Depredation of the California Ridgway’s rail: Causes and distribution

We studied the causes of mortality for the California Ridgway’s rail at multiple tidal marshes in the San Francisco Bay Estuary, California. We radio-marked 196 individual rails and examined the evidence from 152 recovered California Ridgway’s rail mortalities from our radio-marked sample and determined plausible cause of death from a wide array of evidence. We also included 10 additional California Ridgway’s rail mortalities (unmarked) that we encountered during our normal field operations. We assigned a likely cause of death to 130 of the recoveries, of which 127 were determined to be caused by predation. Of those, 103 could be divided into class of cause (avian or mammalian), and avian predators were responsible for 64% of those events. Primary predators identified include domestic or feral cats, red fox, owl, and northern harrier. We did find seasonal differences between avian and mammalian predation rates, with higher proportions of avian predation in the winter and early spring. Time of day and tide height
were correlated with predation events, with a greater proportion of known mortalities found during periods of high tides (over 60% marsh inundation) and during daylight hours. Predation is the primary source of mortality for California Ridgway’s rail. Management actions that try to reduce avian predation may be the most effective at improving rail survival rates, given the proportion of avian predation detected.