Red Knot (Calidris canutus)
The red knot is a master of long-distance migration, with some individuals flying more than 9,300 miles from Arctic breeding grounds in North America to wintering grounds in extreme southern South America.
Red Knot. Photo by Fred Fallon
In order to survive these long journeys, red knots undergo extensive physiological changes. Flight muscles increase while stomach and gizzard masses decrease. For much of the year red knots eat small mussels and other mollusks. During migration they eat fewer hard foods because of their shrunken gizzards, and in spring they seek soft foods like the eggs of the horseshoe crab.
The unassuming red knot is a medium-sized shorebird with a short bill and sexes look similar. In breeding plumage, its face and underparts are a rich chestnut and upperparts are primarily dark. In winter plumage, the red knot is predominantly gray with a white belly. More impressive is their flocking nature.
Red knots migrate in larger flocks than most other shorebirds with 98% of spring migrants and 97% of fall migrants concentrating at important coastal migratory sites. They break their spring and fall migrations into roughly 1,500 mile segments, ending at stopover sites called staging areas where flocks of birds converge to feed. One of the most important staging areas along the Atlantic includes the Delaware coast and the Cape May peninsula.
Horsheshoe crab aggs. Photo by Greg Breese, USFWS
The spring migration coincides with horseshoe crab spawning. Shorebirds, including red knots, arrive at the Delaware staging area thin and sometimes emaciated. They gather in the tens of thousands to gorge themselves on the soft eggs of the horseshoe crab, essentially doubling their body weight so that they may continue their migration to Arctic breeding grounds.
Red knots often arrive in their arctic breeding areas before insects are active and available to eat. The birds eat plant seeds, grass shoots and other vegetable foods. Nests are simply a shallow, lined scrape on the tundra. A typical clutch contains three to four eggs, and is incubated by both sexes for about three weeks. The downy chicks leave the nest shortly after hatching and are able to feed themselves, eating almost exclusively on insects.
Although the flocking of red knots may help protect the birds from attacks by birds of prey, the strong tendency to congregate also leaves them vulnerable to habitat degradation and habitat destruction and, in South America, to hunting pressure.
Surveys during spring migration in Delaware Bay on the U.S. coast indicate a serious population decline. Aerial surveys of wintering red knots in southern South American have shown a 50% decrease from the mid-1980s to 2003.
Their survival depends upon the continued availability of billions of horseshoe crab eggs at staging areas like the Delaware coast. During the 1990’s an increase in harvesting of horseshoe crabs, may have contributed to the decline in red knots.
Another necessity for red knots is the existence of arctic habitat for breeding. Global climate change, which may be greatest at the latitudes where this species breeds and winters, could destroy critical fragile habitat.
To help protect this and other shorebirds species, both New Jersey and Delaware have limited the harvesting of horseshoe crabs along their coastlines. There have also been major efforts to reduce the risk of industrial and maritime accidents (e.g. oil spills) in the Delaware Bay area. Scientists are now looking at climate change and how that may affect red knots and other species that use the tundra to breed.