The song of the blue-winged warbler is one of the high-frequency songs; Albert E. Brand (1938) gives the approximate mean as 7,675, the highest note about 8,050 and the lowest note about 7,125 vibrations per second; this compares with an approximate mean for the black-poll warbler of 8,900 vibrations per second, the highest frequency of any of the wood warblers, and an average for all passerine birds of about 4,000 vibrations per second. _Field marks._--A small warbler with a greenish olive back, yellow forehead and under parts, with a black line through the eye and two white wing bars, is a blue-winged warbler. The female is merely more dull in coloration than the male, and the young even duller. The hybrids between this and the golden-winged warbler are more puzzling, but in a general way they can be recognized; a nearly typical blue-winged warbler with a black throat is probably a Lawrence's warbler; and a golden-winged warbler without a black throat or cheek and with a variable amount of white and yellow on the under parts and in the wing-bars, is probably a Brewster's warbler. But there is an immense amount of individual variation between the two species, due to frequent crossing. _Fall._--Most of the blue-winged warblers move southward during August and September, though a few may linger in the southern part of the breeding range into October. Professor Cooke (1904) says: "Most of the individuals of the species migrate across the Gulf of Mexico, apparently avoiding Florida on the east and Texas and Vera Cruz on the west, as there is no record of the occurrence of this warbler in fall in Texas, and but one in Florida--that of a bird taken at Key West August 30, 1887." But this remains to be proved. Alexander F. Skutch writes to me: "This is another very rare migrant in Central America. It has been recorded only a few times in Guatemala and apparently not at all in Costa Rica. I have seen it only once, on the Finca Mocá, Guatemala, on October 30, 1934." Very little seems to be known about its winter distribution and still less about its winter habits. DISTRIBUTION _Range._--Eastern United States to Panamá.