I put together this matrix of variation in breeding plumage Rose-breasted Grosbeaks, showing the differences between each of the four age-sex classes. In this species it’s relatively easy to distinguish males from females, a little harder to distinguish one-year-old males from older males, and nearly impossible to determine the age of females.
As in many songbirds, the key to aging is to look for the more worn, faded, and drabber wing feathers retained from juvenile plumage (grown in the nest the previous summer). These feathers contrast with the more recently grown wing covert feathers and innermost tertials.
Also like most songbirds, we can’t determine the age of a Rose-breasted Grosbeak after its first year. They replace all of their feathers in late summer in a complete molt, and from that point on are in the “adult” cycle and look essentially the same for the rest of their life.
Link to a one page pdf
This is a great resource, thank you!
Do you know if males keep their summer plumage all year round or molt on their breeding grounds? I’ve been wondering about this for quite some time.
Males molt in late summer into a slightly drabber winter plumage, with black-and-white wings and a rosy breast but most of the feathers have brownish fringes. They don’t molt again in spring. The pale brownish feather edges simply wear off by spring to reveal the clean black of the spring/summer plumage.