Forests are a dominant habitat type in North America and, with 310 breeding bird species, are one of the most rich habitats for birds. And looking at the overall forest birds indicator from the U.S. State of the Birds Report, it appears that things are going well in this habitat, at least in this decade — population levels appear to have almost reached their 1968 levels.
However, the report team decided early on to analyze this habitat in more detail — and we found indicators for the different U.S. forest subtypes that show very clear differences among these types and some cause for concern.
We assigned each species to one of four different forest regions in the country: Eastern, Western, boreal, and subtropical forests.
- Eastern forests occur in roughly the eastern half of the United States; these forests are primarily deciduous.
- Western forests are found in the western half of the United States along the Pacific Coast and in the mountains; they are primarily coniferous.
- The Boreal forest occurs at high latitudes in North America; in the United States, it is found in Alaska and to a limited extent in the lower 48 in northern Minnesota, Michigan, New York and New England.
- Subtropical forests are extremely restricted in the United States, being found only in very southern Florida and in south Texas along the Rio Grande River.
Western and boreal forest birds show relatively stable populations, with current levels essentially the same as in 1968. However, Eastern forest birds show a distinct and continual decline through the present day (although there is the suggestion of some stabilization in recent years). There was insufficient data to construct a useable indicator for subtropical forest birds. Clearly, problems contributing to the continued decline of Eastern forest birds are masked when all forest birds are lumped together.
What is contributing to the declining state of Eastern forest birds? While a relatively high proportion of species in the indicator show declining trends, I see three broad sub-groups of Eastern forest birds that are declining for distinct reasons.
- The first group are those species associated with southeastern U.S. pine forests. Examples include the Red-cockaded Woodpecker (endangered), Brown-headed Nuthatch, and Bachman’s Sparrow.
- The second group are species found in mature deciduous forest, such as Eastern Wood-Pewee and Cerulean Warbler.
- The final group, and the one with probably the most declining species, is composed of those species associated with “second-growth” habitats, those that occur after disturbance and last only a short time. Some examples of the many species in this group are Brown Thrasher, Golden-winged Warbler, Prairie Warbler, and Field Sparrow.
The specific factors contributing to the declines of each of these three sub-groups are complex and perhaps best left for future blog posts. Nevertheless, the forest birds indicator illustrates how complex a single habitat type can actually be. The State of the Birds analyses clearly indicate significant conservation issues confronting our Eastern forest birds in the present day. We can only hope that these problems do not spread to the other forest types before we have a chance to address them.