E. Migration And Meteorological ConditionsThe belief that winds affect the migration of birds is an old one. The extent to which winds do so, and the precise manner in which they operate, have not until rather recently been the subject of real investigation. With modern advances in aerodynamics and the development of the pressure-pattern system of flying in aviation, attention of ornithologists has been directed anew to the part that air currents may play in the normal migrations of birds. In America, a brief article by Bagg (1948), correlating the observed abundance of migrants in New England with the pressure pattern obtaining at the time, has been supplemented by the unpublished work of Winnifred Smith. Also Landsberg (1948) has pointed out the close correspondence between the routes of certain long-distance migrants and prevailing wind trajectories. All of this is basis for the hypothesis that most birds travel along definite air currents, riding with the wind. Since the flow of the air moves clockwise around a high pressure area and counterclockwise around a low pressure area, the birds are directed away from the "high" and toward the center of the "low." The arrival of birds in a particular area can be predicted from a study of the surrounding meteorological conditions, and the evidence in support of the hypothesis rests mainly upon the success of these predictions in terms of observations in the field.From some points of view, this hypothesis is an attractive one. It explains how long distances involved in many migrations may beaccomplished with a minimum of effort. But the ways in which winds affect migration need analysis on a broader scale than can be made from purely local vantage points. Studies of the problem must be implemented by data accumulated from a study of the process in action, not merely from evidence inferred from the visible results that follow it. Although several hundred stations operating simultaneously would surely yield more definite results, the telescopic observations in 1948 offer a splendid opportunity to test the theory on a continental scale.The approach employed has been to plot on maps sector vectors and vector resultants that express the directional trends of migration in the eastern United States and the Gulf region, and to compare the data on these maps with data supplied by the U. S. Weather Bureau regarding the directions and velocities of the winds, the location of high and low pressure areas, the movement of cold and warm fronts, and the disposition of isobars or lines of equal pressure. It should be borne in mind when interpreting these vectors that they are intended to represent the directions of flight only at the proximal ends, or junction points, of the arrows. The tendency of the eye to follow a vector to its distal extremity should not be allowed to create the misapprehension that the actual flight is supposed to have continued on in a straight line to the map location occupied by the arrowhead.A fundamental difficulty in the pressure-pattern theory of migration has no doubt already suggested itself to the reader. The difficulty to which I refer is made clear by asking two questions. How can the birds ever get where they are going if they are dependent upon the whim of the winds? How can pressure-pattern flying be reconciled with the precision birds are supposed to show in returning year after year to the same nesting area? The answer is, in part, that, if the wind is a major controlling influence on the routes birds follow, there must be a rather stable pattern of air currents prevailing from year to year. Such a situation does in fact exist. There are maps showing wind roses at 750 and 1,500 meters above mean sea level during April and May (Stevens, 1933, figs. 13-14, 17-18). Similarly, the "Airway Meteorological Atlas for the United States" (Anonymous, 1941) gives surface wind roses for April (Chart 6) and upper wind roses at 500 and 1,000 meters above mean sea level for the combined months of March, April, and May (Charts 81 and 82). The same publication shows wind resultants at 500 and 1,000 meters above mean sea level (Charts 108 and 109). Further information permitting a description in general terms of conditions prevailing in April and May is found in the "Monthly Weather Review" covering these months (cf.Anonymous, 1948a, Charts 6 and 8; 1948b, Charts 6 and 8).Fig. 38.Over-all sector vectors at major stations in the spring 1948. See text for explanation of system used in determining the length of vectors. For identification of stations, seeFigure 34.Fig. 39.Over-all net trend of flight directions at stations shown inFigure 38. The arrows indicate direction only and their slants were obtained by vector analysis of the over-all sector densities.First, however, it is helpful as a starting point to consider the over-all picture created by the flight trends computed from this study. InFigure 38, the individual sector vectors are mapped for the season for all stations with sufficient data. The length of each sector vector is determined as follows: the over-all seasonal density for the station is regarded as 100 percent, and the total for the season of the densities in each individual sector is then expressed as a percentage. The results show the directional spread at each station. InFigure 39, the direction of the over-all vector resultant, obtained from the sector vectors on the preceding map, is plotted to show the net trend at each station.As is evident from the latter figure, the direction of the net trend at Progreso, Yucatán, is decidedly west of north (N 26° W). At Tampico this trend is west of north (N 11° W), but not nearly so much so as at Progreso. In Texas, Louisiana, Georgia, Tennessee, and Kentucky, it is decidedly east of north. In the upper Mississippi Valley and in the eastern part of the Great Plains, the flow appears to be northward or slightly west of north. At Winter Park, Florida, migration follows in general the slant of the Florida Peninsula, but, the meager data from Thomasville, Georgia, do not indicate a continuation of this trend.It might appear, on the basis of the foregoing data, that birds migrate along or parallel to the southeast-northwest extension of the land masses of Central America and southern Mexico. This would carry many of them west of the meridian of their ultimate goal, obliging them to turn back eastward along the lines of net trend in the Gulf states and beyond. This curved trajectory is undoubtedly one of the factors—but certainly not the only factor—contributing to the effect known as the "coastal hiatus." The question arises as to whether this northwestward trend in the southern part of the hemisphere is a consequence of birds following the land masses or whether instead it is the result of some other natural cause such as a response to prevailing winds. I am inclined to the opinion that both factors are important. Facts pertinent to this opinion are given below.In April and May a high pressure area prevails over the region of the Gulf of Mexico. As the season progresses, fewer and fewercold-front storms reach the Gulf area, and as a result the high pressure area over the Gulf is more stable. Since the winds move clockwise around a "high," this gives a general northwesterly trajectory to the air currents in the vicinity of the Yucatán Peninsula. In the western area of the Gulf, the movement of the air mass is in general only slightly west of north, but in the central Gulf states and lower Mississippi Valley the trend is on the average northeasterly. In the eastern part of the Great Plains, however, the average circulation veers again slightly west of north. The over-all vector resultants of bird migration at stations in 1948, as mapped inFigure 39, correspond closely to this general pattern.Meteorological data are available for drawing a visual comparison between the weather pattern and the fight pattern on individual nights. I have plotted the directional results of four nights of observation on the Daily Weather Maps for those dates, showing surface conditions (Figures40,42,44and46). Each sector vector is drawn in proportion to its percentage of the corresponding nightly station density; hence the vectors at each station are on an independent scale. The vector resultants, distinguished by the large arrowheads, are all assigned the same length, but the nightly and average hourly station densities are tabulated in the legends under each figure. For each map showing the directions of flight, there is on the facing page another map showing the directions of winds aloft at 2,000 and 4,000 feet above mean sea level on the same date (see Figures41-47). The maps of the wind direction show also the velocities.Unfortunately, since there is no way of analyzing the sector trends in terms of the elevations of the birds involved, we have no certain way of deciding whether to compare a given trend with the winds at 2,000, 1,000, or 0 feet. Nor do we know exactly what wind corresponds to the average or median flight level, which would otherwise be a good altitude at which to study the net trend or vector resultant. Furthermore, the Daily Weather Map illustrates conditions that obtained at 12:30 A. M. (CST); the winds aloft are based on observations made at 10:00 P. M. (CST); and the data on birds covers in most cases the better part of the whole night. Add to all this the fact that the flight vectors, their resultants, and the wind representations themselves are all approximations, and it becomes apparent that only the roughest sort of correlations are to be expected.However, as will be seen from a study of the accompanying maps (Figures40-47), the shifts in wind direction from the surface up to 4,000 feet above sea level are not pronounced in most of the instancesat issue, and such variations as do occur are usually in a clockwise direction. All in all, except for regions where frontal activity is occurring, the weather maps give a workable approximation to the average meteorological conditions on a given night.The maps (Figures40-47) permit, first, study of the number of instances in which the main trend of flight, as shown by the vector resultant, parallels the direction of wind at a reasonable potential mean flight elevation, and, second, comparison of the larger individual sector vectors and the wind currents at any elevation below the tenable flight ceiling—one mile.On the whole, inspection of the trend of bird-flight and wind direction on specific nights supports the principle that the flow of migration is in general coincident with the flow of air. It might be argued that when the flow of air is toward the north, and when birds in spring are proceeding normally in that direction, no significance can be attached to the agreement of the two trends. However, the same coincidence of wind directions and bird flights seems to be maintained when the wind currents deviate markedly from a northward trajectory. Figures46and47, particularly in regard to the unusual slants of the flight vectors at Ottumwa, Knoxville, and Memphis, illustrate that this coincidence holds even when the wind is proceeding obliquely eastward or westward. On the night of May 22-23, when a high pressure area prevailed from southern Iowa to the Atlantic coast, and the trajectory of the winds was northward, migration activity at Knoxville and Ottumwa was greatly increased and the flow of birds was again northward in the normal seasonal direction of migration.Further study of the data shows fairly conclusively that maximum migration activity occurs in the regions of high barometric pressure and that the volume of migration is either low or negligible in regions of low pressure. The passage of a cold-front storm may almost halt migration in spring. This was demonstrated first to me by the telescopic method at Baton Rouge, on April 12, 1946, following a strong cold front that pushed southeastward across the Gulf coastal plain and over the eastern Gulf of Mexico. The winds, as usual, shifted and became strong northerly. On this night, following the shift of the wind, only three birds were seen in seven hours of continuous observation. Three nights later, however, on April 15, when the warm air of the Gulf was again flowing from the south, I saw 104 birds through the telescope in two hours. Apropos of this consideration in the 1948 data are the nights of May 21-22 and 22-23.Fig. 40.Comparison of flight trends and surface weather conditions on April 22-23, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on April 23. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:5. Louisville: 9,100 (1,100)16. College Station: 13,300 (1,900)6. Murray: 16,300 (2,700)17. Baton Rouge: 6,200 (1,000)8. Stillwater: 1,900 (500)19. Lafayette: 2,800 (600)9. Knoxville: 15,200 (1,700)21. Winter Park: 6,200 (700)13. Oak Grove: 13,600 (1,700)23. Tampico: 11,100 (3,700)Fig. 41.Winds aloft at 10:00 P. M. on April 22 (CST).Winds at 2,000 feet above mean sea level are shown in black; those at 4,000 feet, in white. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 40.Fig. 42.Comparison of flight trends and surface weather conditions on April 23-24, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on April 24. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:1. Albion: 1,100 (300)14. Mansfield: 4,900 (1,200)2. Ottumwa: 5,500 (900)16. College Station: 700 (100)4. Lawrence: 5,400 (1,400)17. Baton Rouge: 1,700 (400)5. Louisville: 13,300 (2,700)18. Pensacola: migration negligible6. Murray: 9,800 (1,400)20. New Orleans: 1,600 (800)8. Stillwater: 800 (100)21. Winter Park: 2,700 (300)9. Knoxville: 8,000 (900)23. Tampico: 63,600 (6,300)10. Memphis: 7,900 (1,000)24. Progreso: 31,300 (3,900)Fig. 43.Winds aloft at 10:00 P. M. on April 23 (CST). Winds at 2,000 feet above mean sea level are shown in black; those at 4,000 feet, in white. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 42.Fig. 44.Comparison of flight trends and surface weather conditions on April 24-25, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on April 25. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:1. Albion: migration negligible12. Rosedale: 1,100 (100)2. Ottumwa: 4,600 (1,500)14. Mansfield: 1,700 (400)3. Columbia: 1,400 (400)18. Pensacola: migration negligible5. Louisville: 1,700 (200)21. Winter Park: 600 (100)10. Memphis: 6,600 (900)24. Progreso: 27,300 (3,000)Fig. 45.Winds aloft at 10:00 P. M. on April 24 (CST).Winds at 2,000 feet above mean sea level are shown in black; those at 4,000 feet, in white. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 44.Fig. 46.Comparison of flight trends and surface weather conditions on May 21-22, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on May 22. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:2. Ottumwa: 6,900 (1,400)13. Oak Grove: 5,800 (800)5. Louisville: 1,500 (200)14. Mansfield: 2,500 (800)9. Knoxville: 3,200 (500)18. Pensacola: migration negligible10. Memphis: 7,000 (1,200)21. Winter Park: 1,200 (200)Fig. 47.Winds aloft at 10:00 P. M. on May 21 (CST). Winds at 2,000 feet above mean sea level are shown. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 46.On the first night, following the passage of a cold front, migration at Ottumwa was comparatively low (6,900 birds in five hours). On the following night, when the trajectory of the winds was toward the north, the volume of migration was roughly twice as high (22,300 birds in eight hours). At Louisville, on May 21-22, the nightly station density was only 1,500 birds in seven hours, whereas on the following night, it was 8,400 birds in the same length of time, or about six times greater.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.CONCLUSIONS1.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 theZugunruheof 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 produceheavy 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 CITEDAllen, R. P., and R. T. Peterson1936. The hawk migrations at Cape May Point, New Jersey. Auk, 53:393-404.Anonymous1936-1941. 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.1948a. Meteorological and climatological data for April 1948. Monthly Weather Review, April 1948, 76:65-84, 10 charts.1948b. Meteorological and climatological data for May 1948. Monthly Weather Review, May 1948, 76:85-103, 11 charts.Bagg, A. M.1948. Barometric pressure-patterns and spring migration. Auk, 65:147.Bergman, G.1941. Der Fruhlingszug vonClangula hyemalis(L.) undOidemia nigra(L.) bei Helsingfors. Eine Studie über Zugverlauf und Witterung sowie Tagesrhythmus und Flughöhe. Ornis Fennica, 18:1-26.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 during nocturnal migration. Auk, 23:210-217.Chapman, F. M.1888. Observations on the nocturnal migration of birds. Auk, 5:37-39.Davis, L. I.1936-1940. The season: lower Rio Grande Valley region. Bird-Lore (now Audubon Mag.), 38-42.F. [arner], D. [onald] S.1947. Studies on daily rhythm of caged migrant birds (review of Palmgren article). Bird-Banding, 18:83-84.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 Fish, Tallahassee, 1-579 + 14 pp., 58 pls., 72 text figs.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 hiatus. Wilson Bull., 57:92-121.1946. Evidence of trans-Gulf migration. Auk, 63:175-211.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 über die Tagesrhythmik gekäfigter Zugvögel. Zeitschrift für Tierpsychologie, 6:44-86.Pough, R. H.1948. Out of the night sky. Audubon Mag., 50:354-355.Putkonen, T. A.1942. Kevätmuutosta Viipurinlakdella. Ornis Fennica, 19:33-44.Rense, W. A.1946. Astronomy and ornithology. Popular Astronomy, 54:55-73.Scott, W. E. D.1881a.Some observations on the migration of birds. Bull. Nuttall Orni. Club, 6:97-100.1881b.Migration of birds at night. Bull. Nuttall Orni. Club, 6:188.Siivonen, L.1936. Die Stärkevariation des Nächtlichen Zuges beiTurdus ph. philomelosBrehn undT. musicusL. auf Grund der Zuglaute geschätz und mit der Zugunruhe einer gekäfigten Singdrossel Verglichen. Ornis Fennica, 13:59-63.Spofford, W. R.1949. Mortality of birds at the ceilometer of the Nashville airport. Wilson Bull., 61:86-90.Stebbins, J.1906. A method of determining height of migrating birds. Popular Astronomy, 14:65-70.Stevens, Loyd A.1933. Upper-air wind roses and resultant winds for the eastern United States. Monthly Weather Review, Supplement No. 35, November 13, pp. 1-3, 65 figs.Stone, W.1906. Some light on night migration. Auk, 23:249-252.1937. Bird studies at Old Cape May. Delaware Valley Orni. Club, Philadelphia, Vol. 1, 1-520 + 15 pp., pls. 1-46 and frontis.Thomson, A. L.1926. Problems of bird migration. Houghton Mifflin Company, Boston.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 lunar disc on the nights of September 23 and 24, 1896. Science, 6:409-411.Walters, W.1927. Migration and the telescope. Emu, 26:220-222.West, R. H.1896. Flight of birds across the moon's disc. Nature (London), 53:131.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, 62:98-111.1947. Lowery on trans-Gulf migration. Auk, 64:217-238.Winkenwerder, H. A.1902a. The migration of birds with special reference to nocturnal flight. Bull. Wisconsin Nat. Hist. Soc., 2:177-263.1902b. Some recent observations on the migration of birds. Bull. Wisconsin Nat. Hist. Soc., 2:97-107.Transmitted June 1, 1949.23-1020squareUNIVERSITY OF KANSAS PUBLICATIONSThe University of Kansas Publications, Museum of Natural History, are offered in exchange for the publications of learned societies and institutions, universities and libraries. For exchanges and information, address theExchange Desk, University of Kansas Library, Lawrence, Kansas, U. S. A.Museum Of Natural History.—E. Raymond Hall, Chairman, Editorial Committee.This series contains contributions from the Museum of Natural History.Cited as Univ. Kans. Publ., Mus. Nat. Hist.Vol. 1.(Complete) Nos. 1-26. Pp. 1-638. August 15, 1946-January 20, 1951.Vol. 2.(Complete) Mammals of Washington. By Walter W. Dalquest. Pp. 1-444, 140 figures in text. April 9, 1948.Vol. 3.1.The avifauna of Micronesia, its origin, evolution, and distribution. By Rollin H. Baker. Pp. 1-359, 16 figures in text. June 12, 1951.2.A quantitative study of the nocturnal migration of birds. By George H. Lowery, Jr. Pp. 361-472, 47 figures in text. June 29, 1951.Transcriber's NotesWith 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 inFigure 12was added (seebelow) to illustrate the information contained on that sheet. Some text was moved to rejoin paragraphs.There are two notes in the original text indicating that the images for Figures41and45were transposed. The correct images have been placed with the captions and the two notes were removed. Lastly, the cover image was compiled from a copy of the original cover with two of the graphics contained in the article added and the list of UK pulications was moved to the end of the document.Typographical CorrectionsPageCorrection385flght ⇒ flight394diargrams ⇒ diagrams404Determinaton ⇒ Determination411obsever ⇒ observer419Morover ⇒ Moreover425Mississippii ⇒ Mississippi425a ⇒ as430at ⇒ and431inserted "a"("…traveling along a certain topographic feature…")442concensus ⇒ consensus472Stephens, Loyd A. ⇒ Stevens, Lloyd A.
E. Migration And Meteorological Conditions
The belief that winds affect the migration of birds is an old one. The extent to which winds do so, and the precise manner in which they operate, have not until rather recently been the subject of real investigation. With modern advances in aerodynamics and the development of the pressure-pattern system of flying in aviation, attention of ornithologists has been directed anew to the part that air currents may play in the normal migrations of birds. In America, a brief article by Bagg (1948), correlating the observed abundance of migrants in New England with the pressure pattern obtaining at the time, has been supplemented by the unpublished work of Winnifred Smith. Also Landsberg (1948) has pointed out the close correspondence between the routes of certain long-distance migrants and prevailing wind trajectories. All of this is basis for the hypothesis that most birds travel along definite air currents, riding with the wind. Since the flow of the air moves clockwise around a high pressure area and counterclockwise around a low pressure area, the birds are directed away from the "high" and toward the center of the "low." The arrival of birds in a particular area can be predicted from a study of the surrounding meteorological conditions, and the evidence in support of the hypothesis rests mainly upon the success of these predictions in terms of observations in the field.
From some points of view, this hypothesis is an attractive one. It explains how long distances involved in many migrations may beaccomplished with a minimum of effort. But the ways in which winds affect migration need analysis on a broader scale than can be made from purely local vantage points. Studies of the problem must be implemented by data accumulated from a study of the process in action, not merely from evidence inferred from the visible results that follow it. Although several hundred stations operating simultaneously would surely yield more definite results, the telescopic observations in 1948 offer a splendid opportunity to test the theory on a continental scale.
The approach employed has been to plot on maps sector vectors and vector resultants that express the directional trends of migration in the eastern United States and the Gulf region, and to compare the data on these maps with data supplied by the U. S. Weather Bureau regarding the directions and velocities of the winds, the location of high and low pressure areas, the movement of cold and warm fronts, and the disposition of isobars or lines of equal pressure. It should be borne in mind when interpreting these vectors that they are intended to represent the directions of flight only at the proximal ends, or junction points, of the arrows. The tendency of the eye to follow a vector to its distal extremity should not be allowed to create the misapprehension that the actual flight is supposed to have continued on in a straight line to the map location occupied by the arrowhead.
A fundamental difficulty in the pressure-pattern theory of migration has no doubt already suggested itself to the reader. The difficulty to which I refer is made clear by asking two questions. How can the birds ever get where they are going if they are dependent upon the whim of the winds? How can pressure-pattern flying be reconciled with the precision birds are supposed to show in returning year after year to the same nesting area? The answer is, in part, that, if the wind is a major controlling influence on the routes birds follow, there must be a rather stable pattern of air currents prevailing from year to year. Such a situation does in fact exist. There are maps showing wind roses at 750 and 1,500 meters above mean sea level during April and May (Stevens, 1933, figs. 13-14, 17-18). Similarly, the "Airway Meteorological Atlas for the United States" (Anonymous, 1941) gives surface wind roses for April (Chart 6) and upper wind roses at 500 and 1,000 meters above mean sea level for the combined months of March, April, and May (Charts 81 and 82). The same publication shows wind resultants at 500 and 1,000 meters above mean sea level (Charts 108 and 109). Further information permitting a description in general terms of conditions prevailing in April and May is found in the "Monthly Weather Review" covering these months (cf.Anonymous, 1948a, Charts 6 and 8; 1948b, Charts 6 and 8).
Fig. 38.Over-all sector vectors at major stations in the spring 1948. See text for explanation of system used in determining the length of vectors. For identification of stations, seeFigure 34.
Fig. 38.Over-all sector vectors at major stations in the spring 1948. See text for explanation of system used in determining the length of vectors. For identification of stations, seeFigure 34.
Fig. 39.Over-all net trend of flight directions at stations shown inFigure 38. The arrows indicate direction only and their slants were obtained by vector analysis of the over-all sector densities.
Fig. 39.Over-all net trend of flight directions at stations shown inFigure 38. The arrows indicate direction only and their slants were obtained by vector analysis of the over-all sector densities.
First, however, it is helpful as a starting point to consider the over-all picture created by the flight trends computed from this study. InFigure 38, the individual sector vectors are mapped for the season for all stations with sufficient data. The length of each sector vector is determined as follows: the over-all seasonal density for the station is regarded as 100 percent, and the total for the season of the densities in each individual sector is then expressed as a percentage. The results show the directional spread at each station. InFigure 39, the direction of the over-all vector resultant, obtained from the sector vectors on the preceding map, is plotted to show the net trend at each station.
As is evident from the latter figure, the direction of the net trend at Progreso, Yucatán, is decidedly west of north (N 26° W). At Tampico this trend is west of north (N 11° W), but not nearly so much so as at Progreso. In Texas, Louisiana, Georgia, Tennessee, and Kentucky, it is decidedly east of north. In the upper Mississippi Valley and in the eastern part of the Great Plains, the flow appears to be northward or slightly west of north. At Winter Park, Florida, migration follows in general the slant of the Florida Peninsula, but, the meager data from Thomasville, Georgia, do not indicate a continuation of this trend.
It might appear, on the basis of the foregoing data, that birds migrate along or parallel to the southeast-northwest extension of the land masses of Central America and southern Mexico. This would carry many of them west of the meridian of their ultimate goal, obliging them to turn back eastward along the lines of net trend in the Gulf states and beyond. This curved trajectory is undoubtedly one of the factors—but certainly not the only factor—contributing to the effect known as the "coastal hiatus." The question arises as to whether this northwestward trend in the southern part of the hemisphere is a consequence of birds following the land masses or whether instead it is the result of some other natural cause such as a response to prevailing winds. I am inclined to the opinion that both factors are important. Facts pertinent to this opinion are given below.
In April and May a high pressure area prevails over the region of the Gulf of Mexico. As the season progresses, fewer and fewercold-front storms reach the Gulf area, and as a result the high pressure area over the Gulf is more stable. Since the winds move clockwise around a "high," this gives a general northwesterly trajectory to the air currents in the vicinity of the Yucatán Peninsula. In the western area of the Gulf, the movement of the air mass is in general only slightly west of north, but in the central Gulf states and lower Mississippi Valley the trend is on the average northeasterly. In the eastern part of the Great Plains, however, the average circulation veers again slightly west of north. The over-all vector resultants of bird migration at stations in 1948, as mapped inFigure 39, correspond closely to this general pattern.
Meteorological data are available for drawing a visual comparison between the weather pattern and the fight pattern on individual nights. I have plotted the directional results of four nights of observation on the Daily Weather Maps for those dates, showing surface conditions (Figures40,42,44and46). Each sector vector is drawn in proportion to its percentage of the corresponding nightly station density; hence the vectors at each station are on an independent scale. The vector resultants, distinguished by the large arrowheads, are all assigned the same length, but the nightly and average hourly station densities are tabulated in the legends under each figure. For each map showing the directions of flight, there is on the facing page another map showing the directions of winds aloft at 2,000 and 4,000 feet above mean sea level on the same date (see Figures41-47). The maps of the wind direction show also the velocities.
Unfortunately, since there is no way of analyzing the sector trends in terms of the elevations of the birds involved, we have no certain way of deciding whether to compare a given trend with the winds at 2,000, 1,000, or 0 feet. Nor do we know exactly what wind corresponds to the average or median flight level, which would otherwise be a good altitude at which to study the net trend or vector resultant. Furthermore, the Daily Weather Map illustrates conditions that obtained at 12:30 A. M. (CST); the winds aloft are based on observations made at 10:00 P. M. (CST); and the data on birds covers in most cases the better part of the whole night. Add to all this the fact that the flight vectors, their resultants, and the wind representations themselves are all approximations, and it becomes apparent that only the roughest sort of correlations are to be expected.
However, as will be seen from a study of the accompanying maps (Figures40-47), the shifts in wind direction from the surface up to 4,000 feet above sea level are not pronounced in most of the instancesat issue, and such variations as do occur are usually in a clockwise direction. All in all, except for regions where frontal activity is occurring, the weather maps give a workable approximation to the average meteorological conditions on a given night.
The maps (Figures40-47) permit, first, study of the number of instances in which the main trend of flight, as shown by the vector resultant, parallels the direction of wind at a reasonable potential mean flight elevation, and, second, comparison of the larger individual sector vectors and the wind currents at any elevation below the tenable flight ceiling—one mile.
On the whole, inspection of the trend of bird-flight and wind direction on specific nights supports the principle that the flow of migration is in general coincident with the flow of air. It might be argued that when the flow of air is toward the north, and when birds in spring are proceeding normally in that direction, no significance can be attached to the agreement of the two trends. However, the same coincidence of wind directions and bird flights seems to be maintained when the wind currents deviate markedly from a northward trajectory. Figures46and47, particularly in regard to the unusual slants of the flight vectors at Ottumwa, Knoxville, and Memphis, illustrate that this coincidence holds even when the wind is proceeding obliquely eastward or westward. On the night of May 22-23, when a high pressure area prevailed from southern Iowa to the Atlantic coast, and the trajectory of the winds was northward, migration activity at Knoxville and Ottumwa was greatly increased and the flow of birds was again northward in the normal seasonal direction of migration.
Further study of the data shows fairly conclusively that maximum migration activity occurs in the regions of high barometric pressure and that the volume of migration is either low or negligible in regions of low pressure. The passage of a cold-front storm may almost halt migration in spring. This was demonstrated first to me by the telescopic method at Baton Rouge, on April 12, 1946, following a strong cold front that pushed southeastward across the Gulf coastal plain and over the eastern Gulf of Mexico. The winds, as usual, shifted and became strong northerly. On this night, following the shift of the wind, only three birds were seen in seven hours of continuous observation. Three nights later, however, on April 15, when the warm air of the Gulf was again flowing from the south, I saw 104 birds through the telescope in two hours. Apropos of this consideration in the 1948 data are the nights of May 21-22 and 22-23.
Fig. 40.Comparison of flight trends and surface weather conditions on April 22-23, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on April 23. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:5. Louisville: 9,100 (1,100)16. College Station: 13,300 (1,900)6. Murray: 16,300 (2,700)17. Baton Rouge: 6,200 (1,000)8. Stillwater: 1,900 (500)19. Lafayette: 2,800 (600)9. Knoxville: 15,200 (1,700)21. Winter Park: 6,200 (700)13. Oak Grove: 13,600 (1,700)23. Tampico: 11,100 (3,700)
Fig. 40.Comparison of flight trends and surface weather conditions on April 22-23, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on April 23. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:5. Louisville: 9,100 (1,100)16. College Station: 13,300 (1,900)6. Murray: 16,300 (2,700)17. Baton Rouge: 6,200 (1,000)8. Stillwater: 1,900 (500)19. Lafayette: 2,800 (600)9. Knoxville: 15,200 (1,700)21. Winter Park: 6,200 (700)13. Oak Grove: 13,600 (1,700)23. Tampico: 11,100 (3,700)
Fig. 41.Winds aloft at 10:00 P. M. on April 22 (CST).Winds at 2,000 feet above mean sea level are shown in black; those at 4,000 feet, in white. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 40.
Fig. 41.Winds aloft at 10:00 P. M. on April 22 (CST).Winds at 2,000 feet above mean sea level are shown in black; those at 4,000 feet, in white. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 40.
Fig. 42.Comparison of flight trends and surface weather conditions on April 23-24, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on April 24. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:1. Albion: 1,100 (300)14. Mansfield: 4,900 (1,200)2. Ottumwa: 5,500 (900)16. College Station: 700 (100)4. Lawrence: 5,400 (1,400)17. Baton Rouge: 1,700 (400)5. Louisville: 13,300 (2,700)18. Pensacola: migration negligible6. Murray: 9,800 (1,400)20. New Orleans: 1,600 (800)8. Stillwater: 800 (100)21. Winter Park: 2,700 (300)9. Knoxville: 8,000 (900)23. Tampico: 63,600 (6,300)10. Memphis: 7,900 (1,000)24. Progreso: 31,300 (3,900)
Fig. 42.Comparison of flight trends and surface weather conditions on April 23-24, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on April 24. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:1. Albion: 1,100 (300)14. Mansfield: 4,900 (1,200)2. Ottumwa: 5,500 (900)16. College Station: 700 (100)4. Lawrence: 5,400 (1,400)17. Baton Rouge: 1,700 (400)5. Louisville: 13,300 (2,700)18. Pensacola: migration negligible6. Murray: 9,800 (1,400)20. New Orleans: 1,600 (800)8. Stillwater: 800 (100)21. Winter Park: 2,700 (300)9. Knoxville: 8,000 (900)23. Tampico: 63,600 (6,300)10. Memphis: 7,900 (1,000)24. Progreso: 31,300 (3,900)
Fig. 43.Winds aloft at 10:00 P. M. on April 23 (CST). Winds at 2,000 feet above mean sea level are shown in black; those at 4,000 feet, in white. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 42.
Fig. 43.Winds aloft at 10:00 P. M. on April 23 (CST). Winds at 2,000 feet above mean sea level are shown in black; those at 4,000 feet, in white. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 42.
Fig. 44.Comparison of flight trends and surface weather conditions on April 24-25, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on April 25. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:1. Albion: migration negligible12. Rosedale: 1,100 (100)2. Ottumwa: 4,600 (1,500)14. Mansfield: 1,700 (400)3. Columbia: 1,400 (400)18. Pensacola: migration negligible5. Louisville: 1,700 (200)21. Winter Park: 600 (100)10. Memphis: 6,600 (900)24. Progreso: 27,300 (3,000)
Fig. 44.Comparison of flight trends and surface weather conditions on April 24-25, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on April 25. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:1. Albion: migration negligible12. Rosedale: 1,100 (100)2. Ottumwa: 4,600 (1,500)14. Mansfield: 1,700 (400)3. Columbia: 1,400 (400)18. Pensacola: migration negligible5. Louisville: 1,700 (200)21. Winter Park: 600 (100)10. Memphis: 6,600 (900)24. Progreso: 27,300 (3,000)
Fig. 45.Winds aloft at 10:00 P. M. on April 24 (CST).Winds at 2,000 feet above mean sea level are shown in black; those at 4,000 feet, in white. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 44.
Fig. 45.Winds aloft at 10:00 P. M. on April 24 (CST).Winds at 2,000 feet above mean sea level are shown in black; those at 4,000 feet, in white. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 44.
Fig. 46.Comparison of flight trends and surface weather conditions on May 21-22, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on May 22. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:2. Ottumwa: 6,900 (1,400)13. Oak Grove: 5,800 (800)5. Louisville: 1,500 (200)14. Mansfield: 2,500 (800)9. Knoxville: 3,200 (500)18. Pensacola: migration negligible10. Memphis: 7,000 (1,200)21. Winter Park: 1,200 (200)
Fig. 46.Comparison of flight trends and surface weather conditions on May 21-22, 1948. The meteorological data were taken from the U. S. Weather Bureau Daily Weather Map for 12:30 A. M. (CST) on May 22. The nightly station densities and the average hourly station density (shown in parentheses) are as follows:2. Ottumwa: 6,900 (1,400)13. Oak Grove: 5,800 (800)5. Louisville: 1,500 (200)14. Mansfield: 2,500 (800)9. Knoxville: 3,200 (500)18. Pensacola: migration negligible10. Memphis: 7,000 (1,200)21. Winter Park: 1,200 (200)
Fig. 47.Winds aloft at 10:00 P. M. on May 21 (CST). Winds at 2,000 feet above mean sea level are shown. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 46.
Fig. 47.Winds aloft at 10:00 P. M. on May 21 (CST). Winds at 2,000 feet above mean sea level are shown. Velocities are indicated by standard Beaufort Scale of Wind Force. The numbers in circles refer to the stations shown inFigure 46.
On the first night, following the passage of a cold front, migration at Ottumwa was comparatively low (6,900 birds in five hours). On the following night, when the trajectory of the winds was toward the north, the volume of migration was roughly twice as high (22,300 birds in eight hours). At Louisville, on May 21-22, the nightly station density was only 1,500 birds in seven hours, whereas on the following night, it was 8,400 birds in the same length of time, or about six times greater.
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
LITERATURE CITED
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23-1020square
UNIVERSITY OF KANSAS PUBLICATIONS
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Transcriber's NotesWith 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 inFigure 12was added (seebelow) to illustrate the information contained on that sheet. Some text was moved to rejoin paragraphs.There are two notes in the original text indicating that the images for Figures41and45were transposed. The correct images have been placed with the captions and the two notes were removed. Lastly, the cover image was compiled from a copy of the original cover with two of the graphics contained in the article added and the list of UK pulications was moved to the end of the document.Typographical CorrectionsPageCorrection385flght ⇒ flight394diargrams ⇒ diagrams404Determinaton ⇒ Determination411obsever ⇒ observer419Morover ⇒ Moreover425Mississippii ⇒ Mississippi425a ⇒ as430at ⇒ and431inserted "a"("…traveling along a certain topographic feature…")442concensus ⇒ consensus472Stephens, Loyd A. ⇒ Stevens, Lloyd A.
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 inFigure 12was added (seebelow) to illustrate the information contained on that sheet. Some text was moved to rejoin paragraphs.
There are two notes in the original text indicating that the images for Figures41and45were transposed. The correct images have been placed with the captions and the two notes were removed. Lastly, the cover image was compiled from a copy of the original cover with two of the graphics contained in the article added and the list of UK pulications was moved to the end of the document.
Typographical Corrections
PageCorrection385flght ⇒ flight394diargrams ⇒ diagrams404Determinaton ⇒ Determination411obsever ⇒ observer419Morover ⇒ Moreover425Mississippii ⇒ Mississippi425a ⇒ as430at ⇒ and431inserted "a"("…traveling along a certain topographic feature…")442concensus ⇒ consensus472Stephens, Loyd A. ⇒ Stevens, Lloyd A.