This immature bird was photographed at Rancho Naturalista, Costa Rica. It was still being fed by its "parent", a Montezuma Oropendola. The cowbirds are nest parasites and lay their eggs in the nests of other birds who hatch and raise them. Generally the nest parasites kill the true offspring of the parent.

When a friend (Cagan Sekercioglu - here's his photo gallery) looked at the photo above, he thought that it was a White-throated Thrush and sent me a copy of the photo below which is an immature White-throated Thrush that he caught in a mist-net near the town of San Vito in Costa Rica near the Panamanian border. I think the resemblence is pretty amazing. Of course side-by-side, the difference would be obvious; the Giant Cowbird is, in fact, giant.