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The Black-tailed Gnatcatcher and the Dwarf Cowbird

Wilson C. Hanna

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It was not until May 25, 1933, that I found a nest. of the Black-tailed Gnatcatcher (Polioptila melanura californica) in this vicinity parasitized by the Dwarf Cowbird (Molothrus ater obscurus). The nest was four feet from the ground and near the top of a black sage bush growing on a dry hillside, a few miles east of Riverside, California, the type locality for this gnatcatcher. The nest was typical and without the ornaments on the outside which the Western Gnatcatcher always uses. There were four eggs of the owner and one of the parasite; they were fresh and the weight in grams respectively, 1.11 (the largest I have taken), 1.06, 1.03, 1.01, and 2.83. The average of thirty-eight eggs of this gnatcatcher that I have weighed is 0.99 gram and the smallest but 0.82 gram. Another nest, was found in the same vicinity on the same day with three eggs, and when I visited it on May 27, one of the eggs had a hole in it for which I blamed a cowbird.

It must be that the Black-tailed Gnatcatcher has not suffered from the cowbirds to any great extent in any locality, as Friedmann in his book “The Cowbirds” fails to mention them as victims. In this locality one of the probable reasons for their escape has been due to their usual habitat being on the dry bush-covered hillsides or dry washes between 700 and 2000 feet elevation above sea level. Another reason is that they nest rather early, April 10 and May 30 being my records of extreme dates.

The usual complement of eggs of this gnatcaticher is four and I have never seen any more. The brooding bird can often be touched while on the nest.

Colton, California, November 28, 1933

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