by Nick Bolgiano
How have Pennsylvania’s breeding bird populations changed in recent years? The state’s second Breeding Bird Atlas will help to answer this question, but the Breeding Bird Survey (BBS) is particularly valuable for this purpose. The BBS is a collection of routes, each run once during the breeding season, usually during early June. Each route consists of 50 stops one-half mile apart and all birds observed within a three-minute period at each stop are noted. U.S. Fish and Wildlife Service assesses the status of North American birds from the data collected on approximately 2000 routes.
The accompanying graphs give a visual summary of the BBS data within Pennsylvania and the surrounding region. They allow you to see bird distributions, where changes have occurred, and the rate of these changes within the state. Each graph has a map from the first atlas period, 1983-89, and from a recent seven-year period, 1998-2004, as well as a trend line of the mean Pennsylvania birds per route. On the maps, symbols of different type and color represent different densities and appear for routes run three or more times within that time period. Symbols are plotted at route starting points. If starting points of multiple routes are close together, the symbol representing the higher density is shown on top. If a range next to a colored symbol in a map's Mean/route table is given as, say >0-4, this means that the average count is greater than zero and less than or equal to 4.
As you look through the maps, you will notice several patterns. One is that Pennsylvania is a keystone state for a number of species, in that abundance drops off sharply north or south of the state. For some species, particularly forest birds, abundance is much higher within Pennsylvania compared to other places within the region. Conversely, some species are much more abundant just outside Pennsylvania’s borders.
The majority of Pennsylvania’s forest birds appear to be increasing. Among these, the boreal birds whose distributions follow the Appalachians south from their northern heartland have generally fared very well. For species that have special habitat requirements, a route with high density may be close to other routes with much lower density. For such species, be cautious in making conclusions about increases, as the addition of a few routes can have a large influence. Black-throated Blue Warbler may be an example of this.
For several forest birds, decreases in BBS counts have occurred within a broad belt from northwestern Pennsylvania to the Adirondacks. Acid deposition has been blamed in the decline of Wood Thrush within this area. Data for Least Flycatcher, Eastern Wood-Pewee, and Veery suggest possibly similar trends.
Birds of farmlands and of early successional habitat have generally fared less well than the forest birds. Ring-necked Pheasant and Northern Bobwhite either no longer occur as naturally reproducing populations in many locations or are in much lower densities than during the first atlas. Others, such as Red-winged Blackbird, Vesper Sparrow, and Brown Thrasher, dropped sharply in number during the 1970s and early 1980s, but now appear as if they are stable in number.
You can see the great success of some species. Canada Goose is spreading from southeast and northwest, Red-bellied Woodpecker and Tufted Titmouse continue their surge northward, while Eastern Bluebird and Tree Swallow appear to have benefited from more nest boxes being deployed.
These graphs contain much more information than is described here. See for yourself.
711 W. Foster Ave.
State College, PA 16801
BBS maps in taxonomic order:
Great Blue Heron
Great Horned Owl
Great Crested Flycatcher
Northern Rough-winged Swallow
Black-throated Blue Warbler
Black-throated Green Warbler