Birders often associate autumn with drabness once the colorful breeding plumage of spring birds has been replaced by the muted and confusing appearance of nonbreeding and first-year birds in the fall. While this may often be the case, e.g. in warblers, it is not always true. Warbling Vireo (Vireo gilvus), a familiar and widespread Massachusetts breeder, tends to show its brightest yellowish wash in the fall, after it molts to basic plumage in July and August (Gardali and Ballard 2020), which generally occurs when the similar looking Philadelphia Vireo (Vireo philadelphicus) is migrating south through New England, an overlap that can make identification challenging.

In ideal conditions, with good lighting and sustained views, pretty much every Warbling and Philadelphia vireo should be separable in the field (Figure 1). However, this does not always line up with the reality of birds flitting about high up in trees or in deep thickets. Many of the crucial field marks for the two species can be impacted by angles and lighting, so identification should be handled with care in circumstances where views are brief or poor. Some birds may not be identifiable to species.