Date: 7/10/19 4:29 pm
From: DJ Lauten and KACastelein <deweysage...>
Subject: [obol] Re: shorebird question
On 7/10/2019 7:36 AM, <t4c1x...> wrote:
>
> Does anybody know why the heaviest movement of migrating shorebirds in
> spring seems to coincide with a strong headwind from the north?
> Darrel

Hi Darrel

I would say we have some ideas of what is happening, but I am uncertain
any of us truly "knows" exactly what is up.    But here is a few cents......

First, we need to clarify a bit.   There are two related things
happening: one is the normal northward movement of shorebirds heading to
major spring stopover locations on their way to the arctic nesting
grounds.   The second is the weather conditions that coincide with the
time span that the birds are traveling north based on daylight length,
and therefore, hormonal control of migration timing.   So in other
words, shorebirds are going to migrate north in a pretty tight spring
window span no matter the weather conditions.   But they will utilize
weather conditions to make their trip efficient.   It so happens that we
human birders observe this behavior, and in some years along the outer
coast the birds are really concentrated and numerous.   In other years,
not so much.   We see birds, but not as numerous nor concentrated. It
also so happens that when we see the former event, it is almost always
in high pressure, stiff northwest windy conditions.    When it is not
like this, we don't see as many birds, but they are still somewhere,
migrating north - either further out over water, further inland over the
mountains, or higher in altitude.   When there is high pressure and
resulting northwest winds, we see amazing numbers of birds.   My
assessment of the situation is that the high pressure does two things,
it pushes, or keeps the birds at much lower altitudes.    But more
importantly, we may think that flying into the wind is difficult, maybe
costly, but it is exactly what birds do.   When a bird opens it's wing,
it unfolds an airfoil, which creates lift, exactly like an airplane
does. The lift is created by a difference in airspeed over and under the
wing, resulting in a mini low pressure effect that causes lift. When a
bird closes it's wing, gravity instantly takes effect and pulls the bird
down.   This uplift followed by a down pull creates momentum, which
translate into forward motion.   All a bird has to do is open it's wing,
and close it, and it's moving forward. It's as simple as humans walking,
they, and we, just do it.   So while it may  not make sense to us, it
makes perfect sense to a bird.   Ask any pilot which direction they take
off into - into the wind or with it?   And land for that matter.    I
suspect when we do not see these large movements, the birds are finding
the right altitude and location to do the same thing - so they get up
much higher into those winds and just cruise along, flipping open their
wings occasionally to stay afloat.   But if the high pressure is so
strong that it is counterproductive to fly up high, you simply stay near
the ground and head north.    And we silly humans see them and are
totally amazed, and ponder why or how they are doing it!   It is very
amazing, and when it happens we should all watch in awe, but it is
happening every year, just mostly above our heads and/or out of sight.

Hope that helps

Cheers

Dave Lauten





 
Join us on Facebook!