Every year since 1966, the U.S. Fish and Wildlife Service has organized thousands of amateur ornithologists and bird watchers in an annual bird count called the Breeding Bird Survey. In recent years, a shocking trend has emerged. While year-round residents that prosper around humans, like robins, starlings, and blackbirds, have increased their numbers and distribution over the last thirty five years, forest songbirds have declined severely. The decline has been greatest among long-distance migrants such as thrushes, orioles, tanagers, catbirds, vireos, buntings, and warblers. These birds nest in North American temperate woodland forests in the summer, but spend their winters in South or Central America or the Caribbean Islands.
In many areas of the eastern and midwestern United States, more than three-quarters of the migrant songbird species have declined significantly. Rock Creek Park in Washington, D.C., for example, has lost 90 percent of its long distance migrants in the last 20 years. Studies of radar images from National Weather Service stations in Texas and Louisiana indicate that only about half as many birds fly over the Gulf of Mexico each spring compared to numbers in the 1960s. This suggests a loss of about half a billion birds in total, a devastating loss.
The culprit responsible for this widespread decline appears to be habitat fragmentation and loss.
Habitat fragmentation. United States forests big enough to support the nesting of woodland songbirds are being carved into ever-smaller patches, and this is having a major negative impact on the breeding of the songbirds. Many of the most threatened species are adapted to deep woods and need an area of 25 acres or more per pair to breed and raise their young. As woodlands are broken up by roads and developments, it is becoming increasingly difficult to find enough contiguous woods to nest successfully.
By providing a large refuge for the breeding of many species of woodland songbird, the undisturbed woods in the southwest corner of St. Louiss Forest Park play a small but important role in saving our nations songbirds. The almost rabid zeal with which local conservation groups oppose even the smallest infringement of Forest Parks woodland bird habitat seems more than justified, viewed in this light.
Habitat loss. A second and perhaps even more important factor in the decline of our songbirds seems to be the availability of critical winter habitat in Central and South America. The birds are typically crowded into dense overwintering populations in the limited areas that have the right kinds of food. All winter the birds eat, gaining weight which will be important on the long flight back to North American breeding grounds the following spring.
The availability of high-quality winter food is critical. Studies of the American redstart by Peter Marra and Richard Holmes of Dartmouth College and Keith Hobson of the Canadian Wildlife Service clearly indicate that birds with better winter habitat have a superior chance of successfully migrating back to their breeding grounds in the spring. Fully 65% of the birds fortunate enough to acquire sites in wetland forest maintained or gained weight over the winter, while birds forced to settle for less desirable scrub habitat lost up to 11% of their body mass over the winter.
Back in New Hampshire the next spring, the researchers found that the first redstarts to arrive at the breeding grounds from Jamaica and Honduras were coming from the superior wetland habitat.
As spring wears on, stragglers from the scrub overwintering habitats begin to belatedly arrive. Careful census by the research team confirmed that the birds that wintered in the substandard scrub left later in the spring on the long flight to northern breeding grounds, arrived at their summer homes later, and as a consequence had fewer young.
Thus, loss of wetland habitat in the Caribbean and Central America is also contributing to the decline of our migratory songbirds. As the best wetland habitat disappears, overwintering birds fare poorly, and this leads to population declines. Unfortunately, the Caribbean lost about 10% of its wetland forest in the 1980s, and continues to lose about 1% a year.
Any hope of saving our birds is going to require a serious effort to preserve both breeding and overwintering habitats. The future of Americas songbirds lies in the dogged determination of conservationists like those guarding Forest Parks woodlands. We should all applaud them.