David La Puma
Wed, 17 Mar 2010 19:06:06 -0700
Winds are indeed interesting. Several questions arise: Do birds use surface winds to "decide" whether to go or not, or do they use a combination of surface variables such as barometric pressure, rH, etc., and/or how much do they "test the atmosphere" (we do know that this occurs, where birds will take off, and then facing opposing winds soon afterward, will return to the surface) given a range of surface wind conditions?
I'm including surface winds in this preliminary model (actually, I'm using wind vectors, to make linear an otherwise circular variable, which takes into account both wind direction and speed to produce a variable that ranges from head-wind to tail-wind). The initial model does show this as being quite important in predicting migration intensity (high tail-wind vectors appear to be good predictors of high density migration events). I plan to include wind vectors aloft as well, as a separate variable, and yes, I suspect they will be important predictors. Most nocturnal migration occurs within the first few thousand feet, so in the case of tonight, it looks like birds are moving under SSW winds at the surface to W winds up to about 3000 feet ASL (according to the Wallops Island radiosonde). These winds are averaging 10kts, which is a nice tail wind given a southerly component, and is still quite manageable with a westerly cross wind. On average, birds moving over the mid-Atlantic in spring take a SW->NE trajectory, so some west in the wind can still contribute to a tailwind. I suspect that had the winds been more northerly at 900mb (~3000ft) but all else remained constant, you'd still see birds moving tonight albeit at a lower altitude. Cheers David ____________________________________________________ David A. La Puma Villas, NJ Websites: http://www.woodcreeper.com http://badbirdz2.wordpress.com Photos: http://www.flickr.com/photos/woodcreeper On Wed, Mar 17, 2010 at 9:46 PM, Bryan Guarente <dafekt...@yahoo.com> wrote: > The question that I like to ask of all this is which level of winds > matters the most when talking about migration? I am seeing that others are > taking into account the surface wind direction (NWS observations and > forecasts), but what about above that? The winds change direction > dramatically with height quite often, especially in areas where the surface > friction is high like large cities, so how much are the higher level winds > taken into account? If the model predictions are right from this morning > for tonight, then the winds in both of your locations about a kilometer up > in the atmosphere should be out of the WNW. Shouldn't that hinder the > movement of birds tonight? > > Bryan Guarente > Meteorologist/Instructional Designer > The COMET Program > University Corporation for Atmospheric Research > Boulder, CO > > -- NFC-L List Info: http://www.NortheastBirding.com/NFC_WELCOME http://www.NortheastBirding.com/NFC_RULES ARCHIVES: 1) http://www.mail-archive.com/nfc-l@cornell.edu/maillist.html 2) http://birdingonthe.net/mailinglists/NFCL.html 3) http://www.surfbirds.com/birdingmail/Group/NFC-L Please submit your observations to eBird: http://ebird.org/content/ebird/ --