Why so fewer birds in the yard come late summer and early autumn?
One possible cause is Cooper Hawks. We are in east TX and put out a lot of seed ; we had more birds. Fewer in recent months due to the Cooper Hawk(s). Our yard is a buffet for them ,I hear them often, see them occasionally ;they will perch in the under story.  And we hear birds hit the windows as they panic from the hawk ; it happens 0 to 4 times a day. The hawk has developed a technique of diving and after the small birds when they knock themselves out on the windows , the hawk walks on the patio to pick one up. I have been going out after a bird strike and picking up stunned birds and put them in a shoebox for 30 minutes. They usually recover, had a cardinal yesterday. If you are home during the day , listen for window strikes and watch for sudden flights of all the birds. The Sharpshin Hawk is also here but not a problem.......... And there is a degree of randomness to migrations: Some years we get many Goldfinches, Pine Siskins, Robins, and some years a few or none. And birds like Cedar Waxwings are notorious for their unpredictable movements.