Hi Tweets,

Re Vaux's Swift (VASW) Jack Bowling writes:

>I made a slip of the fingers in my previous post. The May 1-2 date of arrival
>for Vaux's Swifts is for *central* BC, not southern BC as I stated. Michael
>Price will no doubt catch this boo-boo. I believe it is mid-April for southern
>BC so the timing of their migration is actually a bit late rather than early!

Jack, I assumed that's what you meant; I was just filling in the SW corner
of the province (your Pacific Northwest, Yankee Geographical Imperialist
Revisionists! :-)))

>For aerial insectivores such as swifts, weather conducive to ascending air
>which carries insects aloft must be crucial to their northward movement.

So a query: why doesn't this stop swallows--the other aerial
insectivores--from arriving pretty much on schedule in February, March and
early April pretty much regardless of conditions other than outright
blizzard and iron-cold Arctic high pressure? What's different about the VASW
diet and feeding style that forces them to come in late if conditions aren't
*just* right for them? What conditions? In the city, I often see them in
feeding flocks mixing in with Violet-green Swallows (VGSW), which arrive in
mid- to late February, so what retards their progress north to such a degree?

And because the atmospherics at this time of the year are so changeable
(transl.--it can be goddam cold even to early May, delaying &/or precluding
aerial insectivore arrival), VASW arrival dates can really vary over quite a
wide range. Even factoring out mis-ID's of VGSW in 'stiff-wing' display at
the early end and non-reporting of returned birds leading to a later than
actual arrival date that year at the later end, the range seems
extraordinarily large: at least a month between earliest and latest. To put
this in context, the average range over the last decade and a half for
northbound arrivals is, as an educated guesstimate, three or four days
either side of the average arrival date. In any given year, about 75%-80% of
returning northbound migrants arrive within *two* days either side of their
average date.

And consider the other swift in Cascadia, Black Swift (BLSW): its average
arrival date for Vancouver BC is 5/21, and it's usually about four days
either side, a much tighter schedule. Why a large range for one and a far
smaller one for the other? I'd guess, and Jack B. would have the useful
intelligence here, that April is the crueller month to VASW because of
possibility--if not likelihood--that irruptions of cold Arctic air could
nuke their insect prey. Well, that first question returns: why doesn't this
affect swallows also? By later May, the weather patterns are more stable,
with less chance of getting blindsided by a late cold snap.

In response to my cautionary note on Vaux's Swift/Violet-green Swallow
flight profiles,
Janet Hardin writes:

>That's precisely why those of us who have helped train Marbled Murrelet
>surveyors are concerned with making certain that observers know the
>difference. Even though for a few years I thought it was *obvious* that
>murrelets and swifts had very different flight styles, I finally saw a
>couple of swifts flying as a pair and behaving much more like murrelets can
>in flight.

Janet, you dodge one in the fact that we don't have a inland-nesting
diminutive shearwater in the Region, otherwise you'd be having to do the
same for Black Swift! '-)

Thanks for mentioning another *obvious* difference which turns out to have
some similarity, if only at one edge. There oughta be a list of all the ways
various species' behavior, song, plumage state, flightstyle, etc. can
suggest other species. I could be authoritative here: I've fallen for 'em all.

Hey, just occurred to me, with all the racket that the adults and young make
around the nest at night, wouldn't this tend to attract the tender
attentions of owls? Is there any known owl predation on murrelets?

Michael Price
Vancouver BC Canada
mprice at mindlink.net