This Blackbird is a general feeder, eating insects, apparently in preference, and wild seed. They derive their name of Cow Blackbird from their keeping about that animal, and finding, either from her parasitic insects or her droppings, opportunities for food. They feed on the ground, and occasionally scratch for insects. At the South, to a limited extent, they frequent the rice-fields in company with the Redwinged Blackbird.
Mr. Nuttall states that if a Cow Blackbird’s egg is deposited in a nest alone it is uniformly forsaken, and he also enumerates the Summer Yellowbird as one of the nurses of the Cowbird. In both respects I think he is mistaken. So far from forsaking her nest when one of these eggs is deposited, the Red-eyed Vireo has been known to commence incubation without having laid any of her own eggs, and also to forsake her nest when the intrusive egg has been taken and her own left. The D. æstiva, I think, invariably covers up and destroys the Cowbird’s eggs when deposited before her own, and even when deposited afterwards.
The Cow Blackbird has no attractions as a singer, and has nothing that deserves the name of song. His utterances are harsh and unmelodious.
In September they begin to collect in large flocks, in localities favorable for their sustenance. The Fresh Pond marshes in Cambridge were once one of their chosen places of resort, in which they seemed to collect late in September, as if coming from great distances. There they remained until late in October, when they passed southward.
Mr. Ridgway only met with this species in two places, the valley of the Humboldt in September, and in June in the Truckee Valley. Their eggs were also obtained in the Wahsatch Mountains, deposited in the nest of Passerella schistacea, and in Bear River Valley in the nest of Geothlypis trichas.
Mr. Boardman informs me that the Cow Blackbird is a very rare bird in the neighborhood of Calais, Me., so much so that he does not see one of these birds once in five years, even as a bird of passage.
The eggs of this species are of a rounded oval, though some are more oblong than others, and are nearly equally rounded at either end. They vary from .85 of an inch to an inch in length, and from .65 to .70 in breadth. Their ground-color is white. In some it is so thickly covered with fine dottings of ashy and purplish-brown that the ground is not distinguishable. In others the egg is blotched with bold dashes of purple and wine-colored brown.
On the Rio Grande the eggs of the smaller southern race were found in the nests of Vireo belli, and in each of the nests of the Vireo pusillus found near Camp Grant, Arizona, there was an egg of this species. At Cape St. Lucas, Mr. Xantus found their eggs in nests of the Polioptila melanura. We have no information in regard to their habits, and can only infer that they must be substantially the same as those of the northern birds.
The eggs of the var. obscurus exhibit a very marked variation in size from those of the var. pecoris, and have a different appearance, though their colors are nearly identical. Their ground-color is white, and their markings a claret-brown. These markings are fewer, smaller, and less generally distributed, and the ground-color is much more apparent. They measure .60 by .55 of an inch, and their capacity as compared with the eggs of the pecoris is as 33 to 70,—a variation that is constant, and apparently too large to be accounted for on climatic differences.