The nestling in the front that is stretching its neck up, is it a brown-headed cowbird? It appears to have a redder gape (inside of mouth) than the other nestlings. No obvious size difference. (Sorry for bad photo quality)
I would agree with you. As the chick matures, notice if the area around the eyes remains unfeathered for longer than the other chicks. Like other blackbirds, this is a noticeable feature (i.e., an otherwise feathered chick with a prominent “naked” eye ring).
UPDATE: It is a Brown-Headed Cowbird nestling. It has a red mouth and it begs more aggressively than the other nestlings.
I would agree with you. As the chick matures, notice if the area around the eyes remains unfeathered for longer than the other chicks. Like other blackbirds, this is a noticeable feature (i.e., an otherwise feathered chick with a prominent “naked” eye ring).
They’ve all fledged now, the Cowbird actually fledged last, much to my surprise.