Departure patterns of Whimbrels using a terminal spring staging area
141 – 146
1 August 17
Bryan D. Watts, Fletcher M. Smith, Barry R. Truitt
Center for Conservation Biology, College of William & Mary and Virginia Commonwealth University, Williamsburg, VA 23187-8795, USA
Many Arctic-breeding shorebirds utilize terminal spring staging areas that are strategically positioned within mid-temperate latitudes. Because these staging areas represent final refueling opportunities and have a direct link to the breeding grounds, the condition of birds and their pattern of departure may influence reproductive performance. A significant portion of Canada’s Hudson Bay and Mackenzie River Whimbrel populations utilize the tidal salt marshes and mudflats of the mid-Atlantic coast of North America as a terminal stopover site. We observed Whimbrels (n = 39,720 individuals) departing from the lower Delmarva Peninsula in Virginia (USA) during the second half of May (2009–2014) to quantify departure dates, time of day and flock size. The majority (90%) of Whimbrels departed the study area during the six-day period of 22–27 May with a clear peak during 24–25 May when mean departure rates exceeded 600 birds/hr. Departure date appears to be highly conserved with median date varying across only three days over six years. Departures peaked approximately 2.5 hrs before civil twilight with 82% of individuals leaving within the two-hr period between 1.5 and 3.5 hrs before twilight. The distribution of departing flock sizes approximated a negative exponential as smaller flocks were more common. Although 50% of all flocks recorded contained less than 35 individuals, 50% of all individuals occurred in flocks that contained 80 individuals or more. The highly synchronous and consistent departure pattern may help to facilitate synchronous arrival on the breeding grounds and reinforce mate fidelity. Group travel may provide benefits such as collective navigation and energetic savings related to flock aerodynamics.