Breeding in the Canadian Artic and wintering in Argentina and Chile, red knots undertake an epic migration journey of around 9,300 miles (15,000 km) twice every year. In order to complete the voyage successfully, red knots (Calidris canutus) require top quality food sources, and previously they have found this in abundance in the shape of horseshoe crab eggs in Delaware Bay. However, it’s been noted that red knot numbers have declined drastically since the turn of the century, with one of the main reasons being the decline in horseshoe crabs that have been harvested for commercial gain. Without sufficient fuel, these medium-sized shorebirds may not make it back to their Arctic breeding grounds, or if they do, they may be too weak to breed successfully, and considering that nearly 90 percent of the red knot population use Delaware Bay on their migration route, the lack of food could result in the species becoming endangered, or extinct.
Recognizing this problem, the US Fish and Wildlife Service (FWS) have proposed that red knots be classified as ‘threatened’ under the Endangered Species Act (ESA). Data reveals that there has been about a 75 percent decline in red knot numbers since the 1980s, with the rate of decline increasing sharply after the year 2000, coinciding with the decline in horseshoe crab populations. Red knots arrive at Delaware Bay just as horseshoe crabs arrive on the beaches to lay their eggs in shallow holes they dig in the sand. Each female lays up to 120,000 eggs in batches, which are then fertilized by the male that hitched a ride on her back to the beach. Shorebirds, including the red knot, eat many thousands of these protein rich eggs in the two week period before they hatch.
When red knots arrive at Delaware Bay, they are quite exhausted and emaciated. They need to rebuild their strength and stock up on fat reserves for the arduous journey ahead. As they feast on the eggs, they undergo a number of interesting physiological changes. As documented by the FWS, these include an increase in fat stores, and an increase in size of the chest (pectoral) muscles and heart, while the gizzard, stomach, intestines, liver and leg muscles of the birds decrease in size, all in preparation for the last leg of their migration.
While the decline of horseshoe crab eggs as a food source is a serious problem for red knots, it is not the only problem they face. The FWS notes that climate change is altering the terrain they breed in and impacting their diets on their home turf. Rising sea-levels and coastal development are other issues. But these are beyond the control of conservationists concerned with the plight of migratory birds. What can (and likely will) change is when harvesting of horseshoe crabs takes place, so that when the red knots arrive they have first choice of the horseshoe crabs’ eggs to fuel up for the last leg of their journey.