I. The data.
Over the last 30 years, a marked decline has been noted in the abundance of those birds which overwinter in the tropics, while other bird groups (permanent residents, short-distance migrators) have remained essentially constant.
Data which shows these patterns has been organized and presented on the North America Breeding Bird Survey, found through the web link:
The important question to now ask is why?
II. Destruction of wintering grounds.
As the birds which show this effect most are those who spend winter in the tropics, some began to wonder if deforestation in wintering grounds could be related to loss of tropical forest communities.
Points which argue in favor of this is the fact that many our migratory songbirds live over a much smaller area in the winter than summer. It is therefore easier to destroy their winter habitat than their summer.
Over time, the theory that loss of tropical migrant birds was related to tropical deforestation became seen as the only answer.
Of course, nothing is ecology is so easy...................
Loss of nestlings happens in two ways:
1) Nest predation by scavenger birds, domestic cats, etc.
2) Nest parasitism by cowbirds, who push eggs out of nests, and lay their own in it, leaving their egg to be raised by the other bird couple.
Both of these processes are far more frequent when nesting occurs in small, isolated forest stands.
Because of this, many birds may have ceased reproduction in many of the most fragmented parts of the upper Midwest (southern Wisconsin, Illinois).
III. The answer?
Both factors are certainly important. We do not know yet which is the
most important, but it is clear that not all of the responsibility for songbird
loss lies at the feet of tropical nations.
Created 2 September 2011, Last Update 02 September 2011
Not an official UW Green Bay site