Warbling Vireo - Vireo gilvus

Warbling Vireo has a partial preformative molt and limited prealternate molts that are generally limited to body feathers and difficult to detect. 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 a specific age is particularly distinct. Sex can generally be determined only by brood patch / cloacal protuberance during the breeding season.

Warbling and Philadelphia Vireos can be very similar in appearance; Warbling Vireo can generally be distinguished by having pale gray lores, and with any yellow on the underparts being brightest on the sides and flanks.

Quick tips

• Look at the outer rectrices - on FCF/FCU (HY/SY) birds they are narrow and pointed, while on DCB/DCU (AHY/ASY) birds they are more broad and rounded.
• From summer through early winter, check the upper mandible lining - on FCF/FCU (HY/SY) birds it is pinkish to whitish, while on DCB/DCU (AHY/ASY) birds it is usually gray.
• Examine the primary coverts, which on FCF/FCU (HY/SY) birds are somewhat paler than the greater coverts, while on DCB/DCU (AHY/ASY) both feather tracts are similarly greenish-gray in appearance.

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