Tuesday, April 17, 2012

Calendar Birds

For arctic nesting Peregrine Falcons we know that late arrival at a breeding site does not entirely preclude a successful breeding season, but a shortened season stacks the odds against those that do arrive later.  Early arrival lengthens the season and may ensure easy access to a breeding territory, but the trade-off can be low food supplies.  Apart from resident lemmings (when present) and early season ground squirrels, Snow Buntings are often the only migrants species present when the falcons first arrive.

Male ground squirrels are among the first resident prey to appear in the spring
Snow Buntings are almost always the first migrant prey, often arriving ahead of peregrines (G.Court)
Under ideal conditions arrival would coincide with no competition for breeding sites, mild weather and plentiful food. To a degree, arrival depends on date of departure from wintering areas, which is prompted by, amongst other factors, day length.  Judgement with regard to when to leave can be very precise even over several years for some Calendar Birds.
 
A falcon known as Island Girl (see Falcon Research Group Blog) that winters in Chile and spends her summers on Baffin Island, Nunavut began her inward migration April 14, 2012.  Her departure date in each of the 4 migration years that she has been tracked has varied only slightly (12/04/2009, 13/04/2010 and 11/04/2011).  With no way to judge what can be highly variable environmental conditions thousands of miles to the north, tundra peregrines simply keep going until they encounter circumstances that preclude further travel.
Variability in snow cover near the Hudson Bay on or about departure day in years Island Girl has been tracked by FRG (green=snow free, blue=snow/ice, black=open water)
By late May snow cover around the Hudson Bay has retreated 300 miles further northward, in some years reaching almost as far as the Manitoba border with Nunavut.  Thousands of peregrines literally stream into the Arctic flying around or over the Hudson Bay at this time of year.
Differing routes taken by Island Girl 2009 - 2011
Chance encounters in the vast Canadian Arctic are extraordinary.  To get to our Baffin Island study site at Steensby Inlet our field crews usually usually fly from Rankin Inlet to Iqaluit, a route that overflies South Hampton Island and the Hudson Strait...a flight path remarkably similar to that taken by Island Girl in 2010.  
Island Girl's roost location (blue) June 2, 2010 (source http://www.frg.org/track_pefa12_2010n_archive.htm) and the approximate photo location (jet) also June 2, 2010



By 2010, we were already well aware of Island Girl's breeding location on south Baffin Island, and en route to Iqaluit, Mark Prostor snapped a shot of the Hudson Strait a little to the east of Salisbury Island and south of her breeding location on June 2.

Conditions in the the Hudson Strait June 2, 2010.  Edge of Salisbury Island can be seen middle right (M. Prostor)

You can follow daily location updates and the FRG crew as they literally chase "Island Girl" and "Felipe" (a satellite tagged male tundra peregrine) north from Chile.

4 comments:

Mark W said...

Interesting Al, so the peregrines diet switches to columbian ground squirrels (as well as avian) when they initially arrive back?

Alastair said...

Hi Mark W,

Where there are ground squirrels (none on Baffin Island), they are used all summer, with some individuals using them to greater degrees than others.

Alastair

Jeff said...

Alastair,
Thanks for the report, the photos and maps really add to it.
I'm amazed by the accuracy of the biologic clock telling these birds when its time to migrate.
Cheers.
Jeff

Alastair said...

Hi Jeff,

The best data for multiple years on the same bird comes from FRG (www.frg.org) - look at Sparrow King and at Island Girl for multiple years. You'll see that Sparrow King's departure varies quite a bit more than Island Girl's, but it's still within a narrow window of time.

Alastair