For at least the last eleven years, a colony of scores of rough-winged swallows have wintered at the Northeast Philadelphia Sewage Treatment Plant near the Delaware River in Philadelphia. It is the only gathering of wintering swallows in North America north of the Gulf of Mexico. All other rough-wings winter on islands and mainlands around the Caribbean Sea and the rim of the Gulf of Mexico where flying insects are abundant and available during the northern winter. That sewage treatment plant is a relatively warm habitat, a human-made mini-ecosystem in the cold of winter. I have read reports of swallows at that plant in winter, but have not been there myself. But it is intriguing to think of this unusual environment, and others like it. And it is enlightening to realize how resilient nature is.
The presence of rough-winged swallows and other kinds of small, insect-eating birds, including a few each of cave swallows, barn swallows, eastern phoebes and a few species of warblers, including palm warblers and yellow-rumped warblers, wintering at that sewage treatment plant, is based on swarms of a kind of insect called midges. Adult midges are small, two-winged flies that fly about that sewage treatment plant in swarms and are eaten by those birds in winter.
Other kinds of small birds that might winter at Philadelphia's sewage plant to eat midges are tree swallows, pine warblers, orange-crowned warblers, common yellowthroat warblers and yellow-throated warblers. A few of each one of these species already winter in the north, though most birds of each species winter in Central and/or South America.
But how can adult midges, being cold-blooded, be active and available to hungry birds in winter? That depends on the warmth of sewage in underground treatment ponds. Sewage water constantly flows from warm buildings through underground pipes to open-air sewage pools in the ground at the treatment plant. Being in the insulating soil, the sewage water stays relatively warm and is a breeding ground for the midges that live at that sewage treatment facility. As fast as the waste water cools, it is supplemented with warmer water that warms the air above it, and midges in that air.
Adult, flying midges don't eat anything. They only live three to five days and there only job is to mate so female midges can spawn thousands of egg masses on the surface of sewage water. But many adults are eaten by birds before they get to mate or spawn. Midge eggs sink into the sewage and bloodworms hatch from them. They are called bloodworms because they are red.
Each bloodworm creates a protective case around itself and joins others of its kind in red, wriggling masses on the bottoms of tanks and their walls. There those red midge larvae consume bacteria, and sewage, which is still loaded with nutrients. Both the bacteria and sewage is suspended in the water and is, therefore, available to the bloodworms. The bloodworms pupate and later emerge as adult, flying midges in great swarms that feed those insect-eating birds around Philadelphia's sewage plant in winter. Bloodworms in more natural habitats feed fish and a host of other aquatic creatures.
Bacteria that bloodworms feed on ingest sewage. We can see that new food chains developed in sewage treatment plants, which is based on human waste. Sewage feeds bacteria, which is consumed by bloodworms that emerge from the waste water as adult, flying midges, many of which are eaten by small birds, some of which are eaten by small hawks and other critters.
There are many kinds of adaptable plants and animals. Two examples of that are chimney swifts and barn swallows that hatch babies in protective niches. Chimney swifts historically nested down the inside of hollow, broken-off trees, but now raise young down the inside of chimneys, which, to them, are like hollow trees. Barn swallows originally nested on cliffs and in shallow caves, but now rear offspring in barns and under bridges, which to them, are like cliffs and caves.
The colony of rough-winged swallows that winter at Philadelphia's sewage plant might, in time, develop a new species of swallows because of their isolation from their mainstream relatives. That group of rough-wings might stay at that plant to nest, which might also contribute to their isolation.
It's inspiring to know that some forms of life are so adaptable that they survive where we wouldn't expect them to. Because of relatively warm sewage, bacteria and midges are active in winter. And swarms of midges attract wintering, insectivorous birds that are farther north, in numbers, than they ordinarily would be. These food chains in a human-made environment created an interesting mini-ecosystem.
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