at the nest. Swallows are not usually troubled by a human presence, as long as it's not right on top of the nest.

Keep a distance--the birds will let you know if you are too close--and you can watch them without bothering them.

You can sometimes see them perched on a wire or a bare tree branch. Swallows avoid perching on limbs that have leaves on them; they are so fast and so maneuverable that it suits them to stay in the open where they can see danger coming. If they can see it, they can almost certainly avoid it.

Swallows are fun to watch. When my dog was young and foolish--instead of middle-aged and foolish as she is now--she used to exhaust herself chasing barn swallows in the park. Her chances of catching one make the lottery look like a sure thing, but it was good exercise. I'm not sure the birds even knew they were being chased. On the rare occasions when she got close enough to alarm one, the bird would, with the merest flick of its wings, rocket straight up and go into a barrel roll that no human pilot could imitate. The same trick works for catching highflying insects.

When they hunt, you frequently see them doubling back with a turn-on-a-dime 180-degree maneuver executed at full speed. At those moments, the bird may be zeroing in on an insect it just missed with a first pass.

Swallows are popular birds. The native people of eastern North America used to hang clusters of hollow gourds on trees around their towns and villages to attract purple martins, which are part of the swallow family. (Several swallows go by the sobriquet "martin." In England they reserve the name "swallow" for truly swallow-tailed birds like our barn swallow. Birds like our tree, roughwinged, cliff, and bank swallows would all be martins. In the U.S. we have purple martins and call everything else in the family a swallow.) If you want to attract martins in a traditional American way, you can buy seeds of the same strains of gourd the Native Americans used. The seeds come complete with directions for turning your homegrown gourds into attractive martin houses.

A more conventional method is to build or buy a martin house and mount it on top of a pole in your backyard. You need a big yard to have much hope of attracting them; some of the colonies in American backyards contain over 200 pairs of birds.

Nesting colonies are the usual thing for swallows. Cliff swallows festoon a suitable wall with rows of nests as regular in size, shape, and position as cells in a honeycomb. Bank swallows honeycomb river banks with rows of nesting burrows.

Tree swallows are not quite as intensely communal, but they do gather in numbers where they can find good feeding grounds and suitable nest sites. Tree swallows nest in holes in trees, and they take quite happily to birdhouses. The number of nest sites available is a major control on the population. Sixty years ago, ornithologists at a research station on Cape Cod raised their nesting population of tree swallows from 4 pairs to 113 pairs in two years just by putting out nest boxes.

Tree swallows are now competing with Eastern bluebirds for the use of the nest boxes on bluebird trails. A bluebird trail is a whole line of nest boxes placed at suitable intervals around a park or preserve. The boxes are too small for starlings, but tree swallows find them ideal. You might get upset about tree swallows moving in if you had spent hundreds of hours building houses and placing them on the preserve, but I doubt if anyone else would mind.

The reason for our fondness for martins and swallows can be summed up in three words: they eat mosquitoes. And all that high-speed flying burns huge amounts of energy, which has to be replenished by more big helpings of mosquitoes. If you calculate the ADMI (Average Daily Mosquito Intake) for a colony of 200 nesting pairs with hungry young in the nest, you can demonstrate with scientific precision that without those birds there would be a six-inch layer of mosquitoes coating every surface in your neighborhood.

These predators may seem like a nice, natural way to control mosquitoes, but understand that the birds have their own interests and that those interests are not the same as ours. Martins like mosquitoes. They will move into your backyard only if it looks like a Mosquito farm. They don't want to annihilate mosquitoes. They want an inexhaustible supply.

You would expect a fast-flying group like the swallows to get around, and indeed they do. They don't live in polar regions, and they are not ocean wanderers--they have never colonized New Zealand or many smaller oceanic islands--but they are everywhere else. A few North American barn swallows get as far south as Tierra del Fuego in winter. And the same species is widespread in the Old World, where it nests in temperate regions and winters in the tropics. My Bird Guide of Thailand says that "over 100,000 roost in Bangkok at night on the electric wires from Nov. till end of April."

About 80 species currently exist, all of them hawk flying insects. All are variants on the pattern of sleek shape, narrow, pointed wings, and big mouths. Of the eight species that regularly nest on this continent, six nest in and around Chicago, and these six have ranges that span the continent. The two that don't live around here are the violet-green swallow, a very close cousin of the tree swallow, and the cave swallow. Violet-green swallows are birds of the western mountains, with a substantial range: mountainous habitat from Alaska to Mexico and from the Pacific to as far east as Texas and South Dakota. Cave swallows are a Mexican species, with a localized population in Texas.

When you see a tree swallow at this time of year it is likely to be in a hurry. Later, as spring turns to summer and the mosquito crop begins to ripen, they settle down. Two months from now, you could go to a good marsh or a wet meadow and watch the tree swallows tirelessly hunting for food for their young. Go on a sunny day and the light will bring out the richness of that blue back. It will also keep down the mosquitoes.

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