Although the forces associated with an ordinary low-pressure area are by no means as intense as those associated with a tropical hurricane, the forces operating are much the same. Consequently birds conceivably might tend to be drawn toward a focal point near the center of the low, where the other factors already mentioned would tend to precipitate the entire overhead flight. Visible evidence of migration would then manifest itself to the field ornithologists.
1. Telescopic counts of birds passing before the moon may be used to determine reliable statistical expressions of the volume of migration in terms of direction and of definite units of time and space.
2. Night migrants fly singly more often than in flocks, creating a remarkably uniform dispersion on a local scale throughout the sky, quite unlike the scattered distributions observable in the daytime.
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.