Like other migratory Catharus species, juvenile Swainson’s Thrushes (Catharus ustulatus swainsoni) molt into a ‘First Basic’ plumage at the end of their first summer, which they wear for one year before attaining their ‘Adult Basic’ plumage. The partial molt that results in First Basic plumage includes some or all of the median coverts (the little olive feathers above the numbered row), and up to five of the (inner) greater coverts, which are numbered 1–9 in the image. In this case, the bird molted only one greater covert: #1, the fresh olive covert without a buffy tip. The retained juvenile coverts (2–9) are paler and more brown, and numbers 2–6 end with a buffy tip. This is called a molt limit, and it helps us to figure out the age of a bird. For all the juicy details of molt limits in North American passerines, check out Peter Pyle’s Identification Guide to North American Birds, Part 1, 1997.
This bird died in a window collision in Montgomery Co., PA, in late September 2016. After taking the photo, I prepared the specimen for the Ornithology Collection at the Academy of Natural Sciences of Drexel University. Legend: primaries (P1–8); secondaries (S1–9). The 9th and reduced 10th primaries are not visible in the photo.