Michael P. asked -

> 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?

My first guess is that there is a vertical resource partitioning happening
between swallows and swifts, swifts being the higher storey specialists. Early
swallows are often hunting right off the water surface, especially in snotty
spring weather. The smaller, lighter prey favoured by swifts may not even make
it off the water surface under the same conditions. Thus, in more benign weather
favorable to carrying the smaller insect prey aloft, swifts can advance their
way northward. If the weather turns sour and the insect supply dries up, they
are stopped in their tracks. Of course, we will never know for sure if they are
actually *forced to retreat southward* under these conditions until someone
puts some kind of transmitter on a swift and tracks it.

> 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.

Yes, the V.-G. stiff-winged display seems to be a home territory thing. The
wide standard deviation of arrival dates is consistent with a high correlation
with wildly varying spring weather. In other words, wildly varying spring weather
leads to wildly varying availability of preferred lighter insect prey and a
consequent wildly varying arrival date for swifts.

> 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.

April is indeed a colder month than May as we all know, with a much higher
likelihood of freezing temperatures, especially at night. I believe the
overnight freezes are the controlling factor here since they would have the
greatest killing power on insect hatches. For instance, spring is behind this
year in central BC with the smaller, shallower lakes only now just beginning
to turn over. However, swallows are back in force now but foraging *just off
the water's surface* mostly along the rivers which are swollen and have been
running open for much longer. Thus, I would surmise that insect prey is more
abundant over the relatively warmer rivers than the still-cold lakes. Again
as to why swallows are apparently more able to handle the effects of cold
weather, I suppose it comes back to the availability of preferred prey. The
early insect hatches are kept closer to the water's surface by the cooler
spring weather, which swallows are able to deal with by flying close to
the water. Swifts apparently either do not like flying close to water or (more
likely) their preferred prey is not available at their preferred foraging
altitudes due to the cooler spring weather. I think everyone would agree that
the only time you see a swift of any kind in the boreal latitudes near the
ground is when they are forced down by inclement weather. Generally, the
finer the weather, the higher the swifts forage, often out of the range of
human eyesight.

- Jack




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