Sufficiency of horseshoe crab eggs for red knots during spring migration stopover in Delaware Bay USA

Abstract

Horseshoe crab (Limulus polyphemus) eggs are a dietary staple of the red knot (Calidris canutus) during its spring stopover on the Delaware Bay. Numbers of knots stopping in Delaware Bay declined in the 1990s concurrent with a decline in horseshoe crabs, leading to the hypothesis that reduced horseshoe crab egg abundance limited the red knot population. Management efforts, including a seasonal harvest moratorium in the Delaware Bay, have been instituted to restore crab populations to levels of sustainable use by multiple users, including migratory birds. Our objective was to evaluate the sufficiency of horseshoe crab eggs in Delaware Bay in May–June 2004 and 2005 for knots to refuel for their migratory flight to the Arctic breeding grounds. We examined egg counts to determine if there were fewer high egg‐density sites later than earlier in the day and season, as migrating birds might deplete this resource. We studied foraging rates at red knot locations to determine if foraging probes increased with time of day and season as birds depleted surface eggs by pecking, then began probing for subsurface eggs. Finally, we experimentally tested whether red knots and their competitors depleted horseshoe crab eggs. Crab egg numbers at knot foraging sites did not decline throughout the day or season in 2004. In both years, we found no evidence that knots switched from pecking to probing with increases in time since sunrise or start of the stopover. Egg numbers were similar in exclosed and accessible plots on crab nesting depressions and in areas of open intertidal zone, but were significantly lower in accessible than in exclosed plots in the wrack line. Our results indicate that horseshoe crab eggs in Delaware Bay were sufficient to support the refueling of the present‐day stopover population of red knots. If an increase in the availability of crab eggs to foraging birds does not result in an increase in knot numbers, managers must prioritize mitigation of limiting factors at other historically important spring stopovers and on the poorly understood breeding and wintering grounds in addition to the Delaware Bay. © 2011 The Wildlife Society.

Weak niche partitioning by migrating shorebirds in a single-food-type environment

“…2,65,66 Nevertheless, some research indicates that the abundance of Limulus eggs in the bay is sufficient for successful refueling of shorebirds. 67 Thus, without knowing whether the eggs are limiting resource for shorebirds, putting observed high overlaps in the context of competition, either intense or relaxed, would be rather speculative.…”

Evaluating a multispecies adaptive management framework: must uncertainty impede effective decision‐making?

“…Shorebird declines have been reported for many arctic-breeding species and across migratory flyways (Gratto-Trevor et al 2011;Morrison et al 2012;Ross et al 2012). Recent evidence suggests that factors other than horseshoe crab eggs have contributed to population dynamics of red knots; these factors include changes in arctic conditions and in wetland habitats range-wide, which have affected shorebirds broadly (Gratto-Trevor et al 2011;Karpanty et al 2011;McGowan et al 2011a;Fraser et al 2013). However, Delaware Bay is a critical stopover for the rufa subspecies (Niles et al 2008), and the management of horseshoe crabs is framed to harvest sustainably, while maintaining adequate stopover resources for migrating shorebirds (ASMFC 1998;McGowan et al 2009).…”

Factors that affect migratory Western Atlantic red knots (Calidris canutus rufa) and their prey during spring staging on Virginia’s barrier islands

“…Third, flock size alone is not directly related to the overall number of birds using the islands. Fourth, prey may have been depleted at some sites during peak migration before red knots were counted [32,33], and fifth, site suitability varies in the intertidal zone. For example, some peat banks are located far above the low-tide line and thus become dry throughout the tidal cycle.…”

“…Historically, much attention was given to red knots using the Delaware Bay spring staging site [29][30][31], as it consistently supports between about 50-70% of the annual spring migrating population of red knots, at higher densities compared to Virginia's barrier islands (on average 291 red knots/km shoreline in Delaware Bay vs. 81 red knots/km in Virginia [29][30][31]). The large number of red knots using the Delaware Bay staging site each spring usually is attributed to the abundance of Atlantic horseshoe crab eggs (Limulus polyphemus [30][31][32][33][34][35][36][37]).…”

“…Although red knots feed primarily on horseshoe crab eggs in the Delaware Bay [30,[32][33][34], they feed on hard-shelled bivalves throughout most of their migration and boreal wintering ranges [38][39][40]. The red knot's reliance on bivalves includes its Virginia spring staging site, where coquina clams (Donax variabilis) and blue mussels (Mytilus edulis) have been described as the most abundant and used prey resources by red knots [31,41].…”