Although sometimes birds in the spring, in order to line their nests, help themselves with small pieces of the soft undercoat that large ungulate mammals like deer shed naturally, this does not appear to be the case here. The birds appear to be one male and two female Brown-headed Cowbirds (Molothrus ater), and these birds do not build their nests, they have the dastardly practice of individually laying their eggs in the nests of other songbirds, usually the smaller ones. .
The Cowbird chick tends to hatch earlier, and it usually outshines its reluctant little “siblings” and Cowbird parents are freed from the rigors of nurturing and raising their own young.
These birds are therefore not looking for nesting material. Instead, they probably use the mule deer (Odocoileus hemionus californicus) as some kind of mobile hunting stand or portable viewing platform.
Although the bulk of their diet is derived from the seeds of grasses, grains and weeds, around 25% of a cowherd’s diet is usually made up of insects like cranes, grasshoppers, beetles, etc. Cowbirds are believed to have evolved alongside the large herds of bison that once roamed. the Great Plains, and birds took advantage of the movement of animals through the prairie, which often caused insects to fly or jump and expose them to the beak of a hungry cowherd.
Cowbirds continue to be associated with cattle and horses, using mammals to reveal insects that lurk in pastures. This mule deer doe probably serves the same purpose, in this case providing both front row perch and flushing services, surprising insects to hide when the doe walks in tall grass.