Northern Prairie Wildlife Research Center
ESTHER E. BURKETT, MICHAEL SILBERNAGLE,
AND RICHARD ZEMDAL
California Department of Fish and Game, 11862 MacGill Street, Garden
Grove, CA 92641; U.S. Fish and Wildlife Service, Kern National Wildlife
Refuge, P.O. Box 670, Delano, CA 93216-0670; U.S. Fish and Wildlife
Service, Ecological Services Division, 2400 Avila Road, Laguna Niguel,
Mammalian predators, primarily red fox and skunk, cause enormous damage by eating eggs and, in the case of the fox, caching eggs for later meals. Red fox predation accounted for the loss of 50% of the California least tern nests at Bolsa Chica Ecological Reserve in Orange County, California, during 1988. Maintaining "soft-catch" leghold traps in the field throughout the 1989 nesting season resulted in increased hatching success.
Nesting islands help to reduce predation by mammals. The islands work best for deterring skunks and cats, although red foxes are capable of swimming to islands. Islands have not proven to be consistently effective where a broad range of predators exists. Islands also create a high personnel load on a yearly basis for vegetation removal. The manual clearing of islands as small as 2 acres can be extremely time consuming without mechanical equipment.
Islands offer no protection from avian predators, which easily fly to the islands from adjacent land. Removal of powerlines, power poles, and other perch sites can aid in alleviating damage from avian predators, as can the use of the perching deterrent "Nix-o-lite." Live-trapping of kestrels by using Balchatri traps has been very successful. Pole traps and direct shooting are utilized where it is virtually impossible to use live-traps.
Chain-link fences have been used successfully at a number of colonies to deter mammalian predators. The height of the fence is critical, as is the cantilevered top. A 7-foot-high fence with a screened cantilever has been successful in deterring mammalian predators. Sand drift can easily destroy fence effectiveness. Mammalian predators seek the weakest point in the fence at which to enter. As is the case with islands, fencing cannot deter avian predators.
One major drawback to fences is the "shadow effect" created by the physical presence of the fence. Terns generally avoid the edge of the fence line, and the highest density is found centralized within the enclosure. Fences also potentially keep chicks from reaching the cover of shoreline vegetation in an island setting. Cover is important to protect chicks from avian predators.
Electric fences generally have not proven consistently effective in reducing mammalian predation, although, when used in conjunction with chain-link fences, they act as a valuable, additional deterrent. The moist coastal environment utilized by the terns is not conducive to electric fence operation. The maintenance of electric fences is demanding.
It is important to document predation very near to the time it occurs. California least terns may abandon a site after continual harassment. Therefore, constant monitoring and vigilance are key components to determine nesting success. Predator control, physical barriers (where feasible), and constant monitoring are the three key components to success.