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 factors. “We haven’t detected an increase in predator numbers like coyotes, ravens or raptors,” he said. “It could be an environmental issue, or something related to food or something we are not finding.” Snowies nest from Limantour to Kehoe Beach and in the nooks and crannies in between. Biologists recorded 50 nests during the season, which starts in May and ended last week. Though the number of nests was the highest since 2018, the nesting success rate was average, with 29 nests hatching and 21 failing. According to Mr. Lau, 12 of the roughly 45 breeding snowies this year were juveniles that hatched here last year, a higher number than usual. But some juveniles didn’t return. One eight-year-old breeder that has returned to Abbotts Lagoon to breed every year since she hatched in 2015 didn’t come back. Mr. Lau said she was one of up to six adults that disappeared. The fragile shorebirds nest in flat, unprotected areas at the foot of dunes closest to the sea. Newly hatched chicks are unable to fly for their first 28 days and depend on adult males for protection. Snowies can breed the first summer after they’re born and live on average for three years, but Mr. Lau said a breeding male in Humboldt County lived to be 23, breeding every year of his life. The season on Point Reyes started off strong, with 16 active nests found at once, a trend usually not observed until midsummer’s nesting peak. But by mid-July, numbers took a nosedive when 37 of the 57 hatchlings had perished. By last week, just 19 of 78 chicks had survived. Although the 24 percent survival rate is lower than the long-term average, survival rates improved toward the end of the summer. Mr. Lau surmised that the recovery may have had to do with the tilling of fields on the ranches, which redirected ravens’ attention. “We did see a decrease in raven activity [at plover sites] in the last month,” he said. “We are not quite sure why that is, but one of our hypotheses is the ranches’ tilling and mowing of fields in the mid-summer, which opens foraging habitat for ravens. We think they get distracted and go to those habitats.” Snowies nest from southern Washington to Baja California, but the majority spend their summers along the California coast; in winter, some migrate along the coast and others stay put. Marin has a smaller population than some neighbors in either direction. Monterey Bay and other beaches in the South Bay have breeding populations sometimes numbering in the hundreds.