Sunday, June 2, 2013

Coincidence - maybe not.

Watching “Island Girl” (a female Peregrine Falcon tagged by FRG in 2009) migrate northward for the 5th consecutive year, it's difficult not to be charmed by her.  By combining efforts across research teams, she and other PTT- or otherwise-tagged birds have disclosed a few more of the nifty migratory tricks used by peregrines flying to and from the breeding grounds.

Small samples have a nasty way of revealing enough information to get folks all hot and bothered, and subsequently flustered when seemingly clear patterns turn out to be no more than mere coincidence when viewed in the context of larger samples.   But just for fun, Mark Prostor (who is a big part of the Arctic Raptors research crew) compared the relative positions of “Island Girl” and “64493” (a PTT-tagged peregrine in 2011 by Arctic Raptors) as they make a final push for their respective breeding territories on Baffin Island and the Melville Peninsular (near Igloolik, Nunavut).

PTT data showed that both birds paused this last week, and despite being more than 700km apart, when viewed in the context of MODIS near-real-time satellite imagery, their stop locations coincided with the snow-line which tends southward nearer to the Hudson Bay and James Bay.  Taking the stop location of each bird relative to their known destinations revealed additional similarities;  both birds positioned themselves at the snowline about 1300km direct line of flight to their breeding sites at remarkably similar compass headings (about 50 degrees).
Stop-locations of "Island Girl" (red) and "64494" (blue) at the snow line (white arc). Note the parallel compass headings (about 50 degrees) from their relative stop-locations at the snow line;  at the time (May 31) both birds had about 1300km to reach their respective nesting location; both birds took a northwest heading near the US/Canada border before resuming their flight to the northeast. 
Earlier flight paths showed another similarity;  both birds veered northwest near the US/Canada border rather than simply maintaining a northeast heading which would have taken them on a much more direct route towards their breeding locations.

Incidentally, both birds will also arrive much later than other tundra peregrines.   Despite extensive snow cover, our spring survey at Rankin Inlet showed that most breeding sites were occupied by pairs by May 22, and tracking by light-based geolocators (which weigh 1 gram) showed that arrival of some birds at Rankin Inlet was as early as May 20.  Breeding latitude likely plays a part (both “Island Girl” and “64493” have further to go than birds breeding at Rankin Inlet) in the explaining the difference.  However, our field crews trapped birds on territory near Igloolik on May 28th.  While it could be coincidence that both PTT-wearing birds will arrive late, just maybe it’s not.

Snow covered sea-ice at Rankin Inlet with Barrier Island cliffs on right.  The region received almost a meter of snow in 3 days less than a week before peregrines arrived on territory.  (Photo E. Hedlin)