Birds and Cat Mortality

As Earth Day approaches, I want to focus on the negative impacts that humans are having on our bird populations. Under the auspices of the Migratory Bird Treaty Act, 836 species of native North American birds are protected. Of those species, 78 are on the Endangered Species List and 14 more are listed as Threatened. Another 144 are listed as a Bird of Conservation Concern. Collectively, nearly a quarter of our bird species appear to be declining. The situation is probably even worse than we realize because we have very poor population data on many less common birds that are not currently listed as species of concern.

Some birds perish because of natural causes (for example, weather, predation by birds of prey, mammals or snakes, starvation, disease). But an awful lot of bird mortality can be attributed to human activities.

Without a doubt, the most significant impact humans are having on bird populations (and other flora and fauna as well) stems from habitat destruction or degradation. This habitat alteration has complex effects for migratory birds that require particular habitats for wintering, for stop-overs during migration and for nesting. It is hard to precisely gauge the impacts of habitat destruction on our bird populations. But added to deaths from natural causes, habitat alteration may be pushing some birds over the brink to local or even global extinction.

We have better measures of bird mortality from some human-related sources. Collisions of birds with buildings, mostly from flying into windows, claim nearly a billion birds a year. Collisions with cell phone and television/radio towers are responsible for five to 50 million avian deaths a year. Collisions with high tension power lines results in over 100 million bird deaths annually in the United States. Collisions with cars kill 60 million birds a year.

Over-use of pesticides can result in dramatic localized kills of birds. One recent estimate claimed that 72 million birds each year perish from exposure to pesticides. This estimate is surely low because pesticides may act slowly in a bird, resulting in death at a distance from a pesticide hot spot. Pesticides may cause neurological problems for birds, leading to death from starvation.

Cats are a potent source of mortality for birds. They have had devastating effects on some oceanic islands, leading to the extinction of some birds. In North America, we really do not have a good handle on the magnitude of the effects of cats on our birds. A recent study in Wisconsin demonstrated that domestic cats in the countryside kill 39 million birds in that state each year.

A recent article by Victoria Sims and her colleagues published in the journal Diversity and Distributions addresses the impact of cats on birds in urban areas in Britain. Only about 3% of cats in Britain remain indoors at all times.

Controversy reigns in Britain about the impact of cats on bird populations. One study claimed that cats account for a third of the deaths of local bird populations. On the other hand, skeptics argue that no conclusive studies have been done to demonstrate significant impacts of cat predation other than on oceanic islands.

When conducting a predator-prey study, an ecologist often begins by documenting the relationship between predator numbers and prey numbers. One expects to see a negative relationship: when predators are scare, prey density should be high; when predators are abundant, few prey will be found. In the field, such relationships between a cat predator and prey abundance have been found for lions, tigers and lynx. Domestic cats are a bit more complicated because they get some of their nutrition from the cat food that their owners provide.

In the Sims study, bird population numbers were obtained from Breeding Bird Survey (BBS) squares (each 1 km on a side) chosen randomly from across Britain. They focused their attention on 30 squares within urban areas. Within each of these urban BBS squares, they censused cats by door-to-door or telephone surveys.

The ornithologists found a positive relationship between the number of cats and the numbers of individual birds (the more cats, the more birds). This result suggests that cats are not having a significant predatory effect on birds. However, the authors claim that cats may have a strong impact in all of their study squares. The lowest cat density in a BBS square was 132 cats. One might expect to see higher bird densities in an urban study square with no cats. So, this study has shone some light on the controversy of the importance of cat predation on birds but is far from the last word.

[originally published on April 7, 2008]