Yesterday and then again today I found this mis-matched pair of birds hanging out in the backyard:
The smaller appears to be a male chipping sparrow and the larger, younger bird is a baby cowbird. As I was reading up on both birds I found (which was also confirmed by Amanda) that cowbirds are known to lay eggs in other birds' nests which those birds then raise as their own. I have a bunch of brown-headed cowbirds around here, so I would not be in the least bit surprised if that is exactly what happened here.
It is the most hilarious thing to watch this huge bird following the little one around with its beak wide open chirping for food.