This open cup bird nest (probably a Song Sparrow) has been parasitized by a cowbird. Notice the larger egg at the top of the photo.
The female cowbird scouts out the nest, and then sneaks in and removes one of the host's eggs and then quickly lays a replacement egg, which the host then raises.
Cowbirds seem to prefer open cup nests, and the nests of other birds that also lay speckled eggs. Eggs do show up in nestboxes also, especially if the entrance hole has been enlarged.
You cannot begin to preserve any species of animal unless you preserve the habitat in which it dwells. Disturb or destroy that habitat and you will exterminate the species as surely as if you had shot it. So conservation means that you have to preserve forest and grassland, river and lake, even the sea itself. This is vital not only for the preservation of animal life generally, but for the future existence of man himself—a point that seems to escape many people.
-Gerald Durrell, The Nature Conservancy
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