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. [Illustration: _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.