I won't pretend to understand all of the dynamic variables involving weather and location and food supplies etc etc etc.
All I know is that the section of Wurzbach on the other side of Babcock by the Medical Center area and near our house is infested with a bazillion nasty birds every fall and winter.
Starting in October they congregate every evening in the trees, on the wires and on the light poles...freaking everywhere you look there's standing-room-only birdage.
And the incessant squawking and screeching and peep-peep-peeping (and sidewalks covered in poop) mark this part of town as an excellent location for anyone who wants to make a movie reminiscient of Hitchcock's classic.
For some reason they almost never cross Babcock to befoul my neighborhood, Thank God.
When they do, I have several techniques to drive the bastards away such as popping camera flashes at them, bouncing shots from my BB Gun off a tree, or just "borrowing" some of the local cats and tossing them over the fence into my back yard.
The worst ones are the Grackles.
A sleek and shiny blue/black bird that's almost as big as a crow, they make the worst noise ever encountered by civilized man.
To me it sounds like someone pushing Alanis Morrisette too hard through speakers with tweeters that are about to fail.
Or like someone dragging the left side of a bicycle across a chalkboard.