Spring migration of red knots along the Virginia barrier islands

Abstract

We conducted low-altitude, aerial surveys from the last week of April through the first week of June (1995, 1996,(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) to assess patterns in bird abundance, migration phenology, and habitat use of red knots (Calidris canutus rufa) staging along the Virginia barrier islands. During most years, knots arrived in early May, reached peak abundance during the third week of May, and were gone by mid-June. The peak period (10 May-3 June) accounted for >90% of detections for all years. The mean high count during the study was 7,373 AE 407 (SE). Decadal comparisons of high counts, accumulated numbers for the period of peak abundance and the season totals provided mixed results. Knots within the study area used peat banks and sandy beaches for foraging. Intertidal peat banks were patchily distributed and represented only 6% of the shoreline. Mean knot densities were comparable between the 2 habitats in the early period of migration but diverged rapidly such that by the third week of May, knot densities were 10-fold higher on peat compared to sand. Knots exhibited wide annual variation in habitat use; the number of knots observed on peat varied from 13% to 91% of the total birds for a given year, presumably reflecting the underlying population dynamics of respective bivalve populations. Potential prey dynamics coupled with the patchy distribution of peat appear to cause year-to-year variation in space use along the island chain. The Virginia barrier islands appear to be part of a terminal staging area for red knots that stretches from New Jersey south through the Outer Banks of North Carolina. Documenting red knot use within portions of this region beyond the well-studied Delaware Bay may lead to a more complete understanding of the sampling process used by red knots to make staging decisions. Ó 2014 The Wildlife Society.

Influence of introduced peregrine falcons on the distribution of red knots within a spring staging site

“…Red knots use patches of exposed peat and open sand beaches as foraging habitat along the barrier islands [ 20 ]. The dominant prey used by knots within these two habitats include blue mussels ( Mytilus edulis ) for peat patches and coquina clams (Donax variabilis) for sandy beaches [ 48 ].…”

“…Intertidal bivalves are recruitment driven where larval settlement patterns are notoriously influenced by fluctuations in the physical environment [ 49 – 51 ], leading to dramatic spatio-temporal variation in standing stocks (e.g., [ 52 , 53 ]). Red knots exhibit wide year-to-year variation in the relative use of the two habitats [ 20 ], presumably reflecting the underlying population dynamics of the two bivalve populations. Annual variation in bivalve stocks may help to explain differences in the magnitude of site-to-site responses to occupancy of specific nesting towers.…”

“…The study area is a terminal spring staging site for several shorebird species that build fat reserves before moving on to arctic breeding grounds [ 40 , 41 ]. Red knots use the active beach zone along the islands for foraging [ 20 ]. The study area is also a restoration site for peregrine falcons and annually supports 8–12 breeding pairs that nest on artificial structures [ 42 ].…”

“…The population uses a network of staging areas to refuel along migration routes [ 18 ]. One strategically critical site is the mid-Atlantic Coast from New Jersey south to the mouth of the Chesapeake Bay where many birds refuel for the last time before making nonstop flights to Arctic breeding grounds [ 19 , 20 ]. The Virginia barrier islands support several thousand red knots during spring migration [ 20 ].…”

“…One strategically critical site is the mid-Atlantic Coast from New Jersey south to the mouth of the Chesapeake Bay where many birds refuel for the last time before making nonstop flights to Arctic breeding grounds [ 19 , 20 ]. The Virginia barrier islands support several thousand red knots during spring migration [ 20 ]. During some years, these birds represent a considerable (>25%) portion of the overall population known to stage along the Atlantic Coast [ 13 ].…”