Dave,
The migration here in SoCal seems to be pretty constant....
1....It does help when Salt Lake area freezes, but many of our ducks don't come from there...Oregon and Nevada are important to us as well...So, freezing in those areas and further north are important as well.
2....As mentioned above, freezing temps on bodies of water up north make birds move, if there's no big open water or spring creeks...Our gadwalls mostly come from Oregon and Nevada per band returns.
3....Moon phases are very important...The birds like to migrate on full moons and northerly winds...The next full moon (Nov 19)...There should be a major push of snows and far north ducks during that time (widgeon, spoons, GW).
4....Ducks and geese make it from SLC in one night...We are around 700 miles SLC valley...Averaging 40-60 MPH, they make the flight in about 10-12 hours...Honkers may fly a little faster...Historically, they get here from SLC around Thanksgiving.
5...Low water may help some, but it the condition probably has to be more long term... Ducks (& geese) will change locations as food and water changes...I know in the early 70's, Wister and the IV were loaded with honkers (more than snows)...Then as the snow population boomed, honkers left...Oceanside had 1000's of birds and fed in local wheat fields...Then, it was Lake Henshaw (early 80's)...Which at the time, was growing alfalfa and potato's on the VID lands surrounding the lake...Then it was the Colorado River and especially the Cibola area...Now, they seem very dispersed thru out SoCal and mostly in unhuntable (golf courses, lakes, city parks, etc.)
Pix from the early 70's & early 80's...Wister (limit 1), O'side (limit 2), and Henshaw (limit 2)
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