Pedernales Falls State Park Prothonotary Warbler Nest Box Study: Bell's Vireo & Brown-headed Cowbird Egg

Prothonotary Warbler Occurrence In Texas

Central Texas draws the western line for the Prothonotary Warbler range. This species occupies much of eastern North America up to the southern boarder of Ontario Canada.
Prothonotarys' nest in natural cavities but also readily use nest boxes located on or near water. Flooded swamps represent ideal habitat but they will also nest along rivers or small bodies of water.
In May 2007 I saw a brightly colored male Prothonotary Warbler at Pedernales Falls State Park bird blind. After some research I learned male birds have been seen building nests in neighboring counties to Blanco County. Further research revealed a pair of Prothonotary Warblers successfully nested at Heard Wildlife Sanctuary in McKinney, Texas.
In December 2007 I decided to undertake a nest box study at the park. January through February 2008 I hiked the entire Pedernales River and tributaries within park boundaries, in an effort to choose the best sites. Habitat selection proved a challenge do to high flood levels along the river.
Five sights were eventually selected where two nest boxes each were placed. Most boxes were attached to trees however two were attached to metal poles. These boxes were monitored from March through June 2008 on a weekly basis. A Breeding Bird Study was conducted at each nestbox location in 2008 and will be continued in 2009.

Sunday, June 8, 2008

Bell's Vireo & Brown-headed Cowbird Egg

After verifying which egg was the Brown-headed Cowbird's I returned today to remove it from the Blue Grosbeak's nest. The cowbird's egg, pictured here, has speckling on it, and lucky for me the grosbeak's eggs are pale blue/white, an easy distinction in this case. Cowbird nestlings typically out-compete their smaller nest mates for food. This species has increased it's numbers in North America thanks to forest clearing creating more edge for it to search out host nests. So wherever there are threatened species nesting, cowbirds are being actively trapped and nests monitored to remove these eggs.

I started out this morning at the Bird Blind where I saw all the usual nesters, however, in the scrubby patch of land adjacent to the trail I heard a Bell's Vireo. Its song matched my recordings, very fast and bubbly like a House Wren. Later two Bell's Vireos came to the oak tree along the open edge of the bird blind. This is a new species for me in the park. It likes shrublands that include willows and streamsides.

I also verified that the Lark Sparrows are tending young in the nest. Viewing the nest from below, it is rather camouflaged to the human eye.