Large populations of deer are edging out forest birds in North America, report scientists in this month's issue of the journal Biological Conservation. The study is the first to evaluate the impact deer grazing can have on nest quality and food resources in areas unaffected by human activities such as forestry or hunting. It also offers general rules for predicting the influence these animals could have on bird ecosystems in the future.

The decline of forest birds has been blamed mostly on such factors as disease, loss of habitat and an increase in the number of animals that prey on bird nests. But according to biologist Sylvain Allombert of the Center for Functional Evolution and Biology in Montpellier, France, and colleagues, few studies have considered the overabundance of deer, whose populations are reaching historic peaks. The white-tailed deer population, for example, is ecologically excessive in 73 percent of its range in North America, and other deer species tip the scales in up to 41 percent of their range. These animals can devastate a forest understory, which is used by some birds for nesting and also serves as a home to insects, worms and other invertebrates that birds rely on for food.

Allombert's team examined the relationship of deer and forest birds on six islands in the Haida Gwaii archipelago, a chain of about 350 islands off the coast of British Columbia. Here, the Sitka black-tailed deer thrives, having been introduced by colonists in the late 19th century. To obtain a solid comparison, the researchers studied islands with a range of deer history: two of the islands had no deer at all, two had deer populations for about 20 years and two had deer for more than 50 years. They also surveyed the birds that were typically dependent on the forest understory, such as warblers, wrens, sparrows, woodpeckers and hummingbirds, and ranked their dependence on the vegetation. Lastly, the scientists took into consideration an existing study of 31 islands that detailed the impact of deer on vegetation.

The team found that the more a bird species relied on the forest understory for nesting and food, the more it was adversely affected by a sizable deer population. For example, on the islands browsed by deer for more than 50 years, bird abundance was 55 percent to 70 percent lower than on the deer-free islands. For those species that had the highest dependence on forest-floor plants, the numbers were dramatic. The fox sparrow and the rufous hummingbird, for instance, were common on deer-free islands but missing on the islands with a long browsing history.

The study has implications for understanding bird populations in such regions and for managing deer abundance. "These trends, when put together with results from this and previous studies, underline the potential role of deer abundance as a factor explaining negative population trends in forest songbirds, a role probably still under-estimated," the authors write. And by monitoring the understory and keeping tabs on bird populations, biologists will have a better means for regulating deer numbers.