Data Zone
Justification of Red List category
This species is listed as Vulnerable because it occupies a small range when breeding, nests on a limited number of islands and islets and is thought to be in on-going decline owing mainly to the impacts of invasive mammalian predators. Predator eradication is currently ongoing however and if a population recovery is evidenced, this species may qualify for downlisting to a lower threat category. Similarly, if the species is confirmed to be breeding at more than 10 locations, downlisting may also be warranted. Until such confirmation however, the species is precautionarily retained as Vulnerable.
Population justification
Birt et al. (2012) estimate the total number of breeding pairs to be in excess of 6,500, which equated to 13,000 mature individuals. Partners in Flight (2019) estimate the population at 15,000, hence it is placed here in the band for 10,000-19,999 mature individuals, assumed to equate to 15,000-30,000 individuals in total.
Trend justification
A number of colonies have been extirpated by introduced predators, particularly throughout the islands of western Baja California however, many of the extirpated colonies have reestablished and recovered in recent decades following the control and removal of invasive predators (Bedolla-Guzmán et al. 2019). Alien invasive species continue to threaten some colonies however the eradication of cats and other invasive predators on many breeding islands since the mid-1990s has reversed numerous such trends, generating population increases, as noted at Anacapa Island, California (4.45 increase in nesting 2011-2014 [Whitworth and Carter 2017, Bedolla-Guzmán et al. 2019]), likely at San Miguel Island (2004-2015 [Whitworth and Carter 2018]) and on a number of islands off the coast of western Baja California (Islas Coronado, Todos Santos, San Martín and San Benito [Whitworth et al. 2020]). A number of other threatening processes are however on-going; the combined effects of which are suspected to be inducing an overall moderate decline in this species's global population.
Synthliboramphus scrippsi breeds on islands off southern California, U.S.A. (San Miguel, Santa Cruz, Anacapa, Santa Barbara, San Clemente, and, formerly, Santa Catalina) and western Baja California, Mexico (San Benito, Todos Santos and the Coronado and San Jerónimo islands) (Chesser et al. 2012, Whitworth et al. 2020). On large islands, the species is confined largely or entirely to offshore rocks (Drost and Lewis 1995). Breeding on San Martín and Cedros Islands, Baja California, is less certain but presumed present (Chesser et al. 2012, Whitworth et al. 2020). In the U.S.A., an estimated 885-2,575 breeding pairs occur, the biggest colony on the island of Santa Barbara holds 500-1,250 pairs (B. Keitt and D. Whitworth in litt. 2003). Small numbers breed on Anacapa (200-600 pairs), Santa Cruz (100-300 pairs), San Miguel (50-300 pairs), and San Clemente islands (10-50 pairs), U.S.A. (Sowls et al. 1980, Hunt et al. 1981, Carter et al. 1992, Drost and Lewis 1995). The Santa Barbara colony has been stable since the 1980s, but may have declined since the turn of the century (B. Keitt and D. Whitworth in litt. 2003). The breeding population size on the islands off western Baja California was estimated between 2002-2008 at 1686-4428 pairs, and despite likely being extirpated on all islands excluding Coronado and San Benito in the past (Bedolla-Guzmán et al. 2019), it has returned to breed at all such locations. Numbers are estimated to include 1117-2933 pairs at Isla Coronado, 262-688 at Todos Santos, 19-49 at San Martín, 24-64 at San Jerónimo, 231-607 at San Benito and 33-87 on the Cedros Islands (Whitworth et al. 2020). The eradication of cats on some such islands (Islas Coronado, Todos Santos, San Martín and San Benito) in the late-1990s has likely resulted in recent population increases in such localities. The species winters offshore from northern California (rarely) south to southern Baja California (Chesser et al. 2012). Some post-breeding birds reach Oregon, USA, and British Columbia, Canada (Gaston and Jones 1998, Karnovsky et al. 2005).
It nests on steep sea-slopes, canyons and cliffs with a sparse cover of herbaceous and shrubby plants (Drost and Lewis 1995). It is a generalist predator and may exploit higher prey concentrations around pelagic convergence lines (Hamilton et al. 2004). Around Santa Barbara, it feeds on larval fish, especially Northern Anchovy Engraulis mordax (Hunt and Butler 1980, Hunt et al. 1981). Breeding effort and performance are lower when anchovy abundance is low (Hunt and Butler 1980). Adults enter and leave the nesting colonies at night when there is little or no moon, typically arriving on the Channel Islands in mid-February. Nesting persists through mid-June with peak nesting from late March to late April (Jones et al. 2005). Clutches consist of two eggs laid approximately eight days apart with replacement of lost clutches unusual. Incubation takes c.34 days (Jones et al. 2005). There are no recent records of them breeding under shrubs on top of Anacapa Island, a habitat used frequently on nearby, rat-free, Santa Barbara Island (Jones et al. 2005).
Invasive mammals were responsible for all known and suspected colony extinctions in the past and pose an ongoing threat by being present in all the larger colonies and former colonies (McChesney and Tershy 1998). Feral cats Felis catus depredate chicks and adults. Heavy cat predation was recorded on North Coronado until the mid-1990s (Jehl and Bond 1975, Drost and Lewis 1995). The cat population at Catalina was most recently estimated at 600-750 individuals (Guttila and Stapp 2010) and eradication of cats and rats on the island has been regarded as ‘not currently feasible’ (Whitworth et al. 2014). Despite this however, many introduced predators have been removed from a number of breeding islands in recent decades (McChesney and Tershy 1998, Keitt 2005, Birt et al. 2012) with post-eradication increases noted (Whitworth and Carter 2017); the eradication of cats on some islands off the coast of western Baja California (Islas Coronado, Todos Santos, San Martín and San Benito) in the late-1990s has likely resulted in recent population increases in such localities (Whitworth et al. 2020). Black Rats Rattus rattus prey on murrelet eggs and adults (Whitworth et al. 2014). Eradication of rats at Anacapa Island in 2001–2002 greatly improved murrelet hatching success in the following years (2003–2010) (Whitworth et al. 2005, 2013), with 191% more eggs hatching and 94% fewer nests being depredated (Newton et al. 2016). Hatching success remains very low on islands including St. Clementine however, a result of ongoing predation by foxes and black rats (Whitworth et al. 2018). House Mice Mus musculus were recently introduced to the Coronados Islands, and Deer Mice Peromyscus maniculatus anacapae (native to nearby Cedros Island) have also been introduced to the San Benito Islands. Deer Mice depredated 46% of all monitored eggs on Santa Barbara (Murray et al. 1983), which is the largest known breeding site for the species (Howard et al. 2014). However, on Anacapa, Deer Mice were established long before the introduction of rats, precluding them from consideration as the cause of declines there (Jones et al. 2005). Another problematic native species is the Barn Owl Tyto alba. Annual estimates of owl predation on Santa Barbara range from 11 to 172 individuals (Thomsen and Harvey 2012, Thomsen et al. 2013, Nur et al. 2013). Population growth modelling based on data collected between 1991 and 2010, resulted in an estimate of a 1.17% population decline owing to owl predation, and projections of a hypothetical 50-80% reduction in predation pressure could potentially result in a 1.15-2.54% positive growth rate, barring an increase in egg depredation from island mouse populations (Nur et al. 2013). This assessment implicated the role of Barn Owls in the suppression of the Scripps’s Murrelet population recovery on Santa Barbara Island although recent evidence suggests that the presence of Barn Owls on Santa Barbara may have an indirect positive influence on Murrelet egg survival through increased predation on deer mice, and resultantly reduced egg predation from the mice themselves (Thomsen and Green 2016). Such interactions however, have been shown to shift with changing El Niño Southern Oscillation (ENSO) rainfall conditions as changing rainfall, vegetation abundance and resultantly deer mouse populations can result in 15-fold increases in the predation of Murrelet by Owls in the absence of mice as a food source (Thomsen et al. 2018). It has recently been evidenced that fecundity is dramatically reduced during drought years due to enhanced egg predation by deer mice and population declines may result (Thomsen and Green 2019). Any increase in the frequency or severity of drought conditions will likely increase the rate of population decline in response.
Various forms of pollution are threatening the species. The potential impacts of large and small oil spills in congregation waters near breeding islands is of great concern (Carter et al. 2000, Burkett et al. 2003). Long Beach Harbour is the largest oil port in western North America, with daily arrivals and departures of large oil tankers (USFWS 2005). Frequent oil spills occur in the vicinity of Long Beach Harbour, but most have not affected waters near Catalina (Carter 2003, Whitworth et al. 2014). As stated by Drost and Lewis (1995), pollution from offshore oil-wells or the Los Angeles oil-tanker lane could potentially extirpate the species from the south California Bight. Organochlorine pollutants are considered not to have a current significant effect on murrelets breeding at Catalina, but may have caused reduced reproduction or population size in the past, especially between the 1940s and 1970s, prior to the end of production and dumping of DDT in southern California waters (Drost and Lewis 1995, Whitworth et al. 2014). The potential impacts of bright lights used by squid fishing boats near breeding colonies is another concern, potentially causing disorientation and elevated predation pressures. Many owl-predated murrelets were found during a period of intense inshore squid fishing around Santa Catalina and disturbance from related activities could discourage nest-visitation and possibly cause collision mortalities (Whitworth et al. 2014).
Human disturbance has been identified as a cause of nest abandonment and a major threat to the species (Howard et al. 2015). The two highest rates of abandonment on record were observed at the Dock and the Bunkhouse plots, which are directly associated with human structures. The human disturbance associated with these sites may also affect the remaining nesting birds. Machinery operations at the dock include high decibel engine noise and vibration of the dock structure, which along with frequent traffic, could be affecting nesting murrelets beneath the decking of these structures (Howard et al. 2015).
Extreme fluctuations of sea surface temperature related to ENSO, are likely to affect food availability for the species, though the impact of such events appears to be small due to their generalist prey (USFWS 2016). In addition, the species's foraging behaviour puts it at risk from gill-net fisheries, although very few incidences have been reported (Carter et al. 2000). No mortality has been reported in nearshore gill-net fisheries in which other seabirds have relatively high bycatch rates (Pacific Seabird Group undated). However, the white seabass gill-net fishery operating around the northern California Channel Islands has very little observer coverage, potentially indicating that bycatch incidence has been underestimated or completely overlooked (Pacific Seabird Group undated).
Conservation Actions Underway
All potential breeding islands in the USA and Mexico have been surveyed, with the exception of the offshore rocks of Cedros Island (B. Tershy in litt. 1999). Over the past 10 years, most invasive mammals, including cats, rats, goats, sheep, rabbits, donkeys and dogs have been removed from almost all islands that are known current or suspected historical nesting sites in the US and Mexico (McChesney and Tershy 1998, B. Keitt and D. Whitworth in litt. 2003, Keitt 2005, Whitworth et al. 2005, Aguirre-Muñoz et al. 2008, Birt et al. 2012, Whitworth et al. 2020). In 2003, fishing and other extractive uses were banned within important areas of the Channel Islands National Marine Sanctuary in an attempt to reduce light and noise pollution in important near-shore staging areas. However, the exclusion zone covers only a small buffer area around some colonies, and may be ineffective at minimising threats (S. Wolf in litt. 2007). A pilot habitat restoration for the species has begun on Santa Barbara Island (Wolf 2008).
24 cm. Small, black-and-white alcid. Black upperparts. White underparts. White underwing linings. Black on head extends to bill. Partial white eye-ring. Shortish, stout bill. Nominate subspecies has more extensive white on face and ear-coverts, forming crescent in front of and over eye. Similar spp. Craveri's Murrelet S. craveri has partial breast- band, black extending under bill and different underwing colour and bill shape. Voice Shrill whistle, sometimes given as series.
Text account compilers
Fjagesund, T., Everest, J., Martin, R.
Contributors
Anderson, O., Benstead, P., Bird, J., Butchart, S., Calvert, R., Capper, D., Gilroy, J., Harding, M., Keitt, B., Miller, E., Moreno, R., Sharpe, C.J., Symes, A., Taylor, J., Tershy, B., Whitworth, D., Winnard, S. & Wolf, S.
Recommended citation
BirdLife International (2024) Species factsheet: Scripps's Murrelet Synthliboramphus scrippsi. Downloaded from
https://datazone.birdlife.org/species/factsheet/scrippss-murrelet-synthliboramphus-scrippsi on 24/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 24/12/2024.