Shorebird predation of horseshoe crab eggs in Delaware Bay: species contrasts and availability constraints

1. Functional responses -- the relationship between resource intake rate and resource abundance -- are widely used in explaining predator-prey interactions yet many studies indicate that resource availability is crucial in dictating intake rates. 2. For time-stressed migrant birds refuelling at passage sites, correct decisions concerning patch use are crucial as they determine fattening rates and an individual's future survival and reproduction. Measuring availability alongside abundance is essential if spatial and temporal patterns of foraging are to be explained. 3. A suite of shorebird species stage in Delaware Bay where they consume horseshoe crab Limulus polyphemus eggs. Several factors including spawning activity and weather give rise to marked spatial and temporal variation in the abundance and availability of eggs. We undertook field experiments to determine and contrast the intake rates of shorebird species pecking for surface and probing for buried eggs. 4. Whether eggs were presented on the sand surface or buried, we demonstrate strong aggregative responses and rapid depletion (up to 80%). Depletion was greater at deeper depths when more eggs were present. No consistent give-up densities were found. Type II functional responses were found for surface eggs and buried eggs, with peck success twice as high in the former. Maximum intake rates of surface eggs were up to 83% higher than those of buried eggs. 5. Caution is needed when applying functional responses predicted on the basis of morphology. Our expectation of a positive relationship between body size and intake rate was not fully supported. The smallest species, semipalmated sandpiper, had the lowest intake rate but the largest species, red knot, achieved only the same intake rate as the mid-sized dunlin. 6. These functional responses indicate that probing is rarely more profitable than pecking. Currently, few beaches provide egg densities sufficient for efficient probing. Areas where eggs are deposited on the sand surface are critical for successful foraging and ongoing migration. This may be especially true for red knot, which have higher energetic demands owing to their larger body size yet appear to have depressed intake rates because they consume smaller prey than their body size should permit.     

Microsatellite DNA markers for the study of horseshoe crab (Limulus polyphemus) population structure

Twenty‐two microsatellite DNA loci were identified and characterized for horseshoe crabs (Limulus polyphemus) collected from two Atlantic coast and one Gulf of Mexico site. These markers revealed a high degree of genetic diversity (8–35 alleles per locus), heterozygosity (25.0% to 100.0%), and allelic heterogeneity (69.8% of comparisons). Considerable regional differentiation was observed as genetic distances (chord) ranged between 0.25 and 0.45, and all F_ST values (0.014–0.092) were significant. These preliminary findings are consistent with patterns of regional differentiation observed using allozyme variation and contradictory to findings of limited gene flow reported for sequence variation at the mitochondrial DNA COI region.     

Among‐year site fidelity of Red Knots during migration in Washington

ABSTRACT Many shorebirds exhibit within‐ and among‐year site fidelity during their annual cycle. Little is known, however, about the migration ecology of Red Knots (Calidris canutus) that migrate along the Pacific Flyway and occur in Washington in numbers that exceed counts elsewhere on the flyway. At two large estuaries in coastal Washington, Grays Harbor and Willapa Bay, we searched for and recorded the locations of Red Knots (N= 547) that had been individually marked with leg flags at their wintering grounds in Baja California Sur, Mexico, during the period from October 2006 to April 2009. In 2010, we resighted 43 Red Knots at Grays Harbor and Willapa Bay that had been observed at these sites in previous years, primarily in 2009. We found a high degree of site fidelity between years, with birds observed in 2010 more likely to return to the same stopover site used in 2009 than to switch stopover sites. For knots that did not switch estuaries between years, the median nearest distance between locations where individuals were observed between years was 1.4 km at Grays Harbor and 0.6 km at Willapa Bay. Our results provide the first evidence of stopover site fidelity by Red Knots of the roselaari subspecies. Fidelity occurred at three spatial scales: coastal Washington, the two estuaries where we conducted our study, and specific mudflat areas within the estuaries. Because our study sites support high populations of bivalves, Red Knots may be returning to the same areas in subsequent years to exploit what we suspect is a predictable food resource. The abundance of Red Knots and high degree of site fidelity suggest that our study sites in Grays Harbor and Willapa Bay are important for the conservation of this species on the Pacific Flyway.     

Residence Probability and Population Size of Red Knots During Spring Stopover in the Mid-Atlantic Region of the United States

ABSTRACT Weekly counts of western Atlantic red knots (Calidris canutus rufa) at their Delaware Bay migration stopover site have suggested a major decline since the 1980s. We estimated red knot spring passage population size in the New Jersey Coast-Delaware Bay region (DENJ; 2004 and 2006) and Virginia (VA; 2006 and 2007), USA, by correcting weekly aerial counts for mean daily residence probability between counts in a Monte-Carlo simulation. We used daily telemetry relocations in mark-resight models to estimate mean daily residence probability. Average daily residence probability was approximately 1.0 in mid-May, 0.96–0.97 in the week of 22 May, and 0.64–0.77 after May 28 in DENJ in 2004 and 2006 and in VA in 2006. Average daily residency was approximately 0.88 in VA in 2007 from 22 May to 5 June. No birds moved from VA to DENJ in 2006 and only 2 birds (5.5%) moved in 2007. Stopover population sizes (±SE) in DENJ were 17,108 ± 1,322 in 2004 and 19,555 ± 831 in 2006, and in VA were 7,224 ± 389 in 2006 and 8,332 ± 718 in 2007, significantly greater than peak aerial counts. Years with similar peak counts had different residence probabilities; hence, adjustments for turnover should be used in the future to assess annual population changes. Our results suggest that VA can support a significant portion of this red knot subspecies during migration in at least some years. Managing red knots for recovery should entail improving our understanding of the use of other Atlantic Coast sites and protecting key coastal habitat from disturbance and development.     

Stopover Ecology of Red Knots in Southwestern James Bay During Southbound Migration

Many shorebirds rely on small numbers of staging sites during long annual migrations. Numerous shorebird species are declining and understanding the importance of these staging sites is important for successful conservation. We surveyed endangered rufa red knots (Calidris canutus rufa) staging in James Bay, Ontario, Canada, during southbound migration in 2017 and 2018. We used mark-resight data and count data in an integrated Bayesian analysis to quantify migration phenology, estimate passage population size, and model the age structure of the stopover population. Many adult red knots arrived in James Bay in a single wave in early August in 2017, whereas adult red knots arrived in multiple smaller waves in July and mid-August in 2018. These waves may correspond with breeding phenology where more red knots bred successfully and arrived in one large event in 2017 and the higher number of earlier arrivals in July 2018 may have been failed breeders. We included a binomial generalized linear model in the integrated analysis to estimate that 20% and 10% of staging red knots were juveniles in 2017 and 2018, respectively. In future applications, this method could provide a metric to assess breeding performance and develop our understanding of its role in population declines. Overall, we estimated that up to 23% of the estimated rufa red knot population staged in southwestern James Bay for an average of 10–12 days. The region is a key staging site for endangered red knots and could be included in conservation planning. © 2021 The Wildlife Society.     

Foraging conditions ‘at the end of the world’ in the context of long‐distance migration and population declines in red knots

The long‐distance migrant red knot (Calidris canutus ssp. rufa– Scolopacidae) alternates between the northern and southern ends of the New World, one of the longest yearly migrations of any bird and paradoxically overflying apparently suitable habitat at lower latitudes. This subspecies is sharply declining, with a major mortality event following 2000, attributed to commercial overharvesting of food resources at its Delaware Bay (USA) stop‐over site. A full understanding of this peculiar migrant requires an assessment of the foraging conditions at its southern hemisphere wintering sites. Here, for a major wintering site in Argentinean Tierra del Fuego (Río Grande), we describe and compare food abundance, diet and intake rates during January–February in 1995, 2000 and 2008. The two main prey types were the burrowing clam Darina solenoides and three species of epibenthic mussels Mytilidae. In the year 2000, food availability and intake rate were higher than those recorded at other sites used by knots anywhere else in the world, contributing to the explanation of why red knots carry out this impressive migration. Intake rate in 2008 on the two main prey types was dramatically reduced as a result of birds eating smaller prey and strongly increased human disturbance; the same year we also found a high prevalence of a digenean parasite in Darina. We suggest that during the strongly enhanced winter mortality in 2000, knots did not yet face ecological problems in their southernmost wintering area, consistent with the previous evidence that problems at northern stop‐overs negatively affected their numbers. However, in 2008 the ecological conditions at Río Grande were such that they would have facilitated a further decline, emphasizing the importance of a hemispheric approach to research and management.     

Accounting for heterogeneity when estimating stopover duration,timing and population size of red knots along the Luannan Coast of Bohai Bay,China

相似文献