Carter et al.: Leach’s Storm-Petrel at Santa Catalina Island 83 BREEDING OF THE LEACH’S STORM-PETREL OCEANODROMA LEUCORHOA AT SANTA CATALINA ISLAND, CALIFORNIA HARRY R. CARTER1,3,4, TYLER M. DVORAK2 & DARRELL L. WHITWORTH1,3 1California Institute of Environmental Studies, 3408 Whaler Avenue, Davis, CA 95616, USA 2Catalina Island Conservancy, 125 Clarissa Avenue, Avalon, CA 90704, USA 3Humboldt State University, Department of Wildlife, 1 Harpst Street, Arcata, CA 95521, USA 4Current address: Carter Biological Consulting, 1015 Hampshire Road, Victoria, BC V8S 4S8, Canada ([email protected]) Received 4 November 2015, accepted 5 January 2016 Among the California Channel Islands (CCI) off southern California, Guadalupe Island, off central-west Baja California (Ainley 1980, the Ashy Storm-Petrel Oceanodroma homochroa (ASSP) is the Power & Ainley 1986, Ainley 2005, Pyle 2008, Howell et al. most numerous and widespread breeding storm-petrel; it is known 2009). Alternatively, these egg specimens may have been from to breed at San Miguel, Santa Cruz, Anacapa, Santa Barbara, and dark-rumped LESP, which are known to breed at the Coronado San Clemente islands (Hunt et al. 1979, 1980; Sowls et al. 1980; and San Benito islands, Baja California (Ainley 1980, Power & Carter et al. 1992, 2008; Harvey et al. 2016; Fig. 1B; Appendix 1, Ainley 1986). available on the website). Low numbers of Black Storm-Petrels O. melania (BLSP) also breed at Santa Barbara Island (Pitman Within this context, we asked the following questions: (1) Were the & Speich 1976; Hunt et al. 1979, 1980; Carter et al. 1992; 1903 egg records the first breeding records of LESP at Catalina and Appendix 1). Leach’s Storm-Petrels O. leucorhoa (LESP) have in the CCI?; (2) Was the 1937 egg record at Catalina really that of a been captured widely in mist nets at Santa Barbara, Santa Cruz and misidentified LESP?; and (3) Did LESP or ASSP breed at Catalina San Clemente islands, but nests have not been documented there without our detection in 1991–1996? To attempt to resolve these (Hunt et al. 1979, 1980; Carter et al. 1992; Carter & Henderson questions, we conducted nest searches for storm-petrels at Ship 2015; Adams et al. 2016; Appendix 1). Rock and Silver Canyon Landing Area in 2014–2015, resulting in the discovery of LESP breeding at Ship Rock. In this paper, we Early historical summaries (Howell 1917, Willet 1933, Grinnell & describe our 2014 and 2015 observations at Ship Rock and Silver Miller 1944) did not note storm-petrels breeding at Santa Catalina Canyon Landing Area, re-examine 1903 and 1937 egg specimen Island (hereafter, “Catalina”), a large island (34 km long; area records to try to verify their authenticity, discuss past efforts that 194 km2) with parts now heavily developed, 32 km southwest of the failed to fully document breeding by storm-petrels at Catalina, Palos Verdes Peninsula. Extensive surveys of seabird colonies in the and summarize historical literature to clarify the earliest known CCI during 1975–1977 (Hunt et al. 1979, 1980) and in 1991 (Carter documentation of breeding storm-petrels in the CCI. et al. 1992) did not find any storm-petrel nests at Catalina, although suitable crevices were noted in 1991 on Ship Rock (Figs. 1A and 2), Ship Rock (2014): On 2 July, two of us (D.L.W. & T.M.D.) off the town of Two Harbors on the north side of the isthmus (H.R. conducted a nest search, using hand-held flashlights (Figs. 2 and Carter, unpubl. data). In 1994–1996, more focused nest searches 3). All accessible areas of the 18 m high rock were searched for for Scripps’s Murrelet Synthliboramphus scrippsi (Whitworth et al. potential breeding crevices and each area was carefully inspected. 2014) and other crevice-nesting seabirds led to greater suspicion Adult storm-petrels were found in five rock crevices (i.e. large of storm-petrel nesting at Catalina. A few burrows were found and enough for storm-petrels but too small for alcids; Carter et al. storm-petrel odor was detected at crevices at Ship Rock; storm- 1992) near the top of the rock (Table 2; Fig. 3; see Appendix 2 for petrel odor also was noted at one small offshore rock in the Silver photographs of two crevice entrances). Gray heads were apparent Canyon Landing Area (Fig. 1A), off the uninhabited south side on all five adults observed in crevices, but their rumps could not of the island (H.R. Carter, unpubl. data). A 1937 storm-petrel egg be seen; we assumed they were ASSP. Two nests apparently had specimen collected at Catalina (exact location not specified) was small chicks (based on adults with hatched eggshells), one nest had rediscovered in 1996; it had been recorded by the collector as an egg, and in the two other nests only an adult was seen. Another ASSP without details (Carter et al. 2008). In 2008, an unidentified nest (number 6) found inside a partly collapsed dirt/guano burrow storm-petrel was finally observed in a crevice on Ship Rock, among contained an adult storm-petrel with gray head plumage (Fig. 4) four other suitable crevices, but an egg was not seen (H.R. Carter, and lacking any white on the rump, along with one quarter of an unpubl. data). eggshell; we also assumed it was an ASSP. Unfortunately, we had not noticed the burrow entrance, and it collapsed when we stepped In 2012–2014, during online and museum searches for storm- near to it. ASSP do not typically excavate burrows (Ainley 1995). petrel specimens collected by Franklin J. Smith in northern However, this dirt/guano “burrow” appeared to be a natural cavity California in 1910–1926 (Carter et al. 2015), one of us (H.R.C.) of some kind under the hard surface crust of old guano, with a small found two egg specimens of the “Socorro Petrel Oceanodroma entrance through which an adult storm-petrel had entered the cavity soccoroensis” collected by Smith at Catalina in 1903 that had not and partly excavated a nest. Afterwards, the burrow was repaired been reported in historical summaries (Table 1). The identification by clearing out the tunnel and placing sections of the crust on top of these egg specimens seemed suspect, because Townsend’s of the partly collapsed section. On 8 August, storm-petrel nests on LESP O. soccoroensis are currently known to breed only at Ship Rock were re-inspected (Table 2). Four nests had chicks (three Marine Ornithology 44: 83–92 (2016) 84 Carter et al.: Leach’s Storm-Petrel at Santa Catalina Island Fig. 1: Locations of breeding colonies of storm-petrels in the Southern California Bight: (A) Santa Catalina Island, with numbered locations mentioned in Table 3, 1 — Ship Rock, 2 — Bird Rock, 3 — Eagle Rock, 4 — Main Island; 5 — Main Island; 6 — Silver Canyon Landing Area (Subcolony 02), 7 — Silver Canyon Landing Area (Subcolony 01), 8 — Church Rock, and 9 — Main Island; (B) eight major Channel Islands in southern California, United States (SMI — San Miguel Island, SRI — Santa Rosa Island, SCZ — Santa Cruz Island, ANA — Anacapa Island, SBI — Santa Barbara Island, SNI — San Nicolas Island, CLE — San Clemente Island, and CAT — Santa Catalina Island), and islands in northwestern Baja California, Mexico (COR — Coronado Islands and TSI — Todos Santos Islands). Storm-petrels have been reported breeding at all islands except for SRI and SNI (Appendix 1, available on the website). Marine Ornithology 44: 83–92 (2016) Carter et al.: Leach’s Storm-Petrel at Santa Catalina Island 85 observed and one inferred from hatched eggshells), and an adult areas of the rock with suitable crevices. Two rock crevices suitable was seen in one nest. At the two nests that had hatched eggshells for nesting by Scripps’s Murrelets were noted, but no evidence on 2 July, chicks > 37 days of age were found. The repaired burrow of breeding was found. Below the light structure, two dried and (number 6) was not checked in order to avoid further disturbance, faded unidentified storm-petrel wings (154 mm long, consistent although it appeared to be intact. with ASSP or LESP; Ainley 1980, 1995) were found near a pellet that had been regurgitated by a gull or owl (i.e. it contained small In addition to the six active nests found at Ship Rock, we also found brownish contour feathers, apparently from the same individual another suitable crevice with strong storm-petrel odor and about storm-petrel as the wings). 10 other crevices that seemed to be of marginal suitability and probably were not used. We estimated a breeding population size of Ship Rock (2015): On 21 April, the five crevice nest sites noted in six to seven breeding pairs in 2014, based on a thorough check of all 2014 were tagged. Subsequently, these nest sites were monitored Fig. 2: Overview of Ship Rock, Santa Catalina Island, April 2012: north side (top) and west side (bottom). Cormorants and pelicans Fig. 3: Crevice-nesting habitat (crevice number 1 being inspected) roost mainly along the top of the rock. (photos by T.M. Dvorak). on Ship Rock, 2 July 2014 (photo by D.L. Whitworth). TABLE 1 Storm-petrel egg specimens from Santa Catalina Island, California Nest description Set Specimen Locality Date Incubation Identity Collector and notes mark number “Socorro Petrel” “in a burrow “Santa Catalina 3 ft. in. Egg laid on bare Island, 27 June 1903 ground for the nest. On an Fresh Positive 2 Franklin J. Smith ROM 1 986 California” island on the Pacific Ocean off San Pedro, California” “San Pedro “Socorro Petrel” WFVZ 27 June 1903 ND ND 1 Unknown Calif” “Collection of E.J.