The parasitic birdie on the top part of this photo is called a Brown-headed Cowbird. This little guy/lady finds other birds' nests and replaces one of the eggs from the nest with its own. This way, the baby Brown Headed Cow Bird grows up around other species of chicks, sustaining itself on their food. Eventually, it grows up and continues the cycle of mating and swapping its eggs with other birds' eggs.
At first, it may seem like this bird is kind of a jerk. But in reality, it helps balance the population of other birds. Nature has giving him/her a place in the world for a reason.
All of this knowledge thanks to my bird enthusiast coworker John Loftus, who one day grabbed me and showed me this particular bird in the photo above, I like to call Marvin, outside our office.