A 18-cm-long bird with a stout, relatively short, pointed bill and sharply pointed tail feathers; breeding male is black below, with a buff to whitish hind-neck, white scapulars, and white rump; early spring male has pale feather edgings; breeding female is buffy with dark streaks on the back, rump, sides, and head; juvenile resembles breeding female but lacks streaks below; fall adults and immatures resemble breeding female but are darker above and richer buff below (NGS 1983). Strongly sexually and seasonally dimorphic with respect to body mass; mean body weights: breeding females, 29.1 g; breeding males, 33.9 g; migrating females, 39.9 g; migrating males, 51. g (Martin and Gavin 1995).