Experimental control of a native predator may improve breeding success of a threatened seabird in the California Channel Islands

Abstract

Predation by native deer mice ( Peromyscus maniculatus elusus) is considered the greatest threat to nest success of Xantus’s Murrelet ( Synthliboramphus hypoleucus scrippsi), a small, crevice-nesting seabird, on Santa Barbara Island, where about half of the known US population breeds. Reduction of mouse predation has been proposed as one way to increase reproductive success on Santa Barbara Island, California. Between February and June 2004, approximately 1650 mice were translocated from the largest and most consistently monitored murrelet colony on the island, reducing mouse population density significantly relative to a control site. During the translocation period, significantly fewer eggs were preyed upon by mice (20.5%) than the average (36.7%) between 1993 and 2005, excluding 2004. Productivity in 2004 (1.11 eggs hatched nest −1) was also significantly higher than the 12-year average (0.93 eggs hatched nest −1). More eggs were laid and hatched in 2004 than in most previous years, but overall, hatching success was not markedly higher, in part because other reasons for nest failure, e.g., abandonment, egg mortality, were important in 2004. Although local mouse removals may effectively reduce egg predation on a limited scale, particularly during periods when risk may be higher because alternative prey for mice are scarce, island-wide eradication of mice is not practical or desirable here because of the mouse’s endemic status. Because a combination of other terrestrial and at-sea factors are known to influence population viability of Xantus’s Murrelet, conservation strategies that incorporate intensive efforts both on land and at sea will likely be the most effective.