Biologists reached the end of a season of ups and downs monitoring snowy plovers on Point Reyes. A high return rate of juveniles from last year spelled the potential for success, but significant losses of nests, chicks and fledglings dampened optimism sparked by promising early numbers. Matt Lau, a biologist in the park, said the reasons for the high mortality rates are not understood but may include predation, nest abandonment, habitat flooding or other unknown…