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Bird migration

Staging areas are critical for successful migration of birds. Delaware Bay is one of these critical areas and is the site where more than 10,000 shorebirds stop over on their migrations to the Arctic, where they will ultimatley breed.

It is an assumption that the attraction of shorebirds to the Delaware Bay staging area has been the abundant and predictable food source.  The eggs produced by the mass spawning of the largest known population of Atlantic horseshoe crabs (Limulus polyphemus).  Because Delaware Bay is the last stopover before the breeding ground, shorebirds are thought to depend on crab eggs for rapid migratory fattening and timely departure to breeding destinations.  Body reserves obtained from crab eggs not only fuel migration, but may sustain shorebirds in the event of inclement weather and food deprivation on arrival at Arctic breeding grounds.  In this way, Delaware Bay crab eggs seem inexorably linked to shorebird migratory and reproductive fitness.

The importance of crab eggs to shorebirds has become a topic of heated debate as renewed commercial exploitation of horseshoe crabs and possible over harvest of the fishery in the mid-to-late 1990s has led conservationists to link declining shorebird populations with dwindling supplies of crab egg.  Lack of sufficient crab eggs might be detrimental to migration, reproductive success, and even survival. Crab eggs are thought to be especially important to long-distance migrants, like red knots (Calidris canutus rufa), that are time constrained to complete a ca.15,000 km trans-hemispheric migration from the Patagonian coast of South America to the Canadian Arctic.

In collaboration with Mike Haramis at USGS in Patuxent MD,, we used stable isotope methods and pen feeding trials to determine the value of crab eggs to red knots and ruddy turnstones.  See our results in the Journal of Avian Biology.

Haramis, G. M., Link, W.A., Osenton, P.C., Carter, D. B., Weber, R. G., Clark, N. A., Teece, M. A. and Mizrahi, D. S. (2007). The value of horseshoe crab eggs to spring migrant red knots and ruddy turnstones in Delaware Bay: experimental evidence from stable isotope, pen feeding trial and lipid studies. Journal of Avian Biology, 38, 367-376.

Stable isotopes as indicators of bird metabolism

In collaboration with Stuart bearhop and others, we looked at the limitations and potential applications of using stable isotopes to determine bird diets

Bearhop, S., Teece, M. A., Waldron, S. and Furness R. W. (1998) The influence of lipid and uric acid upon d13C and d15N values of avian blood: Implications for trophic studies. The Auk, 117, 504-507.

Birds are weighed during stop over on Delaware Bay
Red Knots are measured and counted by Chris Swarth in this photo