Northern Prairie Wildlife Research Center
During both spring and fall migrations, radar studies have demonstrated that weather has a defining role in determining when a bird will actually begin a migratory flight. The primary stimulus for departure is a following wind; in the spring this is a wind from the south, in the fall it is a wind from the north. Clear skies, presumably providing for celestial orientation cues, are of secondary importance, since major flights will occur under an overcast if adequate tail winds are blowing.
In the North Temperate Zone, migrations are concurrent with periods of rapid seasonal change. In the summer, warm, moist air masses dominate, but as fall approaches colder, drier air pushes southward to eventually bring the grip of winter to the land. The battle for domination of air masses is then reversed in the spring as the longer daylengths increase the heat load in the atmosphere, again giving the advantage to the northward expansion of warmer air. It is along this frontal boundary between these air masses that low pressure centers develop and move eastward, steered by the high velocity jet stream aloft. Winds flow in toward these low pressure centers in a counter-clockwise circulation, fed by air spiralling outward in a clockwise direction from intervening high pressure centers within the air masses. Thus, in the southeastern quadrant of a low pressure center, warm moist winds drive a warm front northward into the colder air, the warmer air being pushed gradually above the colder air forming large areas of cloud cover and widespread rainfall. In the northwestern quadrant of a low pressure center, cold dry air pushes a cold front southeastward into the warmer air mass, abruptly forcing the warm, moist air aloft, sometimes with violent and severe consequences.
Since prevailing wind direction determines whether a migratory flight will occur, the patterns of wind circulation around highs and lows affects migratory movement (Figure 12). During fall migration, the best passage of migrants usually occurs the day after the day of cold front passage with brisk north winds, dropping temperatures, a rising barometer, and clearing skies. The intensity of this flight only wanes as migrating flocks become less and less influenced by the prevailing winds following the cold front as it moves eastward. Since wind direction becomes more variable and wind velocity decreases as high pressure begins to dominate, mass migratory flights are curtailed. This is the time birds stop and feed.
|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 spring, weather conditions in the southeastern or warm sector of a low pressure are conducive to movements of birds since the prevailing wind flows strongly from the south. But when these migrating flocks are overtaken by the cold front sweeping in from the west with its abrupt reversal of wind direction, towering clouds, turbulent air, and often torrential rain, migration stops, and the birds are grounded. If northward migrating flocks overtake the warm front, they are also faced with a shift in wind direction now blowing out of the east, increased cloud cover, and precipitation, but since the air is less turbulent, the wind shift less inappropriate, and the rains gentler, they will often continue northward awhile before they land and begin to forage.
The passage of low pressure system and the associated winds, often results in "waves" of migrants grounded by the storm being seen by observers. This is especially the case in the spring. This phenomenon reaches its superlative expression along the Gulf of Mexico if a cold front is positioned along or just off the coast. Then trans-Gulf migrants nearing the end of their flight from Central America and enjoying the advantage of a following wind must struggle against the adverse headwinds until landfall is reached, and the exhausted birds settle immediately to rest and forage in whatever habitat the coastal strand provides. It is a day to be remembered by any bird watcher. Orioles and tanagers by the dozens crowd the scrubby seaside bushes; Blackburnian and Cerulean warblers forage with Indigo and Painted buntings in the lawns of bayside homes. But if there is no front, there are no birds, the migrants having sufficient fat stores to continue flying northward on the following wind until they must stop to eat and drink.
Soaring birds such as hawks, Ospreys, eagles, and vultures are very dependent on proper wind conditions for migration. In the fall, often the best day to observe hawk migration along mountains in the eastern United States is on the second day after a cold front has passed, providing there are steady northwest to west winds to produce updrafts as the strong air currents are forced over the north-south oriented ridges. Migrants also soar on convective thermals that are generated by the differential heating of the Earth's surface. It has been estimated that the normal premigratory fat load of 100 grams in a Broad-winged Hawk would be exhausted in only five days of flapping flight. But by spiralling in the updrafts of one thermal and gliding down to the next to again to take advantage of the rising air currents, its stored fat would last 20 days, more than enough to provide energy for its 3,000 mile journey from the Neotropics.