The first Mishna in Rosh Hashanah says that 1 Elul is the new year for livestock, i.e. I can only tithe one year's "crop" of livestock at a time, so 1 Elul delineates the herds. (Update: we follow the latter opinion, that it's a month later, 1 Tishrei.)
According to an IRS publication:
Most ranchers schedule breeding to allow for late winter/early spring calving. ... Summer born calves are in greater danger of death due to heat stress. Some ranchers also calve in the fall (September to October)
So you'd have calves born in September, October, February, March, April. It would then make sense to put the line at the end of the summer, as that's when fewest calves are born. (Just as the International Date Line is drawn mostly in the middle of the ocean.)
Okay that would be my explanation given one datum.
- Was that trend also true in Mishnaic Israel?
- Is there a similar trend for sheep and goats?
- Is there a more-obvious explanation that I'm missing here?