I read that quote as bein..Early fall when coyote numbers are higher before deer hunters, callers and other folks kill a few off, howling is not as effective. They are less territorial, that's sounds odd, but I believe the stakes are higher closer to breeding season, thus enticing more stern territorial traits being shown, with Howling proving to be alot more successfull.
I think rather than food sources becoming the equation in early season howling numbers, I believe it's from less territorial traits and alot of YOY that are intimidated by howling.
JD