There has been a lot of talk lately about the decline of songbirds in
eastern North America. No one factor explains it all, but here are some of
the big ones:

Eastern North America was once covered by an enormous, virgin forest that
occasionally burned in places, and whose undergrowth the native Americans
kept clear in places by fire. Now, fire is controlled and the undergrowth
is allowed to cover the forest floor. The forest is so split into small
areas that really large areas of forest are very few. These include the
Smoky Mountains and Adirondack Mountains.

One effect of this has been to increase the forest edges at the expense of
"deep forest". Birds that prefer the deep forest, such as bluebirds, have
declined while birds that like edges, such as cardinals, have prospered.

One of the species that likes edges is the parasitic cowbird, which lays
eggs in other birds' nests. The cowbirds have gradually increased over the
years and are now numerous, making it difficult for some species to
reproduce at all.

A few bird species need to nest in burned-over woods. Their populations
have dropped.

Others need to nest in hollows in old, rotten trees. Their populations have
also dropped.

Wetlands have also declined, reducing the population of waterfowl.

Insecticides such as DDT made bird populations drop precipitously a few
decades ago, partly by reducing the food supply locally, but mostly by
accumulating in their tissues and interfering with reproduction. This was
particularly a problem with birds high in the food chain: the birds of
prey.

Some bird species declined in the 19th century because of excessive
hunting, and some were made extinct or so rare that they have not
recovered.

Another problem lies outside North America. The winter feeding grounds of
many migratory birds are now being logged in South America, and the
remaining forest can support only so many birds.

Although it's not as obvious with mollusks as with birds, they can have
analogous problems due to habitat alteration and reduction. If a bay is
polluted or filled in, the estuarine species will suffer. If the influx of
fresh water to a bay is reduced by damming upstream, it can allow the
marine oyster drill to enter a bay and eat the oysters there. And so on.

Ecologists are not happy campers these days. Every few years they discover
another factor that was obscure before, and it's usually bad news. (Not
always: Most people like cardinals, and hunting birds for feathers was
outlawed many years ago.) As to the specific case of the purple martins in
South Carolina: I don't know, but maybe the hints above can help puzzle it
out. Carol, do you have a local chapter of the Audubon Society or Sierra
Club? They would certainly be interested in this problem.

Andrew K. Rindsberg
Geological Survey of Alabama