Chestnut-sided Warbler - Setophaga pensylvanica

Chestnut-sided Warbler has a partial preformative molt, and partial prealternate molts that include some to all greater coverts, and occasionally up to two tertials. Older females and younger males can overlap in aspects of appearance, making it important to determine age before sex; for some intermediate individuals, age and especially sex may not be reliably determined.

Quick tips

• Check the primary coverts - on FCF/FCA (HY/SY) individuals they are brownish-gray with little or no edging, contrasting with darker and fresher greater coverts, whereas on DCB/DCA (AHY/ASY) individuals the primary and greater coverts are more uniformly dark, with distinct edging.
• Look at the tail - the outer rectrices tend to be relatively narrow and tapered to a point on FCF/FCA (HY/SY) birds, but more broad and rounded on DCB/DCA (AHY/ASY) birds.
• Examine the flanks - DCB/DCA (AHY) males have distinct and extensive chestnut markings, somewhat more so than on FCA (SY) males, while DCB/DCA (AHY) females, FCA (SY) females, and some FCF (HY) males have limited chestnut, and FCF (HY) females lack chestnut entirely.
• In alternate plumage (spring/summer), look at the lores and moustache - on males they are black, while on females they are largely grayish.

Species account prepared by McGill Bird Observatory (2016). Last updated by Marcel Gahbauer (Mar 2022)