California Gull Predator Management and Reproductive Success of Endangered California Least Terns in the San Francisco Bay, California

Abstract

Gull predation is known to be an important source of egg and chick mortality for many waterbirds and can have severe impacts on recovery efforts for special status species. We evaluated the effectiveness of California gull management and removal by monitoring tern hatching and fledgling success annually. From 2005 to 2011, nesting success was monitored at a newly established California least tern colony at Hayward Regional Shoreline in San Francisco Bay. No gull management was undertaken prior to 2007. California gulls were the most abundant aerial predator (97%). We recorded 3,769 predatory gull-tern interactions and presumptive take of 26 tern eggs and 23 chicks. Although gull predatory behavior did not change statistically, gull management efforts resulted in significantly improved tern breeding success (measured by number of nests, eggs, chicks, and fledglings). From 2007 to 2011, this colony produced a total of 242 successful nests and 291 fledglings, an average nesting density of 196 nests per ha, and an average of 1.07 fledglings per breeding pair. We discuss difficulties of lethal control of predators, and suggest the importance of human presence for reducing gull predation at the site.

Forster's tern chick survival in response to a managed relocation of predatory California gulls

“…Oro et al () also found that individual yellow‐legged gulls ( Larus michahellis ) specialized on storm‐petrels ( Hydrobates pelagicus ), and gulls accounted for up to 33% of storm‐petrel mortality. Accordingly, seabird productivity tends to increase in studies that lethally remove the gulls that specialize on depredating eggs and chicks (Hario , Guillemette and Brousseau , Riensche et al ; but see Donehower et al ). Culling large populations of gulls had more variable success for improving waterbird populations and requires continual gull control measures (Thomas , Harris and Wanless , Anderson and Devlin ).…”

Habitat Use by Breeding Waterbirds in Relation to Tidal Marsh Restoration in the San Francisco Bay Estuary

“…Although some of this shift in use may be the result of enhancements made to managed ponds, the large decreases observed in the use of specific salt ponds suggest that changes were more likely driven by a reduction in the habitat value of the remaining salt ponds, many of which are high-salinity ponds (> 120 ppt at the time of the 2019 survey) and are still used for salt production. High-salinity ponds can support abundant populations of invertebrate prey (Ephydra brine flies and Artemia brine shrimp) that often are exploited by American Avocets, Black-necked Stilts, and California Gulls (Larus californicus; Carpelan 1957;Herbst 2006;Takekawa et al 2009); however, those ponds are generally above the salinity tolerance of the fish on which Terns feed, which are limited to low-and medium-salinity ponds (Carpelan 1957;Mejia et al 2008;Peterson et al 2018;Riensche et al 2018). Forster's Terns have been shown to preferentially forage in lower-salinity pond habitat (Bluso- Demers et al 2016), and salinity was an important factor that determined overall pond use for both Forster's Terns and American Avocets .…”