On our Central American cruises, we visit Costa Rica and Panama when many birds from the northern hemisphere have arrived at their southern tropical wintering grounds. Birds seem to have an internal compass and studies have proven that different birds read a variety of environmental cues to find the correct orientation. Pigeons are known to navigate by electromagnetic fields present in the earth's crust while sparrows are known to navigate by certain stars. Supplementing any compass bearings a bird may have are the seasonal breezes that naturally carry birds aloft in the correct direction, minimizing effort.
Many types of birds migrate such as songbirds, raptors, shorebirds, and waterfowl. Some birds prefer to migrate during the day, others at night, some alone, others in flocks. Some groups are highly ordered while others are loose congregations. Most songbirds migrate at 500 to 2,000 meters (1,640 to 6,560 feet), but some fly as high as 6,800 meters (22, 310 feet); swans have been recorded at 8,000 meters (26,246 feet) and bar-headed geese at 9,000 meters (29, 527 feet), seemingly to adjust their flight levels depending on air turbulence, choosing their altitude for the least air disturbance.
Occasional “reverse migration” can take place where the genetic programming of young birds fails to work properly. This leads to great rarities turning up as vagrants thousands of kilometres out of range. We recently heard about flamingos being found in Siberia...in the winter!
Birds follow flyways, and there are four flyways in North America: the Pacific, Central, Mississippi, and Atlantic. These routes follow naturally occurring landmarks such as long valleys, mountain ranges, large rivers, and coastline. The importance of Central America becomes obvious when one looks at the routes because all four of the major routes have branches that converge over the Central American isthmus (although no two species follow exactly the same path from beginning to end). Thousands of winter migrants from the north depend on finding food resources in the Central American neotropics over winter. On our Costa Rica and Panama cruise, we are likely to see many migratory birds from the north, such as the scarlet and western tanagers, rose-breasted grosbeak, barn swallows, American redstarts, bobolinks, numerous thrushes, warblers, and sparrows.
The longest migration route of any bird species in the Western Hemisphere lies in Mississippi Flyway. Beginning on the Arctic coast of Alaska, its southernmost point ends in Patagonia. For more than 4,830 kilometers (3000 miles) while over the continent of North America, from the mouth of the Mackenzie to the delta of the Mississippi, this route is uninterrupted by mountains with not even a ridge of hills on the entire route that is high enough to interfere with the movements of migrating birds. The greatest elevation during this migration is less than 610 meters (2,000 feet). The entire region affords ideal conditions, forested and with waterways which supports hosts of migrating birds. The general north-and-south direction of the major rivers is another factor in determining the importance of this route which is used by large numbers of ducks, geese, shorebirds, blackbirds, sparrows, warbler, and thrushes. Interestingly enough, the majority of North American land birds seeking winter homes in the tropics through the Mississippi Flyway take the shortcut across the Gulf of Mexico in preference to the longer, though presumably safer, land or island journey by way of Texas or the Antilles.
Migration is one of the most widely studied areas of bird biology, yet it is still poorly understood and we still have only a handful of theories about how birds accomplish this. Reasons for such marathon migrations are probably due to several factors such as improved foraging areas and abundance of food, water, protective cover, and a sheltered place to nest and breed since changing seasons in the higher latitudes can transform a comfortable summertime environment into an unlivable one in winter. In tropical regions at least 80 percent of the birds are nonmigratory as all the necessary requirements for survival are available year-round.
According to the Smithsonian Migratory Bird Center, a blackpoll warbler could boast of getting 720,000 miles to the gallon if it were burning gasoline instead of reserves of body fat, while the tiny ruby-throated hummingbird, weighing only about as much as a penny, makes the 1,000 km (621 mile), 24-hour spring flight across the Gulf of Mexico from the Yucatán Peninsula to the southern coast of the United States. Death during migration takes a heavy toll and it is estimated that half of all migrants heading south for the winter will not return to breed in the spring with predation and bad weather being two of the natural causes of mortality.
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