"Barn Swallows and Chimney Swifts:Ecological Indicators"
by Lou Jonas
If you're the type of person who does a lot of observing and thinking as you drive across the country on vacation, here's a question you can spend quite a lot of time on. "Why are there so many different kinds of swallows? Why can't one species do everything all other swallows do?"
If your experiences are similar to mine, you will see that in country areas like the Piedmont, where the air is less polluted, and buildings farther apart, as well as there being a few barns here and there, and some banks along road cuts or along streams, the air is divided up into something like "precincts", with each species having a niche fairly well separated from those of other kinds. The layer next to the ground and up to a little above treetop level, is generally patrolled by barn swallows, and by bank and rough-winged swallows when they are present, near banks or gravel pits.
Chimney swifts will generally be higher, quite a distance above the treetops, in this sort of environment.
However, when you near one of the cities with a great deal of pollution, such as Knoxville and other Tennessee cities, you seldom see a barn swallow, and I can't ever remember having seen them in smaller cities with cleaner air, where they sometimes build their nests in sheds, or on a porch ceiling.
In the cities, the chimney swift will work close to the ground as well as higher in the air. Why this change? As near as I can figure, it is due to the barn swallow being more efficient at the fast maneuvering required close to the ground, where there are more objects to run into; yet, when the barn swallow finds it impossible to exist, as it evidently does in dirty air, and is absent, the swift can and does take over the barn swallow's "job".
Where homes are provided for the purple martin, they are apt to become established, but they, too, seem unable to endure the sooty air as well as the swift. They seem to spend most of their flying time at a fair distance from the ground, perhaps because their soaring flight adapts them well to more open spaces.
In hot, dry weather, chimney swifts tend to move out of an area, probably because insects are more numerous in times of occasional rainfall. It was interesting to see the different in Warrenton and Culpeper in the fall of 1970. Culpeper had a good heavy rain, while Warrenton and surrounding areas had been missed for weeks. There were numerous swifts over Culpeper, while they had all apparently moved from the Warrenton area.
Thus the swallows of various sorts serve as good ecological indicators. They indicate habitat conditions. Are the barn swallows getting scarcer, and the swifts flying close to the ground much of the time, around your home? Then ask yourself, "Why?" and "When will I begin to suffer from bad air?".
(We do have an awful lot of mosquitoes, and not very many swallows for a rural area. Could it be all the spraying the farmers do to their crops?)