Nocturnal Migration


Brad Schram
 

After reading the SBCo Birding notes from Joan Lentz today regarding the
fall-out down there, and seeing a clear night with full moon up here, I
decided to do a little survey of nocturnal migrant activity.

I trained my spotting 'scope, equipped with 32x wide angle eyepiece, on the
moon on a crystal clear night from 9:19 PM to 12:01 AM tonight, April 18. I
watched, jotting down the time every time I had a cumulative total of four
birds across the moon. In that time I saw 55 birds cross the moon's disk
going north at various altitudes. Although the air was virtually still at
ground level here in Arroyo Grande, I saw small birds at high levels getting
blown a bit westward in the glide portion of the flap-glide flight pattern
typical of warblers, sparrows, finches, etc. This wasn't pronounced, but it
did occur a few times. Most birds were headed almost due north toward the
north star. A few were angling a bit east of due north, and two were
seemingly going due west. Many were warblers or other birds of that size
and flight pattern (vireos, empids, buntings, etc), some seemed like
thrushes, others like kingbirds, others like grosbeaks or tanagers. Once I
had a low flock (8) of what seemed to be small geese flash across the disk
too quickly to get more from them.

It's an amazing experience, watching the moon and seeing distant birds cross
it in stark silhouette. On such a clear night it looks as if the birds are
laser cut silhouettes--except that their wings are beating. The flight
patterns are quite clear and one can make educated guesses on what types of
birds they might be. Some were so high that they were merely animated
specks taking a relatively long time (maybe one and a half seconds?) to
cross the disk, the lower ones, of course, flashed across more quickly. A
few bats crossed in random directions. I heard no flight notes, although
given the nearby frog chorus and the distant hum of hwy. 101 this doesn't
mean anything.

I predict lots of migrants in SLO county tomorrow morning (4-19)--unless
they all go over us tonight and keep on going!

Brad