The Gray Vireo as a Victim of the Cowbird
The Gray Vireo as a Victim of the Cowbird.-A nest of the Gray Vireo (Vireo vicidor) was located on May 28, 1944, in the Cajon Pass area northwest of San Bernardino, California. The nest was six feet up from the ground near the end of a limb of a pition tree and was suspended from two twigs on separate branches of the limb. This is only the second record that I have for nests in pifions, although I have observed these birds building nests in several different kinds of shrubs in San Bernardino County. A visit was again made to the site on June 4, in company with Fred Frazer, for the purpose of taking some photographs and I then found that the nest contained two eggs of the ’ vireo and one of the Dwarf Cowbird (Molothrus ater obscurus). I remained at the site for some time after taking the photographs and making other observations, but the vireos did not come to the nest. Since the n&t was somewhat disturbed aid the eggs cold, I concluded that the owners had deserted it.
It is believed that this is the first record of the Gray Vireo as a victim of the Cowbird (see Friedmann, Auk, 60, 1943 :350-356 and cited .references) . _
During the past fifteen years it has seemed to me that most of the nests started by this vireo come to some unhappy end, often before the eggs are placed in thein ; at other .times the eggs are destroyed, and in at least one case one egg was removed from a nest. This damage has been blamed on rats, chipmunks, California Jays, or reptiles, but it now seems probable that the Cowbird should receive at least a part of the blame. Since the Gray Vireo often perches on the top of brush or yuccas it is conspicuous, except for its somber color. With loud, repeated calls it flies to its nesting site. We may thus expect that Cowbirds would have no trouble in finding nests of this species.
The eggs from the nest reported above weighed 1.99 and 1.73 grams, respectively, and were more heavily marked than usual. I have found the average weight of thirty-six eggs of the Gray Vireo to be 1.94 grams, with a range from 1.57 to 2.32 grams. The number of eggs in clutches that I feel certain were not disturbed has ranged from three to four, with an average of 3.55.
The weight of the egg of the Cowbird in this nest was 2.42 grams, a little larger than the average of 2.26 grams which I have determiped by weighing 112 eggs, but it is well within the range of 1.65 to 3.03 grams.-WILSON C. HANNA, Colton, California, June 4, 1944.
