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