Subject: [Tweeters] Common Tern migration
Date: Nov 2 21:59:22 2009
From: Wayne Weber - contopus at telus.net


Birders,

Several observers have commented recently on TWEETERS about the relative
scarcity of Common Terns during fall migration in western Washington,
compared with the numbers seen 20 or 30 years ago. For the record, the same
trend has been obvious in and around Vancouver, BC, where Common Terns
probably were and still are more numerous than almost anywhere in western
Washington.

>From the 1960s through the 1980s, Common Terns were considered to be a
fairly common fall transient, being seen frequently from late July and early
August through late October, and sometimes even early November. It was
normal to see groups of dozens, and occasionally even into the hundreds, at
places such as Iona Island, White Rock, and Point Roberts-- often together
with Bonaparte's Gulls, and often pursued by one or two Parasitic Jaegers.
In the last few years, I would estimate that tern numbers, on the average,
are only 10% or 20% of what they were, and they seem to occur less often
early and late in the season.

On the other hand, numbers of Common Terns seen in spring, although they
vary greatly from year to year, have NOT declined markedly. In spring, they
are often seen only in a couple of places (especially Drayton Harbor and
Iona Island), and often for only a few days. However, they occur then in
larger flocks than in the fall. This spring (2009) produced some of the
largest numbers of Common Terns ever reported around Vancouver, with up to
2000 birds in a day at Iona Island, and 900 at Blackie Spit on Mud Bay in
Surrey.

It should be noted that Common Terns nest entirely east of the Rocky
Mountains, and winter mainly from Mexico south to Peru and Argentina. They
seem to be much scarcer as migrants (spring and fall) in Oregon and
California than in Washington and B.C.
Perhaps, for some reason, a smaller percentage are migrating along the
Pacific Coast in the fall than in the past, although it doesn't seem to have
affected the spring movement.

Franklin's Gulls have also become much scarcer fall migrants around
Vancouver, declining from "uncommon to rare" status to "extremely rare" in
the last few years (one or two sightings a year). However, this trend may or
may not be related to the Common Tern trend.

Wayne C. Weber
Delta, BC
contopus at telus.net