Causes of the Red Knot Decline Sarah Karpanty1, Jim Fraser1, Jim Berkson1, Jonathan Cohen1, Eric Smith2

1Department of Fisheries and Wildlife Sciences2Department of Statistics

The Problem: Declining Red KnotsWhy study migratory shorebirds? • Migratory shorebirds are highly sensitive to anthropogenic changes in their stopover habitat because they must replenish their fat reserves in a very limited time window before continuing to Arctic breeding or South American wintering sites. •Many of these migratory bird populations are experiencing rapid global declines for unknown reasons.

The red knot (Calidris canutus rufa), exemplifies the problems observed in many migratory shorebird species. • The red knot embarks on one of the longest of bird migrations (Fig. 1).• The red knot population is declining precipitously and could become extinct by 2010 if current trends continue. • The red knot is a Candidate for listing under the Endangered Species Act.

The Controversy: Management of Horseshoe Crabs for multiple purposes

• It is hypothesized that the red knot population is limited by the availability of horseshoe crab eggs in Delaware Bay for two reasons.

– Horseshoe crab eggs, the main food source for knots at this stopover, have declined in availability due to unregulated harvest of crabs in the 1990s,– The steep decline in the red knot population in the late 1990s was correlated with a decreasing ability of the red knots to gain proper weight during stopover in Delaware Bay.

• Horseshoe crab harvest (Fig. 2) has been severely restricted or eliminated by regulatory agencies to protect the red knot and other shorebirds.

• Management of crabs for commercial bait and for the use of their blood by the biomedical industry must also be considered.

Prediction 1: Red knot distribution should be largely determined by horseshoe crab egg distribution and abundance.• Prey analyses and radio-telemetry showed that red knot distribution is predicted by crab egg distribution at a landscape level (Fig. 3) and micro-habitat level.• Coquina clams, mussel spat, and human disturbance (Fig. 4) also influenced knot distribution. (Karpanty et al.2007 Journal of Wildlife Management 70:1704-1710)

Conclusions & Management Implications• In Delaware Bay, red knot distribution is driven by horseshoe crab egg abundance and distribution.• Red knots are currently not depleting available horseshoe crab egg resources at the beach-level in Delaware Bay.• Red knots are using coastal Virginia as an independent, alternative stopover and foraging on Coquina clams and mussel spat at that site.

Evidence suggests that red knots are not currently limited by the availability of horseshoe crab eggs in Delaware Bay.

We will expand our studies to coastal Virginia, the Arctic breeding grounds, and South American wintering grounds to explore what factors are driving the continuing

decline of the red knot.

Fig 1. Northward and southward migration pathways of the red knot. These birds fly > 20,000 miles annually to complete this round-trip journey from the wintering grounds to the breeding grounds.

ObjectiveTo examine the hypothesis that the red knot decline is caused by a shortage of horseshoe crab eggs at the Delaware Bay stopover site.• We test 3 predictions from this hypothesis.

Fig 2. Horseshoe crab harvest is economically important to commercial watermen ($15 million/yr) and the biomedical industry ($150 million/yr).

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Prediction 2: Red knots should deplete horseshoe crab eggs during stopover in Delaware Bay.• An exclosure experiment (Fig. 5) showed no evidence of bird depletion of horseshoe crab eggs in plots were birds were allowed to feed versus plots from which they were excluded, beach-wide.• We did find a net 86% loss of eggs in areas of the wrack line, one micro-habitat type, where birds were permitted to forage versus areas from which they were excluded.

Prediction 3: There should be little alternative food and no alternative stopover sites available.• In Delaware Bay, we found little red knot food other than horseshoe crab eggs in the preferred bay beach habitat (Fig. 7).• Red knots do use mussel spat and Coquina clams as an important alternative food in Delaware Bay marsh and ocean beach habitats.• We have identified that 1/3rd of all known red knots stopover in coastal Virginia during spring migration and that this may be an independent stopover site from Delaware Bay.

M. Oates

J. Zipp

Fig 3. Red knots (proportion of locations) shift away from peat beach and marsh to sandy Delaware Bay beaches with crab eggs after the May 19th peak in crab-spawning. χ32 = 9.30, P = 0.03

Fig 7. In the preferred Delaware Bay beach habitat, horseshoe crab eggs dominate the prey items. There were more crab eggs in sites used by red knots than random sites without red knots.

Fig 4. Despite regulations limiting human uses of beaches during the peak migration season in Delaware Bay, we still found evidence that red knots were avoiding areas with high levels of human disturbance.

The Red Knot, an imperiled shorebird.

Fig 5-6. We compared rates of egg loss between bird exclosures (left) and areas where birds were allowed to feed. Birds depleted eggs only from the wrack line micro-habitat (right), not beach-wide.