During spring migration, Red Knots time their arrival in the Mid-Atlantic to coincide with the spawning of horseshoe crabs. Red Knots (and other shorebird species) eat the freshly-laid eggs to gain weight for the final leg of their journey, from the Mid-Atlantic to the Arctic, where the birds need to find mates and breed within a fairly short window of time. A major increase in the harvest of horseshoe crabs, due to their increasing use as bait for whelk fishing, reduced the availability of horseshoe crab eggs. While some shorebird populations remained stable or suffered small decreases, the Red Knot population crashed.
A new study, published in the open access journal Ecosphere, confirms the importance of stopover habitats – and the food sources they provide – for migratory shorebirds. The study uses data collected during a multiyear banding project of shorebirds around Delaware Bay. (As readers may remember, I participated in one such banding session earlier this year.) The study used two types of data from the project: captured or recaptured birds with known identities and known weights and resighted birds with known identities but unknown weights. The project has been banding Red Knots and other shorebirds with metal bands since 1997; in 2003, it started tagging Red Knots with colored bands bearing alphanumeric codes that can be read with binoculars or a spotting scope.
The study used mathematical models to determine the relationships between spawning by horseshoe crabs and weight gain by Red Knots and between Red Knots' mass at departure and future survival. (You can read the study's methods section for a technical description of the models.) The results showed that how much weight Red Knots gain depends not so much on the overall horseshoe crab abundance but on the timing of horseshoe crabs are spawning. If horseshoe crabs spawn later than usual, Red Knots are not able to gain as much weight as they normally would because not enough eggs will be available before they have to depart for the Arctic. Weight gain at stopover sites is crucial since Red Knots with a light weight at departure were unlikely to be recaptured or resighted in subsequent years.
While the model found a correlation between weight gain and subsequent survival, it found a stronger relationship between survival and snow depth on the Red Knots' breeding grounds. Red Knots were more likely to survive from one year to the next if there was deeper snow on the ground when they arrived in the Arctic. The scientists offer several possible explanations. One is that greater snow depth means greater overall moisture, which increases the amount of invertebrate prey available for the Red Knots. Another is that lemming populations tend to crash in years with low snow depth, so in those years predators that normally eat lemmings might turn to shorebirds instead. Determining whether those or other hypotheses are correct will require further research in the Arctic.
The study reinforces the link between horseshoe crabs and Red Knots and the importance of Delaware Bay as a spring stopover site. All necessary efforts should be taken to ensure that the bay continues to provide adequate habitat and food for these long-distance migrants. Such efforts should include further restriction of horseshoe crab harvests, particularly in Delaware, which has continued to allow harvesting even though New Jersey has imposed an indefinite moratorium.
McGowan, Conor P., James E. Hines, James D. Nichols, James E. Lyons, David R. Smith, Kevin S. Kalasz, Lawrence J. Niles, Amanda D. Dey, Nigel A. Clark, Philip W. Atkinson, Clive D. T. Minton, and William Kendall. (2011). Demographic consequences of migratory stopover: linking red knot survival to horseshoe crab spawning abundance. Ecosphere, 2 (art69) : 10.1890/ES11-00106.1
* For international readers, the IUCN ranks Red Knot as a species of least concern; the discrepancy is that there are multiple subspecies, and many of them are secure. However, the western Atlantic subspecies – the one that breeds in the North American Arctic, migrates along the Atlantic Coast of North America, and winters in Tierra del Fuego – is in serious danger of extinction.