Detection of prey DNA from faecal samples is a non-invasive method being successfully applied to the dietary analysis of birds as well as other vertebrates and invertebrates. We have employed this technique to examine the significance of Horseshoe Crab eggs (Limulus polyphemus) for Dunlins (Calidris alpina), Semipalmated Sandpipers (Calidris pusilla), Least Sandpipers (Calidris minutilla) and Short-billed Dowitchers (Limnodromus griseus) during northbound migration at their major stopover area in Delaware Bay. This was accomplished through polymerase chain reaction amplification of Limulus DNA from faecal samples using new Horseshoe Crab-specific primers. We detected the consumption of crab eggs in all study species, although results suggest that eggs may be a less important food source for Least Sandpipers than for the other species. Consumption of eggs increases as the migration season progresses, implying a strong preference of birds for this resource at the end of May, when eggs are readily accessible. The results confirm the value of Horseshoe Crab eggs for migrating shorebirds in this crucial stopover area. The Limulus-specific primers we designed can be effectively used for detection of semidigested Limulus DNA from faecal samples and thus they can be employed in the dietary analysis of other predators that seasonally capitalise on Horseshoe Crabs or their eggs.
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