** Andy S. asked

> Calling Jack Bowling!!!
>
> We need your analysis as to why the Stilt Sandpiper, the bulk of which
> usually migrate through the mid-continent, may have shifted its migration
> route west this year. If a weather pattern is responsible, shouldn't we be
> on the lookout for other cosmic stuff that's usually alien to this part of
> the world?

OK. A two step process as always: birds, then weather.

1) Birds - Stilt Sandpipers breed on the Arctic Slope from n. Alaska to the w.
shores of Hudson Bay. In spring, the migration almost totally follows the
Central Flyway with only a few stragglers making it w. of the Rockies. This
past spring one was seen near Merritt, s.c. B.C., a 5-star event. In the fall,
they stay on the w. edge of the Central flyway and as such are more often
encountered w. of the Rockies, although from all reports this year is a very
good one.

2) Weather - The spring weather was snotty across the western 2/3 of the
northern half of N. America, and none moreso than over the Canadian Prairies.
The weather pattern for the bulk of the spring was dominated by a strong upper
ridge extending from the Gulf of Alaska northeastward to the eastern Arctic; a
deep upper trough lay over central Canada. The airflow across the Prairies was
thus from the north or northeast through most of April and May; cold,
northerly, stiff breezes which no bird likes to fight. Conclusion: the birds
were pushed westward by the prevailing winds this past spring (and not only
waders). Theory: numbers of Stilt Sandpipers in the westernmost part of their
breeding range were higher than normal this year. Perhaps someone in Alaska
can comment on this.

Now how about the "fall" migration? Well, about ten days ago, an upper ridge
which had parked itself over the Prairies started to "retrogress" i.e. move
westward. That ridge now lies over the western edge of the Gulf of Alaska.
When the ridge was over the Prairies, a southwesterly airflow persisted over
the west coast. As it backed up westward, the flow veered from the southwest
into the northwest. For most of last week, the jetstream, i.e. the fastest
part of the flow around the ridge, ran parallel to the Aleutians, around the
top of the Gulf of Alaska and then tracked down parallel to the west coast. My
theory is that the relatively elevated population of Alaskan Stilt Sandpipers
(and Upland Sandpipers) which were already on there way through Alaska at this
time got caught in the flow and were carried down the coast. This fits the
recent sighting pattern well.

The flow has changed again since then and we are back into a south to
southwest flow along the coast again. If I were a betting man, I would say
that a pelagic trip out to 50N, 140W longitude right now would run into a bevy
of southbound Arctic breeders such a Red Phalaropes and Sabine's Gulls
starting to stream into the Gulf since this is where the strongest northerly
flow is occurring. Does this mean that the migrant waders should now taper
off? Probably the largest numbers are now going through, but I would like to
be wrong.

- Jack





Jack Bowling
Prince George, BC
jcbowling at mindlink.bc.ca