The eggs of the cowbird hatch in eleven or twelve days. They average .88 by .65 of an inch, the length varying from .95 to .67 of an inch, and the width varying from .72 to .58 of an inch. The ground is a dingy white or gray, and the markings vary through all the shades of brown, sometimes evenly distributed over the surface, and at other times predominating around the larger end. There is so much diversity in the appearance of different specimens, that frequently the investigator is puzzled in distinguishing the true eggs of the towhee, cardinal, and other species from those of the cowbird.

In the breeding season the male grackles, red-winged blackbirds, and the cowbirds of both sexes, nightly congregate to roost together. Early after the breeding season they form into flocks of from fifty to sixty.

The birds have then finished moulting, and the glossy black of the males has been changed into the duller colors of the females and the young. They assemble with the blackbirds of various species where food is most abundant and easy to be procured.

Late investigations of the food habits of the cowbird indicate that the species is largely beneficial. Prof. Beal showed the food of the cowbird to consist of animal and vegetable matter in the proportion of about twenty-eight per cent. of the latter. Spiders and harmful insects compose almost exclusively the animal food, while weed seeds, waste grain, and a few miscellaneous articles make up the vegetable food. Mr. Silloway thinks "it is not improbable that the so-called insectivorous birds displaced by the cowbird are thus kept in check by this natural agent, and their mission performed by the usurper in directions as helpful as the special functions of the sufferers. We may later come to understand that one cowbird is worth two bobolinks after all."