Bluebird Population History in the U.S.
The following article was written by Linda Edwards, UT Arboretum Society Journal The Leaflet Editor, and originally published in the Volume 23, Number 4 issue of The Leaflet.
As most of you know there continues to be a sharp decline in most species of birds in recent years. The Eastern Bluebird numbers have declined also. The apparent reasons for this decline are multiple, are not well understood, and many involve the activities of man. However, not all of the Bluebird population decline can be attributed to man's activities. Records show that when severe winters hit the eastern United States, Bluebird population numbers plummet. During the winters of 1894-95, 1939-40, 1950-51 and in 1957-58 the east was hit hard with severe winter conditions. Many bluebirds died during these periods, however the natural potential growth rate of Bluebird populations is high and many can recover from such population losses within a few years. As many as six pairs of Bluebirds may nest per acre if nesting sites are available and in Tennessee three or four clutches may be reared per season.
The impact of man's activities on top of these natural population losses compound the problem. Documented causes of population decline are the clearing of trees, especially dead and dying ones, and manicured property. Although Bluebirds inhabit fields and open woodlands, their typical foraging strategy is to perch and then attack, so an area with suitable perches is preferred. One of the primary stressors for Bluebird populations is the lack of nesting holes since they are cavity nesters only. This lack of suitable nesting sites has been exacerbated by the use of metal fence posts. Those old semi-rotten posts were ideal nesting sites. House cats have also proved to be lethal predators.
Competition for available nesting sites is another apparent cause of Bluebird decline. The importation of exotic bird species has also affected the Bluebird nesting site availability. Bluebird have always been in competition with native species for nesting sites. Chickadees, nuthatches, wrens, and tree swallows are cavity nesters too and compete with the bluebird for any available hollow. But with the importation of the house sparrow in 1851 and the European starling in 1890 from Europe, competition for nesting sites reached a different level. House sparrows begin nesting early in the season and often take the nesting sites first. Starling do nest later in the season, but are so aggressive and larger than Bluebirds that it is a simple task for a starling to evict a nesting Bluebird from any nest the starling may want for themselves.
So where are the Bluebirds today? Come find them at the Arboretum. We have lots of them. Why? Because there are nesting boxes positioned in appropriate places and appropriate densities to encourage the rebounding population. The trick to prevent the starling from taking over the nest box. This is easy. The access hole in the box needs to be 1 ½ inches in diameter. The starling is to big to enter. Exit the starling. House sparrows are an altogether different problem. They can enter the small hole and must be cleaned out of the nest before they get established. This takes diligence where house sparrows are a problem.
If you want to learn more about Bluebirds, the North American Bluebird Society is a good place to start. Cornell University also sponsors a nation wide 'Bluebird Network' which allows private citizens to establish Bluebird trails and participation includes gathering data on nesting parameters such as clutch size, number of young fledged, predation, parasites, and number of clutches per season.
Check out these web sites: www.nabluebirdsocietv.orq and http://cornell.edu/birdhouse/
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