Avian and Skunk Predation of Ashy Storm-Petrels at Santa Cruz Island, California

Abstract

Avian and mammalian predation of Ashy Storm-Petrels (Oceanodroma homochroa) were documented during monthly nest monitoring in 1995–1998 and 2005–2015 and once-per-year monitoring in 1999–2004 at 5 colonies at Santa Cruz Island, California. Throughout the study period, predation by Barn Owls (Tyto alba) was documented at the sea cave colonies, and at Orizaba Rock, and likely contributed to reduced breeding success at Bat Cave in 1995–1997. Recent predation by Common Ravens (Corvus corax) caused reduced breeding success each year in 2012–2015 at Orizaba Rock (35%–48%) and in 2013 and 2015 at Bat Cave (49%–52%). Raven predation appears to be developing into a larger long-term problem that may require additional management actions, but efforts have focused on deploying artificial nests at Bat Cave to provide more protected sites. Two sea cave colonies where skunks previously had not been detected experienced major adult mortality due to unusual predation events by island spotted skunks (Spilogale gracilis amphiala) in 2005 and 2008. At Bat Cave in 2005, 2 skunks were trapped after they had killed at least 76 Ashy Storm-Petrel adults. Prior to this mortality event, the colony contained 64–97 nests per year in 1995–1997, with low breeding success (49%–59%) related largely to organochlorine pollutants. After the skunks were caught, nest numbers increased from 19 in 2006 to 100 in 2015, with variable but generally higher breeding success (49%–90%). At Cavern Point Cove Caves in 2008, 2 skunks were trapped after killing at least 32 adults. Prior to the mortality event, this colony contained 11–17 nests per year in 1995–1997, with a lower breeding success rate of 20%–47%. Nest numbers increased from 2 in 2009 to 10 in 2015, with a variable but higher rate of breeding success (50%–80%). Skunk predation events may have been related to temporarily higher skunk populations at Santa Cruz due to low numbers of island foxes (Urocyon littoralis santacruzae). Deer mice (Peromyscus maniculatus santacruzae) were not significant predators of Ashy Storm-Petrels at Santa Cruz Island.