Across all six subspecies, red knots are specialized molluscivores, eating hard-shelled mollusks, sometimes supplemented with easily accessed softer invertebrate prey, such as shrimp- and crab-like organisms, marine worms, and horseshoe crab (Limulus polyphemus) eggs. The mollusk prey is swallowed whole and crushed in the gizzard, which in the red knot is the largest (relative to body size) among any shorebird species evaluated. Large gizzards are among this species’ adaptations to a mollusk diet, allowing red knots to grind the hard shells of its prey. Red knots prefer thin-shelled over thick-shelled prey species because they are easier to digest and provide a more favorable meat-to-mass ratio (higher prey quality). From studies of other subspecies, some researchers concluded that red knots cannot ingest prey with a circumference greater than 1.2 inches. For rufa red knots, prey lengths of 4 to 20 millimeters (mm) have been observed. Foraging activity is largely dictated by tidal conditions, as red knots rarely wade in water more than 2 to 3 centimeters (cm) deep. Due to bill morphology, the red knot is limited to foraging on only shallow-buried prey, within the top 2 to 3 cm of sediment. Along the U.S. Atlantic and Gulf coasts, Donax and Mulinia clams and blue mussel (Mytilus edulis) spat are key prey items for the rufa red knot. A prominent departure from typical prey items occurs each spring when rufa red knots feed on the eggs of horseshoe crabs, particularly (but not exclusively) in the key Delaware Bay migration staging area. In Delaware Bay, horseshoe crab eggs are a superabundant source of easily digestible food, and horseshoe crab eggs are a preferred food item across many portions of the U.S. coast. Away from the coasts, the rufa red knot’s diet is poorly known but may include brine shrimp or insects.
Rufa red knots undertake long flights that may span thousands of miles without stopping. As red knots prepare to depart on long migratory flights, they undergo several physiological changes. Before takeoff, the birds accumulate and store large amounts of fat to fuel migration and undergo substantial changes in metabolic rates. In addition, the leg muscles, gizzard, stomach, intestines, and liver all decrease in size, while the pectoral muscles and heart increase in size. Due to these physiological changes, red knots arriving from lengthy migrations are not able to feed maximally until their digestive systems regenerate, a process that may take several days.
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Delaware Bay serves as the principal spring migration staging area for the rufa red knot because of the abundance and availability of horseshoe crab eggs. Delaware Bay provides the final Atlantic coast stopover for a significant majority (50 to 80 percent) of the red knot population making its way to the arctic breeding grounds each spring. Rufa red knots stopping in Delaware Bay depend on horseshoe crab eggs to achieve remarkable rates of weight gain. Weight gains recorded on the Delaware Bay are the most rapid of any recorded for all known stopover populations of red knots in the world and are among the highest rates observed in the animal kingdom. Although a single horseshoe crab egg contains a very small amount of energy, eggs are present in such superabundance that birds can eat enough in two weeks to nearly double their weights.
On the breeding grounds, the rufa red knot’s diet consists mostly of terrestrial invertebrates such as insects and other arthropods. However, early in the breeding season, before insects and other macroinvertebrates are active and accessible, red knots will eat grass shoots, seeds, and other vegetable matter.
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