Red Knots

Red Knots
Plowing for horseshoe crab eggs. Photo by Beth Starr.
Across the lonely beach we flit,
One little sandpiper and I;

And fast I gather, bit by bit,
The scattered driftwood, bleached and dry.
The wild waves reach their hands for it;
The wild wind raves, the tide runs high,
As up and down the beach we flit,
One little sandpiper and I.

~ Celia Thaxter

There are at least twenty-two species of sandpipers that have been identified in North America, ranging from the 6-inch least sandpiper to the 26-inch long-billed curlew. Red knots are a medium-sized, bulky sandpiper, averaging just under 10 inches in length, and are the largest sandpiper in the genus Calidris in North America. The red knot was first described by Linnaeus in 1758 as Tringa canutus, possibly referring to the story of King Canute and the tide, wherein the king exasperated by his flattering courtiers who claimed he was so great, he could command the tides of the sea had his throne carried to the seashore and sat on it as the tide came in, commanding the waves to advance no further. When it was obvious his demands were futile, he declared, "Let all men know how empty and worthless is the power of kings, for there is none worthy of the name save Him, whom heaven, earth, and sea obey." Why the little red knot was bestowed with so royal a name is a mystery. Later, it was reclassified into Calidris, possibly attributed to a reference Aristotle makes in his zoological text, The History of Animals: "Then there is the scalidris, with plumage ashen-grey, but specked." Seems to describe many of the birds in Calidris.

The red knot, in non-breeding plumage, is quite a drab shorebird. The bill is long, thick, straight, and black; the legs are black or dull green. The upper parts are pale grey with blackish primaries and a white stripe across the wing. The head is grey, with white areas above the eye and on the throat. The breast and tail are pale grey, but the flanks, belly and undertail feathers are white. This basic plumage is similar between sexes and among subspecies. Juveniles are similar to adults in winter plumage, but gray back feathers outlined in white and black give a scaly appearance. Essentially, the red knot is a little gray bird in the winter. Yet in breeding plumage, with its russet head and breast, it is one of North America's most colorful sandpipers.

There are six subspecies of red knots, which are distinguished by differences in geographical range, morphological traits, and annual cycles. Two of these subspecies can be found in North America; both differ from other subspecies in their overall lighter plumage and longer bills. Differences between these two similar subspecies can be seen through genetic analysis. The subspecies found wintering on the Texas coast is the rufa red knot, Calidris canutus rufa.

Red knot diets depend on their environment. During the winter, when they shelter in warmer regions such as the Gulf Coast and South America, they feed primarily on marine invertebrates, such as snails, crustaceans, and especially small mollusks, which they swallow whole and crush in their muscular gizzard. Because of the hard work this shell processing requires, knots have the largest gizzard, relative to body mass, of all the shorebirds. Instead of regurgitating indigestible parts of prey, as do many bird species, the red knot excretes these parts in the feces. Researchers have used fecal content to examine food consumption habits. During the breeding season, when they nest on the arctic tundra, red knots subsist on the seeds of sedges, horsetails, and grass shoots until insects, spiders, and small crustaceans are available for the taking. On marine coastlines, the birds feed using three strategies: pecking, plowing, and probing. Pecking is for surface prey that is detectable by sight. Plowing involves poking the bill about an inch or so down into a sandy, wet tidal flat, with the tip of the bill pointed slightly forward. Then, with rapid up-and-down motions of the head, the knot moves forward slowly, seeming to plow a small furrow in the sand; direction appears aimless. Probing involves repeated deep penetration of the sand (or mud, or other pliable substrate), often to the full length of the bill. This method is not aimless. Specialized sensory organs on their bill tips, called the Herbst' corpuscles, enable the knots to detect buried prey by discerning water pressure differences that build up in the sediment when a spot is probed repeatedly.

One prominent departure from the knots' predilection for mollusks occurs on the shoreline of Delaware Bay, one of the few staging sites on the knots northward migration to the breeding grounds in the Arctic. During migration, knots do not have the ability to process large amounts of hard-shelled prey because their gizzard has diminished for more energy-efficient flight. However, they still need to refuel, substantially. This means they need high-energy, easily processed prey. For this reason, long-distance migrants arriving in Delaware Bay with small gizzards rely on one very specific menu item: horseshoe crab eggs. Tens of thousands of horseshoe crabs come ashore to spawn in May, and the knots gorge themselves. This is one of the conditions they must encounter to reach the breeding grounds.

The rufa red knot is one of the longest-distance migrants in the animal kingdom. On wingspans of 20 inches, these birds fly more than 9,300 miles from south to north every spring, and repeat the journey in reverse every autumn. Knots are a classic "jump" migrant, undertaking flights up to 5,000 miles at a time between staging areas. To reach the breeding grounds in the Arctic, they need to encounter favorable habitat, food, and weather conditions at each of their stops, and instinct has evolved to tell them just the right time to leave in order to take advantage of these narrow seasonal windows. They tend to concentrate in huge flocks at the same staging areas every year, the same locales they've used for hundreds of years. It is estimated that nearly 90 percent of the entire population of rufa red knots can be present on the Delaware Bay shoreline in a single day. Though the subspecies of red knots have only slightly different physical features to distinguish them, their migration strategies are strikingly different. This makes them an apt species for studying the development of avian migration. Consequently, they have become a focal research species for integrating research and conservation strategies for long-distance migrants across all the major flyways in the world.

Together with songbirds, shorebirds have higher Basal Metabolic Rates (BMR) than other birds. (BMR is the cost in calories your body uses just to keep you alive, not including playing, thinking, reproducing, etc.) Their high BMR is likely due to the large organs they need for endurance flights. Red knots spend part of the year in the Arctic, where it costs a plethora of calories just to stay warm. However, they live the other part of the year in temperate and tropical areas, where if they spent the same amount of calories on warmth as the do in the Arctic, they would overheat and die. Knots solve this problem by lowering or increasing their BMR to accommodate their environment, and they do this by changing the size of their internal organs. It's a useful sort of physical sub-routine, running in the background so the birds can focus on more important affairs, such as what happens when they do finally reach the breeding grounds.

Males arrive first in early May and tidy up the nest space; they are highly faithful to their breeding territories from year to year. Red knots are seasonally monogamous, and pair bonds form soon after the females arrive. Males display with aerobatic displays and songs. They rise from the ground in a steep, rapid ascent and, upon reaching the desired altitude, their flight changes to an essentially broad figure eight pattern. There is much gliding and wing quivering as the flight song commences, and the whole display sometimes lasts over an hour. The song-flight display of one bird often stimulates neighboring birds to display, until at times, three or four knots display simultaneously.

Despite their close-knit winter flocking behavior, breeding pairs maintain territories and nests about three quarters of a mile apart from each other. This territorial behavior is rarely seen outside the breeding season and is only enacted by the males. In general, nests are located on sparsely vegetated, dry, sunny, slightly elevated tundra locations, often on windswept ridges or slopes with low cover. The nest is a cup-shaped depression in the ground lined with dried leaves, grasses, and lichens. The female lays three to four buff- or olive-colored eggs with brown markings. A mated pair will have one brood per season. If the clutch is lost, the pair will not replace it. Both mom and pop incubate the eggs equally. During early incubation, neither put much effort into nest defense; they simply fly away when flushed, sometimes taking several hours to return. During late incubation, they remain immobile when approached by potential predators, unless perilously close. Incubating individuals have been lifted by hand from the nest. If flushed from the nest, the on-duty parent performs distraction displays, running around crouched with wings drooped to sides or weakly flapping and producing a high-pitched squealing noise.

*Fun Fact: Knots use waxes secreted from the preen gland for feather maintenance. Before the northward migration to the breeding grounds, the wax changes in its chemical composition. This shift appears to reduce the birds' scent, making them less easily detected by predators. Ideal for incubating parents employing the "No bird here!" strategy.

Eggs hatch in early to late June, after 21-22 days of incubation. The downy, precocious chicks leave the nest almost immediately, but not before mom high-tails it back south, leaving dad to raise the youngsters until they fledge, about 20 more days. After hatching, dad and the kids move away from the higher nesting terrain to lower, wetland habitats. The chicks forage for themselves, eating mostly insects. Long-tailed jaegers (a seabird in the skua family) and arctic foxes are common egg and chick predators in the Arctic, especially in years with low lemming populations. However, even newly hatched chicks recognize and respond to parental alarm calls.

Most juveniles will make their first migration south just a few short weeks after they hatch. They won't return for two years, taking that long to sexually mature. Away from the breeding grounds, they must learn to avoid large falcons (such as the peregrine falcon), harriers, accipiters, smaller falcons (such as merlin), short-eared owls, and great black-backed gulls. Flock aerobatics reduce the likelihood of becoming dinner. When approached by raptors, red knot flocks develop tight formations and exhibit amazingly synchronous, aerobatic maneuvers in much denser flocks than other shorebirds. Other than their affinity for compact flocks during migration and roosting, there is very little known about the social structure of red knots. There is no indication of a social hierarchy. Coastal activities are dictated largely by the tides. When roosting, knots sleep with eyes closed for brief periods, so that someone is vigilant at all times. If undisturbed, flocks may roost throughout the high water period, with bouts of preening erupting here and there.

In 1999, 1,400 to 2,800 rufa red knots were identified at several sites in Texas, including Mustang Island Beach Airport and Padre Island National Seashore. The status of this population is uncertain, but actual numbers are believed to have dropped substantially since the 1980s. The populations wintering in South America decreased by more than 50 percent from the mid-1980s to 2003. Calidris canutus is currently listed by the IUCN as Least Concern. However, Calidris canutus rufa is currently a candidate for the Endangered Species Act, and as North American migratory birds, they are protected under the Migratory Bird Treaty Act.

Red knots were heavily hunted for both market and sport during the late 1800s and early 1900s. An 1893 account, by George H. Mackay, observed that a single shipment of knots from Cape Cod to Boston (the results of a single night's harvest) averaged over 4,000 knots. Bent Life History of the Red Knot, published by the Smithsonian Institution in1929, noted that excessive shooting and market hunting had reduced the species to "a pitiful remnant of its former numbers." It is unclear today if the population has regained its historical numbers.

The red knots' tendency to concentrate at traditional coastal staging sites during migration makes them vulnerable to the loss of key resources. Delaware Bay, where they rely on an abundance of horseshoe crab eggs, is one such area. Increased fisheries for horseshoe crabs in this particular area led to a population decline in red knots in the 2000s. Crab harvests are now regulated explicitly to stabilize and recover red knot populations, and it does seem to be working, though knot numbers remain at low levels relative to earlier decades. Global warming may also impact this species, as it is expected to have the greatest effect at polar latitudes, possibly causing a contraction of Arctic breeding habitat.

The key to conservation of this species is the identification and regulation of important migration staging and wintering areas, possibly by including these habitats in the Western Hemisphere Shorebird Reserve Network. In southernmost Brazil, a major migration staging area of knots at Lagoa do Peixe has been made a national park by the Brazilian government. There are some benefits to having the birds around, besides how unexpectedly interesting they are; shorebird migration brings many birders to the coast, which in Delaware Bay alone, generated nearly $36 million in one season.

*Fun fact: a group of knots is called a tangle.

"To stand at the edge of the sea, to sense the ebb and flow of the watch the flight of shore birds that have swept up and down the surf lines of the continents for untold thousands of to have knowledge of things that are as nearly eternal as any earthly life can be."
~ Rachel Carson

Where I learned about red knots, and you can too!

Audubon Society

Cornell Lab of Ornithology

The Birds of North America Online


U.S. Fish & Wildlife Service

Animal Diversity Web

Classic Collection of North American Birds


Classics Archive


The Viking Network