Horseshoe Crab Eggs Determine Red Knot Distribution in Delaware Bay

Abstract

A decline in red knots (Calidris canutus rufa) has been attributed to horseshoe crab (Limulus polyphemus) egg shortages on the Delaware Bay, an important foraging area for migrating knots. We studied the movements and distribution of 65 radiotagged red knots on Delaware Bay from May to June 2004 and related movements to the distribution and abundance of horseshoe crab eggs and other prey and to other habitat characteristics. The number of horseshoe crab eggs was the most important factor determining the use of Delaware Bay beaches by red knots (logistic regression cumulative Akaike's Information Criterion adjusted for small sample size [A/Cc] w = 0.99). The knots shifted from emergent marsh and peat‐beaches to sandy Delaware Bay beach when crab eggs became abundant, which also suggested the importance of crab eggs. While red knots used sandy beach zones more than expected, given their availability, 44% of red knot low tide locations were in bay and coastal emergent marsh. The abundance of Donax variabilis (A/Cc w = 0.95) and Mytilus edulis (A/Cc w = 0.94) spat, both food for red knots, had a relationship with red knot use of sandy beaches. Levels of disturbance and the abundance of laughing gulls (Larus atricilla) also were important factors in red knot sandy beach use, although secondary to prey resources (A/Cc w < 0.4). These results are consistent with the hypothesis that the abundance of horseshoe crab eggs on sandy beaches is driving movement and distribution of red knots and that there is little alternative food during the migratory stopover in Delaware Bay. Our findings that red knots disproportionately use Delaware Bay sites with abundant eggs and that there is a lack of surplus eggs at areas used and unused by red knots support the continuation of management for sustained yield of horseshoe crabs and other food resources at this stopover.

The effect of benthic prey abundance and size on red knot (Calidris canutus) distribution at an alternative migratory stopover site on the US Atlantic Coast

“…We found red knots in Virginia near the best food resources, as in Delaware Bay (Botton et al 1994;Karpanty et al 2006). Selection of larger prey items likely represents the fact that smaller organisms provide too little energy to be worth the handling time (Zwarts and Blomert 1992).…”

“…This decline has sparked concern about the supply of horseshoe crab (Limulus polyphemus) eggs which are a key food for red knots and other shorebirds during northward migration at this stopover site (Berkson and Shuster 1999;Tsipoura and Burger 1999;Baker et al 2004;Haramis et al 2007;Niles et al 2009). The red knot distribution in the Delaware Bay can be predicted by the distribution and abundance of horseshoe crab eggs (Botton et al 1994;Karpanty et al 2006), and a decline in horseshoe crab abundance has been related to decreased annual survival and lower pre-departure body mass of knots stopping in Delaware Bay (Baker et al 2004). Based on high depletion rates of horseshoe crab eggs by red knots foraging on experimental feeding trays, Gillings et al (2007) supported the hypothesis that horseshoe crab eggs are a red knot population-limiting factor.…”

Prey Distribution, Physical Habitat Features, and Guild Traits Interact to Produce Contrasting Shorebird Assemblages among Foraging Patches

“…The strong relationship between benthic invertebrates as prey and feeding shorebirds as predators is predictable based on many earlier studies of feeding shorebirds (e.g., [25]–[31]). The direct contribution of sediment size to explaining shorebird feeding distributions after already accounting for its influence acting indirectly through benthic invertebrate prey seems likely to reflect the ability of some sediments to modify the availability of the benthic prey independent of their abundance [32]–[34].…”

Tachypleus syriacus (Woodward)—a sexually dimorphic Cretaceous crown limulid reveals underestimated horseshoe crab divergence times

“…The new analysis also supports previous claims of morphological and ecological stasis within Limulidae, revealing a widespread distribution for both Limulinae and Tachypleinae throughout the Mesozoic with an apparent high degree of niche conservatism, as the earliest crown-group limulids occur in environments identical to those of their modern counterparts (Diedrich 2011). How these Mesozoic and Cenozoic species have responded to changing climate, as well as the patterns of survival and extinction exhibited by stem-group limuloid clades, are valuable sources of information for modern conservation efforts, especially given current fears over declining horseshoe crab populations in both Asia (Shin et al 2009;Mishra 2009) and America (Shuster and Botton 1985;Faurby et al 2010) and the subsequent impact on migrating shorebirds (Baker et al 2004;Karpanty et al 2006;Niles et al 2009) and loggerhead turtles (Seney and Musick 2007;Witherington et al 2009) that rely on the American populations as a source of food. The fossil record shows how these creatures have responded to habitat loss in the past and can be used to predict how modern species will respond to current and future environmental changes and guide conservation efforts.…”