At the altitudes at which birds fly, wind speeds often exceed 20 miles per hour. A head wind can halt a bird's froward progress or even blow it backward, whereas a tail wind can easily double its speed. High winds can prevent small birds from migrating. Strong crosswinds can cause birds to drift far off course and may be disastrous for land birds carried over the ocean; such winds are often the reason that birds are sometimes found far outside their normal range.

As in most aspects of bird behavior, there is variation in the speed at which different species fly. Most small songbirds, for example, fly at air speeds (speeds without any influence of wind) of only 20 miles per hour. Waterfowl, and especially the larger shorebirds, maintain speeds of about 40 miles per hour or even more. Migrating birds, however, generally propel themselves at quite moderate speeds, which means that the wind has an enormous impact on their progress. It is therefore no surprise that birds are supreme interpreters of weather and wind.

Long flights over water or other inhospitable terrain provide the ultimate tests of migratory strategy. In many cases, selecting a departure day on which there are tail winds may make the difference between life and death. In spring, millions of small birds fly northward across the Gulf of Mexico.

Most of the time at that latitude, moderate southerly winds blow across the gulf, aiding the migrants. Occasionally, however, a cold front penetrates the gulf from the north. With the northerly winds behind them, these fronts often do not reach Yucatan, a major departure point for the Gulf crossing. Thus, birds can embark on the trip in fine weather with southerly winds only to run into problems out over the water. Trouble will likely come in the form of rain showers and, if the birds penetrate the front, potent head winds.

Under these conditions, what would have been a relatively easy trip can turn into a disaster, and birds that would have arrived on the northern Gulf Coast with fat to spare, arrive exhausted at the beaches, if they can make it at all. Thousands can die of starvation or dehydration while cresting on offshore oil rigs or in the water itself, as evidenced by the corpses that sometimes wash up on beaches. Clearly, it pays to be able to judge the weather.