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US Fish & Wildlife Service FieldNotes
SONNY BONO SALTON SEA NWR: Deterring Predators,Improving Habitat Keys to Tern Nesting Success
Region 8, September 17, 2008
Black Skimmers join a pair of Gull-billed terns on an artificial island at Sonny Bono Salton Sea NWR. (Photos: USFWS)
Black Skimmers join a pair of Gull-billed terns on an artificial island at Sonny Bono Salton Sea NWR. (Photos: USFWS) - Photo Credit: n/a

by Chris Schoneman, Sonny Bono Salton Sea NWR
The Sonny Bono Salton Sea National Wildlife Refuge (NWR) in Southern California’s Imperial Valley is home to a very diverse array of birds throughout the year. Refuge management practices are focused on providing productive habitat for well over 400 species of birds. Wintertime is the refuge’s greatest period of use, as birds fly south to enjoy the valley’s mild winter climate, but some special status birds visit the refuge during the scorching summer months to nest on a small number of islands adjacent to the Salton Sea. These birds are the Western Gull-billed Tern (Sterna nilotica vanrossemi ) and the Black Skimmer (Rynchops niger), both federal Birds of Conservation Concern. For the tern, this refuge provides the largest nesting colony in the United States with recent peak numbers of only 180-201 pair. About 40 pairs nest on the San Diego NWR. Primary habitat at the Salton Sea site consists of a single 30 acre brackish water pond with 5 man-made islands. Numerous management challenges exist at this site for refuge staff to maintain and hopefully improve nesting habitat for these special status birds, including nest predation by mammalian predators, island erosion, and most recently island domination by the larger Caspian Tern.

Refuge staff have implemented several management techniques in order to increase the nesting opportunities for these birds in recent years, with varying success.

First, it was discovered that raccoons were able to swim out to these islands and feast on eggs during the night, eliminating up to 55 nests at once. So we put up an electric fence around two 30 acre ponds during the nesting season to discourage predators. That effort seems to have helped slow down the incursions by raccoons. Since the beginning of this effort, no evidence of mammalian predation has been seen.

Along with the threat of nearby raccoons was the threat of island erosion, shrinking the limited amount of available nesting space on the refuge. In 2005, staff began testing ways to provide artificial nesting “islands” that would not erode and might be used in the Salton Sea, away from mammalian predators. We started with sections of Styrofoam filled “floating dock”. These cubical sections can be bolted one to another, making as long or wide of a floating area as needed. Our first island was about 8’x16’ in size and was covered with sand and gravel to provide nesting substrate. In April 2005, the floating island was launched just in time for nesting to begin to determine if birds would be attracted to it. For this preliminary test the island was placed in the refuge pond that is the primary nesting location for terns, safe from wave action of the Salton Sea. That season the floating dock island attracted loafing birds but no apparent successful nesters. The next season, Spring 2006, we received help from biologists at United States Geological Survey in Davis, California to construct a wooden nesting platform on 4"x 4" posts in the Salton Sea. The concept was to duplicate waterfowl hunting blinds that were built on stilts in the Salton Sea many years before, which had survived the punishing periodic wave action by being raised above the waves on posts. Again, birds were seen loafing on this new structure throughout the summer, but without signs of successful nesting.

In 2007 we tried once more with the floating island, this time in the Salton Sea, but with a sturdy anchor chain that made the float pivot in the direction of waves and a splash guard that deflected water from crashing onto the float. Again, even with the best of intentions and a great deal of biological and engineering expertise put toward the project, the float served as a mere loafing structure at best and took on waves on windy days throughout the breeding period. Meanwhile, the original floating dock in their primary nesting pond began to attract Gull-billed Terns in April of that year, eventually reaching a maximum density of about 31 nesting pair. Successful nesting was well established on the floating dock by the end of the season. And the wooden nesting platform built on stilts in the Salton Sea away from raccoons and above the waves began to attract Black Skimmers. Unfortunately, by the time nesting was observed, waves had demolished all ramps from the platform, leaving potential nestlings stranded 5' above the water in the scorching sun. We were able to document nesting activity on the platform, however, because of difficulty making regular and clear observations, nesting success was not documented. We suspect the height above water to avoid waves made conditions too hot for chick survival.

Finally, in the Spring of 2008 we moved the float from the Salton Sea into the primary nesting pond near the floating dock island and away from the punishing waves of the Salton Sea. Here, in the relative still water of the pond, Skimmers began nesting on it in July after rebuilding ramps and adding a new surface of barnacle shells and sand. About 40 pair nested on this float.

A complicating factor in all our attempts to produce alternate nesting sites for these birds began to emerge in 2007. A larger than normal population of Caspian Terns over-wintered at the Salton Sea the previous winter, establishing a breeding population in early Spring that was about double the previous year (almost 1,600 breeding pair). These larger, more competitive birds occupied nesting sites that were normally occupied by Gull-billed terns and Black Skimmers in past years, forcing the Gull-billed terns and Skimmers to nest on margins of islands or onto our artificial sites. The net result was, and continues to be less optimal nesting area for these two species of conservation concern.

Although we have not yet found a way to provide quality nesting space for these birds outside of their historical nesting pond, away from raccoons and surrounded by the protective waters of the Salton Sea, we have learned a great deal about what will work to provide quality artificial nesting conditions in a sheltered wetland, including types of nesting substrate/soil, ramp construction, use of decoys and call attractants, and attractive sizes of nesting areas.

Contact Info: Chris Schoneman, (760) 348-5278, x227, christian_schoneman@fws.gov