Experimental assessment of taste aversion conditioning on Steller’s Jays to provide potential short-term improvement of nest survival of Marbled Murrelets in northern California
Excessive predation on eggs of Marbled Murrelets has been linked to poor recruitment of young into the murrelet population, especially in the California, Oregon and Washington populations. Corvids have been implicated as the most influential egg predators on murrelets. Fragmented forests provide the only remnant murrelet nesting habitats in California, but these forests also support a high population density of opportunistic corvids like Steller’s Jays. This increases predation risk on murrelet eggs. Effective reductions of egg predation require manipulation of the population density or predation behavior of egg predators. We tested conditioned taste aversion techniques for Steller’s Jays, exposing jays to murrelet-colored and sized eggs that had been treated with carbachol. In laboratory tests on 28 temporarily captive jays, aversion conditioning effectively induced subsequent aversion to the murrelet-mimic eggs. Attack latencies on murrelet-mimic eggs compared to control eggs between the initial and repeat exposure increased by several hours, and the strength of aversion remained constant over the range of retention periods tested (8 weeks). Field trials were conducted on murrelet-mimic eggs and control eggs deployed across a systematic grid on a 428 ha area in murrelet breeding habitat in Redwood National Park. The percentage of murrelet-mimic eggs that were attacked by corvids in the effectiveness assessment (the second field deployment following initial treatment) was reduced by 37% to 72% (depending on assumptions regarding corvid predation) in comparison to control eggs. Attack rates on murrelet-mimic eggs were 12% lower than attack rates on control eggs during the initial deployment, suggesting that the density of egg deployment (1 murrelet mimic egg / 2 ha) resulted in many jays having multiple encounters with treated murrelet-mimic eggs within their territories (they were already aversely conditioned when encountering a second egg within the first period of exposure). We concluded that corvid predation on murrelet-colored eggs can be reduced significantly, and that an egg density of 1 treated mimic egg per 4 ha should be sufficient to treat all corvid territories within a forest. We suggest that conditioned taste aversion treatment maybe a very cost-effective emergency plan to improve reproductive success of murrelets in the Pacific Northwest region.