On Oct 13, 2011, at 9:37 AM, Roger Applegate wrote:Having "permanent" routes that are used year after year can cloud that fact that changes in habitat specific to the route can show a declining trend while all that has happened is that the birds have moved to another location. This location could just be far enough from the route that they are no longer detectable on the route, but they are still there and trend may really be stable or increasing.
This is certainly true for a single route, but when you composite the data from a large number of routes these local effects increasingly average out. I deliberately excluded species that were only found on a small number of routes specifically to avoid this problem, as I stated repeatedly in my write-ups. Anyone who attempts to extrapolate any larger trend from any study at any single location is on very shaky ground; there is nothing unique to a single BBS route in this regard. There is the major bias that these are roadside routes, hence they will reflect habitat changes in areas that are accessible by road and do not sample large roadless tracts. This likely exaggerates the effects of direct human impacts via land use changes.
The BBS has flaws, as does any real world dataset. That's life in the natural sciences. But what do we have for long-term trends in such a large suite of species over such a large area that is better? Christmas Bird Count data make the BBS look precise and flawless by comparison! We have to work with the data we have.
In the question of Tennessee versus continental trends in the article Than posted: The overarching pattern is similar -- increases in most species, especially woodland species. Raptors doing well, neotropical migrants doing so-so. The species-by-species details vary, which is not surprising when you consider that their "eastern" region includes Florida and Maine as well as Tennessee! They also found substantial increases in wetland species; I did not feel that most of these were sampled well enough in Tennessee to make meaningful conclusions.
Bill Pulliam Hohenwald TN
