A survey done of Western and Clark”s grebes on Clear Lake by scientist Floyd Hayes revealed that there was a limited successful hatch on Clear Lake during the 2013 nesting season
A brood survey by motorboat on July 12 recorded 8,250 adults and no juveniles, representing a minimum estimate of the potential breeding population size. Another brood survey by motorboat on September 13 recorded 1,630 adult grebes (Both Western and Clark”s grebes) and 44 juvenile grebes within 100 yards of the motorboat for an overall ratio of 0.027 juveniles per adult. An additional 6,242 adult grebes and 25 juveniles were observed outside of the transect area for a total population of 7,872 adult grebes and 69 juvenile grebes. The ratio of juveniles to adults within the transect area to the total number of adults grebes counted, it was estimated a total number of 211 juveniles for the lake. The relatively low productivity is probably due to nest destruction by high winds and a high rate of avian predation on eggs.
The survey party recorded 34 nest disturbances during 40 hours of observation, for an average of 0.9 disturbances per hour. Natural disturbances were caused by American crows, California gulls and river otters. Humans in motorized watercraft caused the remaining disturbances. The survey crew observed 22 instances of one or more eggs within a nest being preyed upon by, including 21 by American crows and one by a gull. In each successful case of predation the nests were unattended by grebes. There were more than 100 nests set adrift by strong winds and subsequently abandoned by the grebes.
