_Field marks._--The Tennessee warbler has no prominent wing-bars and no very conspicuous field marks. It might be mistaken for one of the small vireos, but its bill is much more slender and acute. The male has a gray crown, a light line over the eye, and a dusky line through it; the upper parts are bright olive-green and the under parts grayish white. The female has a greener crown and more yellowish under parts. For more details, see the descriptions of plumages. _Fall._--The fall migration starts early in August, but is quite prolonged, many birds lingering in the northern States until early in October and in the southern States all through that month. During some seasons and at certain places the Tennessee warbler is exceedingly abundant, sometimes far outnumbering any other species, but it is very variable in its abundance. Mr. Trautman (1940) says that at Buckeye Lake, Ohio, "during some years not more than 20 individuals could be recorded in a day in the southward migration, nor more than a 100 in the season. In other years the bird rivaled the Myrtle Warbler in numbers, and as many as 1,000 individuals could be seen in a day and several thousands during a migration. * * * Throughout the southward migration the species did not confine itself to the upper sections of the taller trees as in spring, but was found in almost equal numbers in smaller trees and brushy thickets, in bushes and saplings along fence rows, and in weedy fields." Professor Cooke (1904) says of the fall migration route: "The principal line of migration is from the Mississippi Valley across the Gulf of Mexico to Mexico and Central America. The eastern part of this route probably extends from the southern end of the Alleghenies across northwestern Florida to the coast of Yucatan and Honduras." A. H. Howell (1932), however, gives several records for central and southern Florida, and says: "In autumn, Weston reports a large migration on October 26 and 27, 1925, when 31 birds were killed at the lighthouse near Pensacola on the two nights, and large numbers seen on the morning of October 26 in vacant lots in the city." Dickey and van Rossem (1938) say of the migration in El Salvador: During the fall migration of 1925, Tennessee warblers arrived in the vicinity of Divisadero on October 13. No advance guard, that is, individuals arriving ahead of the main flights, was observed in this case. On the above-mentioned date they were suddenly found to be present in numbers, and from then on were common in every lowland or foothill locality visited. In point of relative abundance this was by far the most common warbler (resident or migratory) throughout the coastal plain and in the foothills, but it was greatly outnumbered by _Dendroica virens_ above 3,000 feet.