INFLUENCE OF WEATHER
It is thought by some that the weather has little to do with the time of arrival of migratory birds. It is assumed that if the bird is physiologically prepared for migration it departs, irrespective of the weather. Even if this were the case, weather can influence the progress of migration by not only controlling the advance of the seasons but also by helping, hindering, or even stopping bird flight (Welty 1962).
Some scientists believe that birds not only avoid bad weather at the start of a journey but usually finish the journey in good weather (Nesbit and Drury 1967b). Contrary to what many observers believe, the arrival of birds in an area, whether they stop or continue on, is more often controlled by the weather at the point of departure than at the point of arrival. During the peak of migration, suitable weather may occur at an observation site, but strong migratory movements may be arrested before the birds arrive there because the weather was not suitable at the point of departure or somewhere in between. In addition, if there is good weather at the point of departure as well as farther down the migration route, the migrants, once air-borne in a favorable weather pattern, may continue on right over an expectant observer and the whole flight will be missed. Nesbit and Drury's (1967b) radar study on air-ground comparisons found, with few exceptions, ground observers missed the largest movements observed on radar. Observation of a large wave of arrivals indicated migrants had been stopped by a meteorological barrier, and people were actually not reporting maximum migration but an interruption to migration. Therefore, when migration is proceeding normally under safe conditions, very little movement is visible to the ground observer but a large arrival of birds on the ground often indicates something is not in order and the migrants have been forced to stop for one reason or another.
The question is frequently asked: "How can I identify weather conditions suitable or unsuitable for migration?" It is almost impossible to discuss separately the effects of different weather factors on migration because barometric pressure, temperature, wind, and other meteorological phenomena are very closely related.
On the North American continent, air masses generally proceed about 600 miles per day from the west to the east. These air masses vary in pressure, temperature, humidity, and wind. The wind within these masses travels in either a clockwise (anticyclonic) or counter-clockwise (cyclonic) direction. Cyclonic air masses contain relatively moist warm air with low barometric pressure centers and are designated "lows"; anticyclonic air masses are characterized by dry cool air with high barometric pressure areas and are called "highs." Where these air masses meet, a "front" is formed, and the rapidity with which this front moves through an area depends on the temperature and pressure gradient on either side of the front.
An understanding of frontal systems, with their associated wind, temperature and humidity, is one of the keys to understanding when birds migrate. You must not only watch the fronts in your area but the progress of nearby air masses as well because the birds migrating through your area have started their journey to the north or south of you depending on the season. The weather conditions at point of departure will dictate if and when birds will be passing through your area in the near future.
Figure 12. A hypothetical weather system that could be ideal for mass migrations of waterfowl in the fall. The strong southerly flow of air created by counter-clockwise winds about the lows and the clockwise rotation of air about the highs, aids the rapid movement of waterfowl from their breeding grounds in the Canadian prairies to wintering areas in southern United States.
During fall migration, the best passage of migrants usually occurs 2 days after a cold front has gone through. That is, the low has passed and it is being followed by a high characterized by dropping temperatures, a rising barometer, and clearing skies. The 24 hours just after a low has passed are not always conducive to a good passage of birds because winds are often too strong and turbulent in the trough between the two air masses. Hochbaum (1955) correlated mass movements of ducks through the prairies with weather systems and noted the combination of weather conditions described above was ideal for mass migrations of ducks during November. During this period, observers at Delta, Manitoba, south to Louisiana recorded a tremendous flight of ducks as the proper conditions of barometric pressure, temperature, wind, and cloud cover passed across the central United States and Canada. An example of the type of weather system that is often associated with mass movements is illustrated in [Fig. 12.]
Records of lapwings on Newfoundland and the Gulf of St. Lawrence appear to be the result of a particular series of meteorological events (Bagg 1967). The lapwing is a European species rarely found in the New World. If cold air moves into western Europe from the east, lapwings move westward into England, Wales, and Ireland. Occasionally, the development of an anomalous weather pattern over the North Atlantic including an elongated low from Europe to eastern Canada causes some birds to be literally "blown" in the counter-clockwise airstream across the Atlantic to the Gulf of St. Lawrence.