I haven't determined what the voltage is when the BOD puts the AT85 into reset.
I didn't know the 7805 would not disconnect when voltages dropped below 7 volts. But it doesn't really matter as long as it doesn't start passing 5 volts until it gets at least 7. Funny thought about a bird flying over. I tried using my hand to simulate a bird and nothing happens. I think the solar cell is providing almost 500mA at peak so clouds, birds, hand passing overhead, etc. don't lower the output enough for the circuit to stop.
I have no idea how to enable/disable the BOD.