"What's a bigger threat to migratory birds in the Western Hemisphere — loss of habitat, or 'outdoor' cats? And how much of a problem are cats to migratory birds, anyway?"
This is an excellent question — and the answer to it depends on the scale which you are considering.
At the hemispheric scale (in other words, at the scale of entire flyways of migratory birds), there is no doubt that habitat loss — coupled with associated factors like habitat degradation and fragmentation — is by far the biggest threat to migratory birds.
The loss, degradation and fragmentation of habitat is happening in all parts of migratory birds’ ranges, including breeding, wintering and migratory stopover areas.
However, at more local scales — such as that of a park, forest or other area with a distinct population of birds — many other factors come into play, some of which may be as big a threat if not bigger than habitat loss. One of these factors is elevated levels of predation on birds, including adults, young and nests.
In many areas, particularly those in or near cities and suburbs, levels of both native and non-native predators are greater than scientists feel was the case prior to colonization. And among non-native predators, the domestic cat is clearly the most common and widespread.
Estimates of the number of birds domestic cats kill every year are imprecise — but the total maybe as many as several hundred million birds annually in the United States alone. The toll on other native wildlife, such as small mammals, may be far higher.
Given the high number of pet, stray and feral cats in the United States, this dynamic is a serious contributor to the loss of migratory birds, at least in some areas. We recommend strongly that all cat owners keep their cats indoors — or, if outside, in an enclosure or on a leash.
Solving the problem of the loss of migratory birds is something that everyone can contribute to. And paying attention to your cat is one small step on the road to conservation success.
David Mehlman is the director of the Conservancy's Migratory Bird Program.