Here we see the problem the little Chipping Sparrow mother has. She is being followed by two fledgling Cowbirds who think she's their mother. The tragedy is that she thinks she's their mother too. They follow her all of their waking hours. As soon as she finds a morsel of food, they rush up to her and flutter their wings, their way of saying "feed me." In the photograph above, the Sparrow is in the lower-left corner, one of the Cowbirds is sitting on the branch while the other is on the ground in the bottom center of the picture.
Here are the same three birds. The second Cowbird is nearly hidden in the lower-right corner of the picture. Cowbird mothers find nests made by other species. They throw out this bird's eggs and lay their own in their place. The victim mother then hatches the Cowbird's eggs, thinking they are her own. She bonds with the hatchlings and raises them as her own. The tragedy is that the victims expend considerable energy and time raising Cowbirds, but not their own kind. Some species have diminished in numbers because of the Cowbirds parasitic behavior.
Here the two juvenile Cowbirds can be seen clearly.
Here is a different juvenile Cowbird, no doubt also raised by a mother of a different species. It is a little older than the two shown above.
