Technical Report

Nocturnal predation of California least terns at Navel Base Ventura County, Point Mugu

Predation events have rarely been observed and quantification of impacts have consequently been difficult, especially for predators active at night. Using night-capable cameras placed throughout a California least tern (Sternula antillarum browni) colony we examined rates of predation by a complex of potential predators. We identified and quantified direct and indirect effects for each potential predator observed in the colony at night. Small mammals, European starlings (Sturna vulgaris), great horned owls (Bubo virginianus), an opossum (Didelphis virginiana), and coyotes (Canis latrans) were observed in the colony. Coyotes were the most frequent potential predator and were observed on 12 nights in each of the two nesting seasons of investigation. Great horned owls, an opossum, and coyotes predated on terns at night (n = 34, total from both seasons) and coyotes alone were observed predating more than other species (n = 31, total from both seasons). Predator presence in the tern colony resulted in an average of 15.5 ± 3 minutes ( ± SE) of temporary nest desertion (n = 41 flushing events). Anthropogenic disturbances resulted in an average of 150 ± 25 minutes of temporary nest desertion (n = 13 flushing events). The average number of nests observed during these disturbances was 20.3 ± 1.3, and varied with time in the season (2 – 41 nests). All observable nests flushed during every anthropogenic disturbance while 86 ± 0.02 % of observable nests flushed during predator caused disturbances. No association was found between abandoned nests and distance to predated nests in either season. Although coyotes were associated with the greatest number of observed predations, the total number of predations could have been greater if coyotes were not present. Predators such as red fox (Vulpes vulpes), black rats (Rattus rattus), or California ground squirrels (Spermophilus beecheyi) may have greater access to the colony in the absence of coyotes.