The migration is also prolonged or very irregular, for according to the dates of departure given to me by Alexander F. Skutch (see under _Winter_), the last of these warblers do not leave Central America until the very last of April, or the first of May, after the first arrivals have reached New England; some of these records, however, may apply to one of the western races. After the birds reach the United States, the migration fans out northward and northeastward and seems to be more rapid. Of this Frederick C. Lincoln (1939) says: "Coming north from the Tropics these birds reach New Orleans about April 5, when the average temperature is 65° F. Travelling on northward much faster than does the season, they reach their breeding grounds in Manitoba the latter part of May, when the average temperature is only 47°. Encountering progressively colder weather over their entire route, they cross a strip of country in the 15 days from May 11 to 25 that spring takes 35 days to cross. This 'catching up' with spring is characteristic of species that winter south of the United States and of most of the northern species that winter in the Gulf States." _Territory._--Soon after their arrival on their breeding grounds the males begin to select their territories and then to defend them. Dr. S. Charles Kendeigh (1941) made a study of the territories of birds in a prairie community in northwestern Iowa, and writes: A special study of the Yellow Warbler indicated that territorial requirements included suitable nest-sites, concealing cover, tall singing posts, feeding areas in trees, and space, and that when certain of these factors were lacking, territorial relations became confused and the behavior of the birds was modified. * * * These warblers possessed territories that averaged about 150 feet in diameter, or approximately two-fifths of an acre. Even in locations where trees were included, the territories appeared to be of about the same size. The limits of the territory often did not coincide with the boundaries of the thicket in which the nest was located but extended over the neighboring grassland and often included parts of neighboring thickets. These territories were defended by the males partly by singing, although in shrubby areas lacking trees they were handicapped by lack of singing posts from which to proclaim their ownership and to advertise themselves. A few made use of fences from which to sing and also of tall posts and wire from an abandoned electric line that extended through the area. The rôle of the female in defense of territory was not determined.