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Jeannie Carl: What was that?

One beautiful spring morning as L.B. Morris’s first-graders got off the bus, tree swallows were swooping over our heads chasing insects. One of the first-graders looked up and exclaimed, “holy cow!”

Not missing a beat, I looked him in the eyes and said, “That’s a bird … not a cow!”

I am used to people rolling their eyes at me, but there is something very insulting when a first-grader does it.

Other students wanted to know what those birds were as they lined up and headed for the pavilion. I started to give an impromptu lesson about tree swallows and why they were swooping overhead. As we settled at the pavilion, two barn swallows swooped in and headed for the rafters.

A chorus of voices rang out, “tree swallows!”

I was happy that they were listening, but I had to tell them, “No. These birds are barn swallows.” Again, with the eye rolling. Admittedly, before working at the center, I would not have known the difference either.

Tree swallows eat mostly insects, but they will eat berries during bad weather when the insect population is low. They feed from dawn to dusk, usually foraging no more than 40 feet from the ground, chasing any insect that fly. They chase prey in the air, with acrobatic twists and turns, amazing young and old alike.

Tree swallows breed in fields and marshes, throughout northern North America. For nesting they need to find trees with existing cavities, or they may rely on man-made nest boxes. Nest boxes account for only a small fraction of tree swallow nest sites. Natural cavities, where most tree swallows build their nests, have been disappearing for the past 200 years as people clear the land, manage woodlands, cut down older trees and remove dead trees.

The female does most of the nest building, taking between a few days to two weeks to finish the job. The nest is often made entirely of grasses, but may include pine needles, mosses, rootlets, aquatic plants, animal hair and artificial materials like cellophane or cigarette filters. While creating the nest, the female uses her body to push against the nest material to shape it into a cup, about 2-3 inches across and 1-2 inches deep, and lines it with feathers of other bird species. In some populations the male gathers most of the feathers, and in others the male and females split the duty evenly.

Once on a hike, I opened a nest box for the hikers to look inside. We found a clutch of four pale pink eggs. I was able to show off a little by telling the hikers that the eggs were recently laid because the eggs turn white within four days of being laid. Of course, right away I was asked how long until the eggs hatch, and they usually hatch in 20-23 days.

I did caution the hikers that opening boxes regularly was not a good idea for a few reasons.

Tree swallows form pairs to breed, but often a male attends two mates in separate nest sites. Though an individual swallow may have the same mate several years in a row, it is probably faithful to the site rather than the mate. This means that it is common for the chicks in a nest to have different fathers in the same clutch. Nest predators include rat snakes, raccoons, black bears, chipmunks, mink, weasels, deer mice, feral cats, American kestrels, grackles, crows and Northern flickers.

Outside the nest, adults are hunted by hawks, falcons and owls. The swallows commonly swarm and divebomb predators while giving alarm calls.

During the early months of this year, I kept “Lazarus” at home to care for him while we were closed to the public.

After returning him to the center when we could open again, I welcomed him “home” by putting him on his perch just outside the front door. Within seconds, a pair of tree swallows called back and forth between themselves and bombarded him in an effort to drive the predator from the area!

Little did this pair know but Lazarus was back to stay.

Every year, we have a nest box building program either in January or February well before breeding season so boxes can be put out early to allow a period of weathering before mates start looking for a place to raise their young. So, by putting out nest boxes, we can do something to help them and they help us by cutting down on the number of insects.

Contact us for more information on these programs. Tree swallows make good neighbors.

Jeannie Carl is a naturalist at the Carbon County Environmental Education Center in Summit Hill. The center rehabilitates injured animals and educates the public on a variety of wildlife found in the area. For information on the Carbon County Environmental Center, visit www.carboneec.org.

A tree swallow sits on the top of a wooden fence. JEANNIE CARL/SPECIAL TO THE TIMES NEWS