In spring, some 40,000 Red Knots leave their wintering grounds at the tip of South America and fly 9,000 miles to their Arctic breeding sites in the far northern reaches of Canada. As winter approaches, the same birds and their newborns make the return trip.
Bird Migration
Animal migrations are not limited to birds. California Gray whales migrate from their arctic feeding grounds to the lagoons of Mexico to give birth each winter, an impressive 5,000 mile swim each way. Even more mysterious is the annual migration of millions of Monarch butterflies from North America to the high mountains of central Mexico, performed by a unique "Methuselah" generation.
Throughout the world, most bird species engage in some type of migration. Elevational migrants simply leave their high-altitude breeding areas to spend their winters in more temperate lowlands, often within or near the same ranges where they summer. Short-distance migrants generally remain within the same continent, but the hardiest migratory birds are the long-distance migrants, whose semi-annual trips are often intercontinental.
Although almost all birds move within known ranges, migration is quite different from dispersal or irruption. Dispersal is the movement of young to their own territories within established ranges, while irruption is the unpredictable and irregular departure from breeding grounds, generally due to unusually harsh winter conditions. Migration, on the other hand, is a predictable mass movement of a species between summer breeding grounds and warmer wintering sites.
Of the approximately 650 bird species that nest and breed in North America, about 500 of these are involved in some level of migration. Many of these migrations are fascinating and incredible, and that of the Red Knot may be the most incredible of all.
The Red Knot (Calidris canutus)
A shorebird sandpiper, the Red Knot is a medium-sized bird (10 inches in length) with a wingspan of about 20 inches. There are six subspecies found around the world, of which Calidris canutus rufa winters in South America and breeds in the Canadian Arctic. Winter plumage is uniformly grey, but the birds' face, throat and breast show reddish cinnamon color in breeding plumage.
Red Knots are found in marine habitats, particularly along inlets and bays, in the winter, and their breeding range is the tundra of the high Arctic. Knots probe for crustaceans and bivalves in intertidal areas, and also feed on insects and spiders in their breeding grounds. During their northbound spring migration, horseshoe crab eggs in the Delaware Bay region are the primary food source needed to complete their journey. Reduced numbers of horseshoe crabs, mostly due to over-harvesting for fishing bait, has raised concerns for the rufa population, but attempts to list the birds as endangered have not yet succeeded. In New Jersey, some protection has been achieved through restrictions on crab harvesting and beach access.
Migration of Western Hemisphere Red Knots
In February, Red Knots leave their wintering grounds at the far tip of South America, in Chile and Argentina. Flying in flocks of thousands, the birds make one of the longest and most remarkable migratory trips, some 15,000 kilometers to their Arctic breeding grounds. After a week-long non-stop flight across the Atlantic Ocean, their most important refueling stop, in May, is Delaware Bay and the coastlines of New Jersey and Delaware. Here the birds fatten up before undertaking the last long segment of their journey.
Males arrive at the breeding grounds before the females, immediately establishing and defending their territory. Females typically lay four eggs which incubate in about 22 days, and both male and female birds share nest incubation. Young Knots are precocial, hatched with downy feathers and almost immediately begin foraging with the parents.
The females leave on the southbound migration in July, before the young have fledged. The males leave a few weeks later, after the young have fledged, but without them. The fledglings continue feeding at the Arctic feeding grounds for some time, and then, incredibly, take flight and follow the same 9,000 mile route the adult birds embarked upon weeks before. They stop at the same feeding grounds, and they eventually arrive at the same South American beaches to rejoin their parents.
Miraculous Migrations
Migrations of mammals, insects and birds are impressive, often incredible and miraculous. One of the most impressive of all is that of the Red Knot, especially the 9,000-mile unaccompanied trip made by newly-hatched fledglings.
For related information about migrations and other birds, see the following articles:
- The Arctic Tern, the Bird that Migrates the Farthest
- The Razorbill, North America's Rarest Atlantic Auk
- The Atlantic Puffin, a Colorful Pelagic Bird
- Tour Mexico's Monarch Butterfly Preserve
- Whale Watching in the Lagoons of Baja California
References:
Elphick, Chris, Dunning, Jr., John B. and Sibley, David Allen (2001), The Sibley Guide to Bird Life & Behavior, Alfred A. Knopf, ISBN 0679451234
Harrington, Brian, 1996, The Flight of the Red Knot, W. W. Norton, ISBN 0393038610
http:en.wikipedia.org/wiki/Red_Knot
Bob Bowers - A lifelong naturalist and amateur ornithologist, Bob's avocation is studying, photographing and writing about birds.
Join the Conversation