How Long Does Migration Take?






Arctic Tern, Sterna paradisea

Birds begin and end their various journeys at different locations, so the time it takes to travel that distance varies, and their speed depends on the environmental conditions. Weather can slow or accelerate the trip considerably. Canada Geese leave their southern U.S. wintering grounds in early February and arrive in northern Canada the end of April. They more or less follow the 350 F (16 C0) isotherm (a line of equal temperature on a weather map) northward. The Gray-cheeked Thrush, wintering in northern South America arrives in Louisiana in late April and a month later arrives in northern Alaska. This makes sense because if they were to fly in front of the isotherm, they would confront freezing conditions and a dearth of food.

             Most birds fly at about 15-50 mph (25-83 kph) during migration and travel from 100 to 500 miles (166-833 km) in a day. Larger birds tend to fly faster than smaller ones; ducks and geese average 40-50 mph (66-83 kph) while flycatchers average more like 17 mph (28pkh). Smaller birds might travel 100-250 miles (166-416 km) in a day while ducks may go twice that far.

            We can’t go any further without talking about the longest migration in the world, that of the Arctic Tern. A medium-sized bird, there is nothing spectacular about its appearance, but its lifestyle of breeding in the arctic and subarctic and wintering in the Antarctic have made it famous. They are circumpolar breeders in the arctic from May to August where summer days are long. The birds typically lay two eggs and defend their nest vigorously. Although they feed mainly on fish and crustaceans from the ocean, they will eat insects during the nesting season. After the young are fledged, the colonies leave the breeding area to winter in the Antarctic region from November to March when the photoperiod is again long. From about April to May the terns migrate northward again, traveling an average of about 300 miles (500 km) per day, although individual birds have been observed to fly as much as 400 miles (670 km) per day. Some individually tracked birds flew as much as 66,000 miles (80,000 km) annually. As they follow the seasons and photoperiodic changes, Arctic Terns see more sunlight than any other animal.

Arctic Terns can live over thirty years, so during their lifetimes some of these birds may travel the equivalent of three round trips to the moon! An impressive accomplishment, but how many of them actually live to be thirty years old? I found an old banding study that estimated the annual mortality rate of Arctic Tern adults at 18 percent. With that death rate, if we start with a population of 1000 birds in the arctic and 18 percent of this group dies each year, 30 years later only one bird will have survived long enough to have made thirty round trips