Red-eyed Vireo - Vireo olivaceus

Red-eyed Vireo has a partial preformative molt, but no prealternate molts. The prebasic molts are unusual in that usually only some coverts, tertials, and rectrices are replaced prior to migration, with the remainder of flight feathers replaced during winter. Differences in colour and wear of feathers by age can be subtle, and especially by late spring and summer, many intermediates should be considered M-FCF (AHY) unless evidence for either FCF (SY) or DCB (ASY) is particularly distinct. Conversely, because of the suspended prebasic molt, some FCF (SY) and DCB (ASY) individuals can be distinguished as late as during fall migration. Sex can generally be determined only by brood patch / cloacal protuberance during the breeding season.

Quick tips

• Check the iris colour - a grayish-brown appearance indicates FCF (HY/SY), whereas from summer to early winter a reddish iris indicates DCB (AHY/ASY), but note that by mid-late winter that colour is also possible on younger birds and can no longer be used to identify older ones. However, some FCF (SY) individuals retain their iris colour longer and can be aged by that criterion as long as it persists.
• Examine the shape of the primary coverts and outer rectrices - on FCF (HY/SY) birds, both are relatively narrow and pointed (and faded by spring/summer), while on DCB (AHY/ASY) birds, both are broad and rounded (and relatively fresh in spring/summer).
• From summer through early winter, examine the upper mandible lining - it is pinkish-gray to white on FCF (HY) birds, and dark gray on DCB (AHY) birds; the juvenile colouration persists into spring for some individuals, and in those cases can be used to age them as FCF (SY).
• From spring through fall migration, look for a molt limit between the tertials and secondaries - on FCF (SY) birds there is often a cline of wear throughout the secondaries and tertials, while on DCB (ASY) birds the tertials may be similarly pale, contrasting with uniformly darker middle secondaries.

Species account prepared by McGill Bird Observatory (2018). Last updated by Marcel Gahbauer (Mar 2022)