Data Zone
Justification of Red List category
This species has undergone rapid declines in the past, mainly through predation by introduced cats. While other threats continue to locally cause the loss and degradation of habitat within the small breeding range, the population has been showing signs of recovery since the eradication of cats from the principal breeding island. For these reasons, the species is listed as Near Threatened.
Population justification
The vast majority of the population (>95%) breeds on Natividad (Keitt 1998). On the San Benito islands, there used to be at least several thousand pairs (B. Tershy and B. Keitt in litt. 1999), but only 250-500 pairs were estimated in 1991 (Everett and Anderson 1991), and during the 2017-2019 breeding season, 121 breeding pairs were estimated (Sánchez et al. 2021a). On Guadalupe, the population was estimated at 2,500 pairs in 1991 (Everett and Anderson 1991), although may now be extirpated here (Sánchez et al. 2021a). In 1927, nesting was reported in Isla Rasa, Gulf of California, but the species is believed to be extirpated from that island by introduced rodents (Velarde et al. 2015). Nesting has been reported again since 2010; however, the size of the nesting population has not been determined (Velarde et al. 2015).
The total population estimated in 1998-1999 was around 80,000 pairs, and therefore 160,000 mature individuals. However, it is likely that the methods used in 1998-99 overestimated the population size, as counts of nests in selected areas were extrapolated to the whole surface of the island used by birds, without considering that there were large areas without any burrows.
In 2016, aerial photography and GIS were used to count the total number of occupied and unoccupied burrows in Natividad Island during the breeding season (Albores-Barajas et al. 2016). The total number of burrows counted, including occupied and abandoned burrows, was 56,400, but with a relatively low occupancy (between 75% and 50% depending on the area). The counts gave a figure of 37,900 (± 8500 SE) breeding pairs, equating to about 75,800 mature individuals. This figure was based on the initial occupancy of burrows in February 2016. Of these initially occupied burrows, 4,500 (about 12%) were later abandoned (Albores-Barajas et al. 2016). During the 2017-2019 breeding season, through further surveys on nest or burrow densities, 118,920 breeding pairs were counted on Natividad (Sánchez et al. 2021a), roughly equating to 238,000 mature individuals. Given the small populations present on other islands off Mexico's Pacific coast, the population is therefore placed in the band 200,000-299,999 mature individuals.
Trend justification
The species declined dramatically in the past owing to predation, in particular by introduced cats. On the principal breeding island, Natividad, there was a 15% decrease in habitat and a 13-20% loss in burrows between 1970 and the mid-1990s, with an estimated population decline of 4% per annum (Keitt 1998). However, eradication of cats from Natividad in 2002-2003 suggests that immediate threats to the species have now been significantly reduced and recruitment to the population may have increased. Nevertheless, the current trend has not been quantified. Although the counts from 2016 are much lower than in 1998-1999, data from 1998-1999 apparently overestimated the population; thus, the data do not seem to be comparable. However, as the eradication of cats from Natividad achieved a 90% reduction in mortality and a rapid recolonisation of cat-free areas (Keitt and Tershy 2003, Keitt et al. 2006), it is tentatively suspected that the population is currently stable or increasing slowly. Nevertheless, due to the species' longevity, the rapid declines of 4% per year in the large majority of the breeding population up until 2002 equate to a reduction of c.24% over the past three generations (40.5 years). Notably, bycatch in fisheries, habitat loss to infrastructure developments and habitat degradation through anthropogenic disturbance and introduced mammals and birds remain ongoing threats, and while their impacts on the population size have not been quantified it is possible that the population is still declining locally.
Puffinus opisthomelas breeds on five islands or small islets off the Pacific coast of Mexico (Guadalupe, San Benito East, San Benito Middle, San Benito West and Natividad) and on Isla Rasa in the Gulf of California. During the non-breeding season, it disperses along the Pacific coast from Monterrey and San Francisco bays (U.S.A.) and rarely British Columbia (Canada) in the north to the Gulf of California and further south to Nayarit, Jalisco and Colima (Mexico) (Carboneras 1992, Keitt et al. 2020).
Breeding takes place in burrows in sandy soil and rocky crevices. Birds attend colonies for at least 10 months of the year, arriving nocturnally to reduce predation by Western Gulls Larus occidentalis (Keitt et al. 2004) and ravens. Eggs are laid in March and hatching begins in early May (Keitt 1998). During the non-breeding season, individuals have been recorded at sea individually or in groups of up to 15 individuals (Velarde et al. 2015).
Historically, predation by feral cats Felis catus reduced the Natividad population, causing over 1,000 deaths per month. Population models revealed that even small cat populations lead to dramatic reductions in annual population growth of shearwaters, with one cat capable of depredating an estimated 40.5 birds per month (Keitt et al. 2002). Following the publication of these findings, cats were eradicated from the island and shearwater mortality decreased by more than 90%, to fewer than 100 per month (Keitt and Tershy 2003). It is possible that cat predation caused the extirpation, or at the very least, significantly reduced populations on the main island of Guadalupe (Keitt et al. 2006). The presence of habitat inaccessible to cats provides hope that this species may still breed in small remnant populations on Guadalupe and observed calls from prospecting adults in the cliffs of Melpomene Arroyo, indicate that birds are readily available to recolonize the main island in the absence of cat predation (Keitt et al. 2006). On all islands, introduced herbivores (donkey, goat, sheep and rabbit) have caused erosion (reducing soil for burrows), soil compaction, trampled burrows and destroyed vegetation, and rabbits may also have displaced birds from burrows (Keitt 1998, McChesney and Tershy 1998, B. Tershy and B. Keitt in litt. 1999, Donlan et al. 2000, Tershy et al. 2002). Introduced rodents in Isla Rasa, in the Gulf of California, have been successfully eradicated, but no population monitoring has been carried out since first nesting record occurred in 2010 (Velarde et al. 2015). No mammals have been reported since 2013 on San Benito Oeste (Sánchez et al. 2021b).
Between 2014 and early 2016, the birds on Natividad were affected by the presence of a wire fence installed around the landfill adjacent to the colony on the trajectory used by birds when moving from colony to sea. Several carcasses were found during the survey in February 2016 and a study using thermal cameras revealed that the fence caused the mortality of c. 6-7 birds per night, and an estimated 195 deaths per month during the breeding season (suggesting an impact roughly equivalent to the presence of five feral cats) (Albores-Barajas et al. 2016). Upon reporting the mortality levels to the local community and Biosphere Reserve, the fence was immediately removed in April 2016 (Albores-Barajas et al. 2016).
The development of infrastructure on Natividad has destroyed at least 15% (26,532 burrows) of the breeding habitat on the island (Keitt et al. 2003) and further development of the fishing community continues to have detrimental effects on the seabirds (Keitt and Albores-Barajas unpublished data). The human population at Natividad Island is increasing, with more houses built over the last few years. Although the houses are concentrated on the town area, there is generally a greater human presence on the island. Occasionally, mountain biking races are organized on the island and the track runs through the colony. Although the bikes tend to stay within the race track, this may cause considerable disturbance to the burrowing seabirds. Activities such as the building and maintenance of roads, off-road driving through the colony, dumping of garbage and humans walking through the colony all pose threats to the population (Keitt et al. 2003).
Other ongoing threats include incidental capture in artisanal fisheries, dogs excavating burrows (McChensey and Tershy 1998), as well as predation by raven Corvus corax and Western Gull Larus occidentalis. Ravens have increased due to the provision of water on the island in association with human settlements, and quickly learned to scavenge birds killed and injured by the landfill fence (Albores-Barajas et al. 2016). Predation may also occur, however the shearwater's nocturnal habit reduces this interaction and the impact at present is likely negligible, but worth monitoring. Light pollution reduces the protection offered by nocturnal behaviour, and nightly predation pressures by gulls are higher in lit areas; Black-vented Shearwaters avoid attending the colony when the light level is above –2.2 log10L because of threat posed by gulls. Hence, anthropogenic light sources that illuminate colonies could affect seabirds by increasing predation and decreasing access to the colony, leading to decreased foraging efficiency and ability to provide for chicks (Keitt et al. 2004).
Conservation Actions Underway
Natividad is a core area of the Vizcaino Biosphere Reserve, where there is some active management. In 1997-1998, goats and sheep were removed with the cooperation of the local fishing community (Keitt 1998). Cats were controlled in 1998, and eradicated by 2002 (B. Tershy and B. Keitt in litt. 1999, Keitt et al. undated). In 2016, the perimetral fence of the landfill was removed (Albores-Barajas et al. 2016). In 1998-1999, introduced herbivores were eradicated from San Benito (B. Tershy and B. Keitt in litt. 1999) and no invasive vertebrates were found on the island until 2006 (B. Keitt in litt. 2008). On San Benito Oeste, in 2006, a subspecies of the Cactus mouse Peromyscus eremicus (subspecies cedrosensis) was accidentally introduced; mouse eradication was successfully implemented in December 2013 and no mammals have been reported since (Sánchez et al. 2021b). Guadalupe is designated as a biosphere reserve (S.N.G. Howell in litt. 1998), and the process of eradicating the cats is underway.
Conservation Actions Proposed
Produce an up-to-date estimate of the population size. Assess the population trend. Quantify the impact of bycatch and other threats on the population size. Monitor the population trend.
Prohibit future road construction on Natividad. Control the access of visitors to middle and west San Benitos islands (B. Tershy and B. Keitt in litt. 1999). Remove rubbish or manage in a more efficient way (B. Tershy and B. Keitt in litt. 1999, Albores-Barajas et al. 2016). Prevent future introductions of native (gulls and ravens) and non-native predators. Ensure all breeding islands remain free of cats and exotic rodents (B. Tershy and B. Keitt in litt. 1999, Velarde et al. 2015). Protect breeding burrows from disturbance by dogs. Raise awareness for the species and its habitat with the aim of reducing anthropogenic disturbance to breeding colonies.
34 cm. Medium-sized shearwater. Blackish-brown above and dirty white below. Transition at side of head from brown to white is poorly demarcated. Indistinct dusky collar may cross entire chest. Dusky brownish undertail-coverts and brownish thighs. White does not extend on to rump. Similar spp. Townsend's Shearwater P. auricularis is smaller with quicker wing-beats, clean cut black-and-white plumage and white flank patches. Pink-footed Shearwater P. creatopus is larger and flies higher. Audubon's Shearwater P. lherminieri is smaller.
Text account compilers
Fernando, E., Everest, J., Fjagesund, T., Martin, R., Hermes, C., Stuart, A.
Contributors
Albores-Barajas, Y., Anderson, O., Benstead, P., Bird, J., Calvert, R., Capper, D., Dell’Omo, G., Howell, S., Isherwood, I., Keitt, B., Moreno, R., Soldatini, C. & Velarde, E.
Recommended citation
BirdLife International (2024) Species factsheet: Black-vented Shearwater Puffinus opisthomelas. Downloaded from
https://datazone.birdlife.org/species/factsheet/black-vented-shearwater-puffinus-opisthomelas on 23/12/2024.
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 23/12/2024.