I have a bird feeder and over the past few weeks, there has been a bird - I think it is some kind of a medium-sized sparrow or finch - that acts like a baby bird wanting to be fed by a junco.
It follows the junco around making noise, fluffing its wings and opening its mouth, and the junco will give it a seed. They sit on the feeder doing the same behavior. The junco is clearly not this other bird's mother, or is it?
I think we can say the junco is the bird's adoptive mother. The baby is most likely a young cowbird. Cowbirds lay their eggs in other birds' nests and leave the job of raising their offspring to them.
Most of the time the parents will oblige, even though it can be unsettling to see a baby bird that is bigger than its parent.