Walking through one of our pastures (near Lookout CA – Modoc County) after a light snow, Leonard and I watched an American goldfinch (Spinus tristis) searching for seeds. American goldfinches are agile and acrobatic, even hanging upside down to pluck seeds from weeds, grasses and wildflower remnants sticking above the snow. The seeds are plentiful this year so Mr. Goldfinch is not going hungry – at least while the snow is not deep.
Watching the bird eat, I thought about the diet of a nestling American goldfinch. American goldfinches are strict vegetarians. The only insects in their diet are those accidentally ingested while eating seeds.
The mother American goldfinch fills her crop with seeds and regurgitates the seeds once back at the nest. By regurgitating portions of the seeds in her crop, she can assure that every nestling gets fed. The regurgitated, partially digested seeds are called “milk”.
Brown-headed cowbirds (Molothrus ater) parasitize the nests of other bird species. The cowbird lays its eggs in the nest of another bird and then leaves. The host birds then raise the cowbird chicks along with their own. This strategy may work with other birds, but not the American goldfinch. Cowbird nestlings in goldfinch nests rarely survive more than a couple days because the goldfinch “milk” does not contain enough protein. Cowbirds are not adapted to a completely vegetarian diet.
Carduelis tristis is a synonym for Spinus tristis.