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September 2013


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Jane Stein <[log in to unmask]>
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Vermont Birds <[log in to unmask]>
Thu, 19 Sep 2013 11:39:15 -0400
text/plain (43 lines)
Very, very, very broadly, and depending on all sorts of variables, 
mid-morning and mid-afternoon.  That's because the big numbers are 
Broad-Wings, which basically vacate the continent en mass over a very 
short period of time in early fall. In Mass., something like 90 percent 
of the BWS seen over the entire 30 years of data have been seen between 
Sept. 10th and 20th.

BWs use thermals to get height, then glide as far as they can before 
picking up another thermal.  Their "goal" is to get as far as possible 
towards central America as fast as possible while expending as little 
energy as possible-- so soaring and gliding rather than flapping.

The result of that is that when weather conditions are good -- those 
crisp fall days with lots of sun -- very strong thermals are created 
that take the birds in the middle of the day so high that they're often 
out of view.  The "mid-day lull" is a well-known cliche in 
hawk-watching.  In mid-morning and mid-afternoon, the sun angle is lower 
and the thermals not as strong, so they don't get as much height and 
that keeps them at a lower height.

But again, that's not a rule, just a statistic.  You can have huge 
flights visible at any time of day.  One of the truly addictive things 
about hawkwatching during BW season is you can have an empty sky for 
hours, and then they start coming in and just pour through so you get 
hundreds or thousands over the course of a couple of hours, and then the 
spigot shuts off completely and the sky is empty again for the rest of 
the day.  I once experienced an estimated 8,000-bird flight compressed 
into an hour and a half, and no more than 100 birds of various species 
spread over the rest of the day.


On 9/19/2013 9:41 AM, Willem Leenstra wrote:
> Is there a time of day when the numbers are higher than others? Broadly
> speaking, that is, morning vs. evening?
> On 9/19/2013 1:34 AM, Jane Stein wrote:
>> Mt. Wachusett