3. The nocturnal migration of birds is apparently preceded by a resting or feeding pause during which there are few migrants in the air. It is not to an important degree a non-stop continuation of flights begun in the daylight. 4. Nightly migrational activity in North America varies from hour to hour according to a definite temporal pattern, corresponding to the _Zugunruhe_ of caged European birds, and expressed by increasingly heavy flights up until the hour before midnight, followed by a pronounced decline. 5. The visible effects of the time pattern are subject to modification at a particular station by its location with respect to the resting areas from which the night's flight originates. 6. Quantitative and directional studies have so far failed to prove that nocturnal migrants favor narrow, topographically-determined flight lanes to an important degree. 7. Flight densities on the east coast of Mexico, though of first magnitude, have not yet been demonstrated in the volume demanded by the premise that almost all migrants returning to the United States from regions to the south do so by coastal routes. 8. Heavy flights have been recorded from the northern coast of Yucatán under circumstances leading inevitably to the conclusion that birds migrate across the Gulf of Mexico in considerable numbers. 9. There is reason to believe that the importance of the Florida Peninsula as an April and May flyway has been over-estimated, as regards the numbers of birds using it in comparison with the numbers of birds using the Mexican and Gulf routes. 10. The amount of migration is apparently seldom sufficient to produce heavy densities of transient species on the ground without the operation of concentrative factors such as ecological patterns and meteorological forces. 11. The absence or scarcity of transients in some areas in fine weather may be explained by this consideration. 12. A striking correlation exists between air currents and the directional flight trends of birds, suggesting that most night migrants travel by a system of pressure-pattern flying. LITERATURE CITED ALLEN, R. P., AND R. T. PETERSON 1936. The hawk migrations at Cape May Point, New Jersey. Auk, 53:393-404. ANONYMOUS 1936-1941. Tables of computed altitude and azimuth. U. S. Navy Department Hydrographic Office. U. S. Govt. Printing Office, Washington, D. C., vols. 3-5. 1941. Airway meteorological atlas for the United States. Weather Bureau Publ. 1314. U. S. Dept. Commerce, Washington, D. C. 1945-1948. The American air almanac. U. S. Naval Observatory. U. S. Govt. Printing Office, Washington, D. C., 3 vols., issued annually.