Why is it that when we hunt pheasants on opening day we'll sometimes see a rooster that's clearly about eight to ten weeks old? Same reason, I'd suspect, that bluegill produce a "rolling spawn". (BTW, I'm going to use that term as if I came up with it.
) Species don't reproduce because it's a particular time of year. They reproduce because for thousands of generations that particular reproductive strategy has worked to push DNA forward in time. For bluegill, and equally for pheasants, certain environmental influences such as climate and available forage make it advantageous to take a second or third go at it. If this strategy has worked before for a particular species, then it becomes part of the hardwiring.
It would also stand to reason that the further north you go, the more stress it would be on the individual to attempt a late spawn. The energy expended in the attempt might decrease the odds of surviving the long winter ahead.
