In the first week of the red knot stopover on Delaware Bay, there were just enough horseshoe crab eggs for the migratory birds to refuel for the rest of their journey to the Arctic.
The birds eat horseshoe crab eggs, relying on beaches having a high enough density of them that some will be near the surface, said researcher Larry Niles of Niles and Smith Conservation Services on the Conserve Wildlife Foundation blog.
“All together it looks like a normal early migration and a modest horseshoe crab spawn, just barely enough for the birds in the bay,” Niles reported.
The data covers May 12 to May 19.
“However, we are still short of about half the population. Our bay wide count won’t take place until next week on May 22 and 26,” he wrote. “At this point it looks like we have about 14,000 knots in the bay, of which 8,000 are in New Jersey. In the last five years we have had a bay wide population of about 24,000 red knots.”
Niles also studies migratory shorebirds ruddy turnstones and sanderlings, and he said said their situation is similar. He predicted southerly winds will bring the rest of the flock north in the next few days.
Red Knots need to reach at least 180 grams in weight to make it to the Arctic and breed successfully, Niles said. Some birds have more catching up to do than others.
“This week we caught birds that weighed 93 grams which is 30 grams below fat-free weight. These birds had just arrived from a long flight, probably from Tierra del Fuego, Chile, or Maranhao, Brazil,” Niles wrote. “In the same catch, we weighed red knots as high as 176 grams or only 5 grams from the 180-gram threshold. These birds are probably from Florida or the Caribbean wintering areas and so arrive earlier, resulting in them having more time to gain weight.”
Niles has said the Delaware Bay population of horseshoe crabs needs to be increased. While New Jersey has had a moratorium on killing them for about a decade, other nearby states continue to take them for conch bait, especially during the months they spend in the Atlantic Ocean.
He has estimated the population is only about one-third of numbers from 20 years ago in the bay.
While New Jersey and Delaware have done beach replenishment to give the crabs spawning habitat, there are simply not enough crabs to use the bigger beaches in a way that benefits red knots.
The problem is decreasing density of eggs, which “means fewer eggs reaching the surface because crabs are not digging up existing eggs to lay their own,” according to Niles. Red knots require eggs to be brought close to the surface in order to eat them.