Chapter VIII: Part II: The Nature of Nocturnal Migration (3)

The evidence adduced from the present study gives support to the hypothesis that the continental pattern of spring migration in eastern North America is regulated by the movement of air masses. The clockwise circulation of warm air around an area of high pressure provides, on its western edge, tail winds which are apparently favorable to northward migration. High pressure areas exhibit a centrifugal force outward from the center, which may tend to disperse the migratory flight originating at any given point. In contrast, the circulation of air in the vicinity of a low pressure area is counterclockwise with the force tending to be directed inward toward the center. Since the general movement of the air is from the high pressure area toward a low pressure area, birds starting their migrations with favorable tail winds, are often ultimately carried to a region where conditions are decidedly less favorable. In the vicinity of an area of low pressure the greater turbulence and high wind velocities, combined with the possibly slightly less buoyant property of the air, cause birds to descend. Since low pressure areas in spring generally precede cold fronts, with an attending shift of the wind to the north, an additional barrier to the northward migration of birds is imposed. The extreme manifestation of low pressure conditions and the manner in which they operate against bird flight, are associated with tropical hurricanes. There, the centripetal force of the wind is so great that it appears to draw birds into the "eye" of the hurricane. A classic example of this effect is seen in the case of the birds that came aboard the "West Quechee" when this vessel passed through the "eye" of a hurricane in the Gulf of Mexico in August, 1927. I have already discussed the details of this incident in a previous paper (1946:192). There is also the interesting observation of Mayhew (1949), in which a similar observation was made of large numbers of birds aboard a ship passing through one of these intense low-pressure areas.

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.

CONCLUSIONS

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
Yucatan 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,
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ANONYMOUS
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1945-1948. The American air almanac. U. S. Naval Observatory.
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1948_a_. Meteorological and climatological data for April 1948.
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1948_b_. Meteorological and climatological data for May 1948.
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BAGG, A. M.

1948. Barometric pressure-patterns and spring migration.
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BERGMAN, G.

1941. Der Fruhlingszug von _Clangula hyemalis_ (L.) und
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BRAY, R. A.

1895. A remarkable flight of birds. Nature (London), 52:415.

CARPENTER, F. W.

1906. An astronomical determination of the height of birds
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CHAPMAN, F. M.

1888. Observations on the nocturnal migration of birds.
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DAVIS, L. I.

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F. [ARNER], D. [ONALD] S.

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GATES, W. H.

1933. Hailstone damage to birds. Science, 78:263-264.

HOWELL, A. H.

1932. Florida bird life. Florida Department Game and Fresh Water
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LANSBERG, H.

1948. Bird migration and pressure patterns. Science, 108:708-709.

LIBBY, O. G.

1899. The nocturnal flight of migratory birds. Auk, 16:140-146.

LOWERY, G. H., JR.

1945. Trans-Gulf spring migration of birds and the coastal
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MAYHEW, D. F.

1949. Atmospheric pressure and bird flight. Science, 109:403.

OVERING, R.

1938. High mortality at the Washington Monument. Auk, 55:679.

PALMGREN, P.

1944. Studien ueber die Tagesrhythmik gekaefigter Zugvoegel.
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POUGH, R. H.

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RENSE, W. A.

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SIIVONEN, L.

1936. Die Staerkevariation des Naechtlichen Zuges bei _Turdus ph.
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SPOFFORD, W. R.

1949. Mortality of birds at the ceilometer of the Nashville
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STEBBINS, J.

1906. A method of determining height of migrating birds.
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STEVENS, LLOYD A.

1933. Upper-air wind roses and resultant winds for the eastern
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STONE, W.

1906. Some light on night migration. Auk, 23:249-252.

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THOMSON, A. L.

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VAN OORDT, G.

1943. Vogeltrek. E. J. Brill, Leiden, xii + 145 pp.

VERY, F. W.

1897. Observations of the passage of migrating birds across the
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WALTERS, W.

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WILLIAMS, G. G.

1941-1948. The season: Texas coastal region. Audubon Mag., 43-50.

1945. Do birds cross the Gulf of Mexico in spring? Auk,
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WINKENWERDER, H. A.

1902_a_. The migration of birds with special reference to nocturnal
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Transmitted June 1, 1949.

[]
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Transcriber's Notes

With the exception of the typographical corrections detailed below
and some minor corrections for missing periods or extra punctuation
(item 28 in List of Figures), the text presented here is that
contained in the original printed version. A transcription of the
Data presented in Figure 12 was added to illustrate the information
contained on that sheet. Some text was moved to rejoin paragraphs.
The list of UK publications was moved to the end of the document.

In writing variables for formulae, superscripted characters are
shown using a caret (^). So, X squared would be X^2. Subscripts are
shown using an underscore. Carbon dioxide is CO_2. Where several
superscript or subscript character(s) are required or to aid in
clarity, they are placed in braces (ex., H_{2}O for water and
[theta]_{Npt.} for theta degrees from the North point).

Emphasis Notation

_Text_ = Italics

Typographical Corrections

Page Correction

385 flght => flight
394 diargrams => diagrams
404 Determinaton => Determination
411 obsever => observer
419 Morover => Moreover
425 Mississippii => Mississippi
425 a => as
430 at => and
431 inserted "a"
("...traveling along a certain topographic feature...")
442 concensus => consensus
472 Stephens, Loyd A. => Stevens, Lloyd A.