Song Sparrow has a partial to incomplete (rarely complete) preformative molt, but lacks prealternate molts. Because the preformative molt can be complete and many intermediates are difficult to age in spring, caution should be exercised with ageing unless retained juvenile feathers are visible. Sex can be determined only by brood patch / cloacal protuberance during the breeding season.
Juvenile plumage is very similar to that of Lincoln's and Swamp Sparrows; Song Sparrow tends to have a proportionately longer tail, and distinctly wide malar stripe.
- Look at the tail - uniformly narrow and pointed rectrices OR a mix of narrow and broad rectrices are indicative of FCF (HY/SY), while uniformly broad and more rounded rectrices are NOT useful as they are common among both FCF (HY/SY) and older age classes.
- In fall, examine the iris in good light - it is grayish-brown in FCF (HY) birds, compared to chestnut-brown in DCB (AHY) birds.
- Look at the inner secondaries - a contrast between them and more faded outer secondaries is indicative of FCF (HY/SY), while a uniform appearance is typical of DCB (AHY) until mid-fall, but thereafter should only be considered evidence of M-FCF (AHY) because a minority of individuals may undergo a complete preformative molt.
- Check the outer primary coverts - they are relatively narrow and pointed with minimal gray edging on FCF (HY/SY) birds, and relatively broad and rounded with reddish-brown edging on DCB/M-FCF (AHY) birds.
Species account prepared by McGill Bird Observatory (2016). Last updated by Marcel Gahbauer (Mar 2022)