Large bodies of water constitute real barriers to soaring birds dependent on thermals and air currents. Good examples of these barriers include the Mediterranean Sea between Europe and Africa and the Great Lakes in North America. Because these water areas do not create good thermals (generally a warm surface, such as a large field on a sunny day, is needed to create the necessary rising air currents for thermals to form) for birds to soar on, migrants are forced to travel around them on up-drafts created where land and water meet. The shoreline, then, may appear to be the guiding line, but more than likely the birds are simply following air currents created by onshore winds replacing the rising air from the surrounding warmer land surface and being deflected upward by the shoreline. These conditions often concentrate our buteos (broad-winged, rough-legged, red-shouldered, and red-tailed hawks) into restricted areas where, on good days, numbers observed can be spectacular. Similar conditions exist over the Bosphorus at the eastern end of the Mediterranean Sea where literally thousands of storks, eagles, and buzzards can be observed on a good day.
While extensive water areas may alter the migratory path of soaring birds, mountain ridges, especially if parallel to the line of flight, are often very conducive to migratory travel. Systematic coverage of the Appalachian ridges indicates all of them aid the migration of soaring birds. Apparently the highest and longest ridges deflect the horizontal winds upward better than the shorter ridges less than 1,000 feet high, and more birds are seen, on the average, along the higher ridges (Robbins 1956).
In general, nocturnal migrants are not influenced by topography as much as diurnal travellers. Radar observations have played an important role in establishing this difference. Bellrose (1967) found that waterfowl migrating at night through the Midwest were not influenced by major river systems, but in the evening or after daybreak ducks and geese tended to alter their course along the rivers. Drury et al. (1961) recorded massive fall and spring movements from the New England area out over the Atlantic Ocean without any apparent regard for the coastline. Until nocturnal migration could be "watched" on a radar screen, many bird observers assumed the guiding effect of the coastline on migratory travel was more restrictive than it really is.
In summary, topography may help or deter a migrant in its passage. It affects different birds in different ways. In North America, migratory movements are continent wide, and no evidence has indicated any particular part of the landscape influences all birds in the same manner. Certain bird populations may use general areas in migration, but they are usually not rigidly restricted to them because of topography.
PERILS OF MIGRATION
The migration season is full of peril for birds. Untold thousands of smaller migrants are destroyed each year by storms and attacks by predatory animals. These mortality factors, and others, help keep bird populations in check. Perils of migration are among these causes.
Storms
Of all the hazards confronting birds in migration, particularly the smaller species, storms are the most dangerous. Birds that cross broad stretches of water can be blown off course by a storm, become exhausted, and fall into the waves. Such a catastrophe was once seen from the deck of a vessel in the Gulf of Mexico, 30 miles off the mouth of the Mississippi River. Great numbers of migrating birds, chiefly warblers, were nearing land after having accomplished nearly 95 percent of their long flight when, caught by a "norther" against which they were unable to make headway, hundreds were forced into the waters of the Gulf and drowned. A sudden drop in temperature accompanied by a snowfall can cause a similar affect.
Aerial Obstructions
Lighthouses, tall buildings, monuments, television towers, and other aerial obstructions have been responsible for destruction of migratory birds. Bright beams of lights on buildings and airport ceilometers have a powerful attraction for nocturnal air travelers that may be likened to the fascination for lights exhibited by many insects, particularly night-flying moths. The attraction is most noticeable on foggy nights when the rays have a dazzling effect that not only lures the birds but confuses them and causes their death by collision against high structures. The fixed, white, stationary light located 180 feet above sea level at Ponce de Leon Inlet (formerly Mosquito Inlet), Florida, has caused great destruction of bird life even though the lens is shielded by wire netting. Two other lighthouses at the southern end of Florida, Sombrero Key and Fowey Rocks, have been the cause of a great number of bird tragedies, while heavy mortality has been noted also at some of the lights on the Great Lakes and on the coast of Quebec. Fixed white lights seem to be most attractive to birds; lighthouses equipped with flashing or red lights do not have the same attraction.