In these photos, the bird on the right is a baby cowbird begging for food. The bird on the left, the smaller one, is its parent, a sparrow.
These great photos are from a blog called Back Yard Biology.
How did a sparrow come to parent a cowbird? Cowbirds lay their eggs in other birds’ nests:
[Cowbirds] feed on insects, including the large numbers that may be stirred up by cattle. In order for the birds to remain mobile and stay with the herd, they have adapted by laying their eggs in other birds’ nests. The cowbird will watch for when its host lays eggs, and when the nest is left unattended, the female will come in and lay its own eggs. The female cowbird may continue to observe the nest after laying her eggs. If the cowbird egg is removed, the female cowbird may destroy the host’s eggs (see “Mafia hypothesis” for an explanation of this behavior).
It looks like a cowbird deposited an egg in the nest of the male and female cardinals that live out back (Chuck and Charlene) because they’re working very hard at feeding a big, bruising, noisy! cowbird baby.