>An overall analysis of migration counts in western N.
>Amer. showed no significant trends, although a decline at a New Mexico site
>from 1987-1990 was almost significant.

That would probably be our (HawkWatch) Monzano site - which in the
1994 season saw nearly 5,000 Swainson's hawks pass over in one late
afternoon rush! Normally they count hundreds per season, so this
represented about 10 years worth at once - a real anomaly with
no explanation that we can think of other than luck. Just toss
that puppy right out of the dataset :)

Hundreds roosted at the site that evening, and everyone got up early
to see them rise out of the trees and leave the next morning.

> Until relativley recently, western
>raptors were not thought to follow leading lines (topographic features such
>as mouintain ridges and coastlines) as do eastern raptors, but there have
>been several discoveries of flyways through mountain passes or along ridges
>in western N. Amer."

Our sites, at least, get steady numbers of Swainson's hawks along
our ridges, but the number is so low in comparison to their population
that I (and others) question whether any significance can be
attached to counts. They migrate in flocks in the lowlands, and
most of them live to the east of any signficant US site in
the West. They stage in huge flocks where grasshoppers are
abundant, and these places aren't consistent year-to-year as the
grasshoppers are cyclic. And since they move in loose flocks
in the mountains, as well, variability is going to be extreme.
We count hundreds of Swainson's hawks in the Goshutes, as well,
but only perhaps a hundred small kettles in a season, and each
kettle could easily go one or two ranges to the west and would
tend to do so as a group, which increases variability.

On the other hand, systemmatic counts have been undertaken in the
Mexican state of Veracruz for three years and huge numbers of
Swainson's hawks pass overhead - 200,000-300,000. 2,000,000
raptors and turkey vultures in one season (about 1/2 being TVs).

400,000 in one day!

This poses problems of its own. As one of our observers said,
"on the 400,000 (estimated!) bird day, the sky was nearly black
horizon-to-horizon, and I freaked out - how the hell do you estimate
numbers and species with any kind of accuracy at all!"

I freak out when we hit about 300 at the Goshutes!

But this would seem to be the best place to get a handle on
annual variation and long-term trends of this species, since
they don't gather in more managable, but still large, kettles
like broad-winged hawks do in the east.

>Secondly, SWHA is one of the most abundant
>raptors in the prairies and populations of other 'common' raptors are also
>declining in the prairie ecoregion, despite the fact that they have a
>different prey base.....This suggests that whatever is affecting populations
>of these species is pervasive and large-scale"

This would suggest something other than the massive deaths in Argentina
being the root cause of the decline, as other, more sedentary species
are also declining (doesn't prove it, of course, as each species
could be declining for specific reasons). It is also possible
that the deaths in Argentina are too recent to have showed up in
the study period cited - I haven't seen anything specific on
this other than comments by Steve Hoffman, of HawkWatch, to the
effect that conversion of pampas to cultivation is fairly recent,
at least in the areas visited by the researcher who reported back
last winter.

It's scary, though, that this year's reports pinpoint the majority
of poisoned birds as being adult. If they were kids, the poisoning,
while gross, might not have a large effect, simply hastening the
death of immies, most of which are doomed to die before breeding.
But once they reach adulthood, Swainson's hawks probably live several
years on average (red-tails do, I don't know of numbers on Swainson's
but could look it up if I weren't so lazy). So mass killings of
adults will likely lead to a much greater drop in productivity.


- Don Baccus, Portland OR <donb at rational.com>
Nature photos, on-line guides, more at http://www.xxxpdx.com/~dhogaza