Reprinted from a Dec 2, 2007 column in the Morgantown Dominion Post
Every year the National Audubon Society coordinates two major bird counts. Note that these counts are not a census. A census is a true count of the total population and this is rare as wildlife goes. You may say, "why can’t you count all the birds?" But when you think about it, the fact that we can’t count every bird in a species shouldn’t be a surprise. If we can’t do a true census on human beings who have social security numbers, addresses, pay taxes, etc., then how can we expect to make total counts of birds?
So, for most wildlife species, we get indexes to the population, and comparing these from one year to the next gives us trend data. The Audubon Society’s Christmas Bird Count and the spring Breeding Bird Survey are giving us trends that for certain species are on the decline. These counts show that the populations of many songbirds have dropped by 50% over the past 40 years. Birds that have declined are many warbler species, vireos, flycatchers, thrushes, crows and others.
Apparently there are several reasons for this decline. Take the wood thrush for example. A study done at Cornell University suggests that acid rain is the culprit. They note that wood thrush declines are 5 percent per year in areas of the Northeast where acid rain deposits are high. The suggestion is that acid rain cause’s calcium depletion in the soil and this impact’s earthworms, snails, millipedes and other insects that are mainstays in the diet of the wood thrush.
Another factor is the loss of large areas of forests, leaving fragmented wood lots. Apparently some bird species will not nest in the fragmented forests we are creating in the name of progress. Take our own area for example. Our forests have been dissected for housing developments, malls, golf courses, etc. Not too many years ago as you drove to Pittsburgh along Interstate 79, between Washington and the exit to the Airport, there were large expanses of forests. Today they are rapidly disappearing into industrial sites, roads, condos, etc.
And any time one discusses Neotropical migrants (the warblers, vireos, flycatchers, thrushes and others), the loss of their wintering tropical rain forests comes to the fore. Present ongoing research continues to determine the full extent of the loss of rain forests on our birds.
Another cause of bird declines is West Nile Virus (WNV) that came on the scene in 2002-2003. Some species were hit especially hard. Crows dropped by 45 percent, but have now rebounded a bit. Chickadees became locally extinct, but the last few years their numbers have crept back up.
West Nile Virus also hit robins and cardinals, but these two species develop antibodies against the disease, and their numbers are also coming back. (Interestingly crows and chickadees do not develop any immunity to West Nile Virus). While on the subject of this virus, let me mention some results from another new study on the carriers of WNV. Turns out that raccoons, opossums, foxes, and skunks have high infection prevalence for WNV. Apparently when these carriers have high prevalence rates for WNV, the local birds suffer. For example in Ohio more than 60 percent of the raccoons had antibodies for WNV. This correlated to bird problems in the same area.
Most of the above research is tied directly to the Audubon bird surveys. These surveys are important because they are done year after year, using the same methods. Thus, they provide trends on bird numbers, and help us to assess causes for declines when they occur.
