The river's tree swallows are also showing signs of reproductive problems, a side effect of the chemicals documented in laboratory animals.

Nest boxes are meant to attract bluebirds, tree swallows and house wrens.

Phoebes take bugs on the wing, and so do the few tree swallows who live around here, and of course the bats.

Ducks and grebes have set down for a short stay and tree swallows have moved into nesting boxes.

We paused to catch our breath and watch what must have been a million tree swallows pass by on their way from summer to winter.

The trail has 40 birdhouses to attract tree swallows and bluebirds.

Then there were the tree swallows, which had not been known to breed in the park since 1868.

Being highly social outside of the breeding season, tree swallows may form flocks of several thousand birds near roost sites.

An earlier study of tree swallows had the opposite result, showing that individual begging intensity increases with number of offspring.

"They stayed for about a week, but then got chased out by some tree swallows," he said.