Philadelphia Vireo - Vireo philadelphicus

Philadelphia Vireo has a partial preformative molt, but is believed to have no prealternate molts. Age can generally be determined by plumage, although by spring some individuals may appear sufficiently intermediate that they should be considered M-FCF (AHY) unless multiple characteristics indicate a particular age. Determination of sex is limited to brood patch / cloacal protuberance during the breeding season.

Philadelphia and Warbling Vireos can be very similar in appearance; Philadelphia Vireo can generally be distinguished by having contrastingly dark gray lores, and with yellow across the underparts, brightest on the centre of the breast.

Quick tips

• Look at the outer rectrices - on FCF (HY/SY) birds they are narrow and pointed, while on DCB (AHY/ASY) birds they are more broad and rounded.
• From summer through early winter, check the upper mandible lining - on FCF (HY/SY) birds it is pinkish to whitish, while on DCB (AHY/ASY) birds it is usually gray.
• Examine the primary coverts, which on FCF (HY/SY) birds are somewhat paler than the greater coverts, while on DCB (AHY/ASY) both feather tracts are similarly greenish-gray in appearance.
• Check for p10, which is 2-5 mm shorter than the primary coverts on FCF (HY/SY) birds, and even shorter or entirely absent on DCB (AHY/ASY) birds.

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