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Black-vented Shearwater Puffinus opisthomelas



Justification

Justification of Red List Category
This species declined in the past owing to road building schemes and principally predation by introduced cats. However, these threats have ceased and the population may now begin to increase. Given the species's longevity it is retained as Near Threatened on the basis of the past declines.

Population justification
The total population estimated in 1998-1999 was around 80,000 pairs, and therefore 160,000 mature individuals. In 2016, aerial photography and GIS were used to count during the breeding season the total number of occupied and unoccupied burrows in Natividad Island. The counts gave a figure of 37,858 (±8510 SE) breeding pairs, and therefore about 75,600 individuals. This figure was based on the initial occupancy of burrows on February 2016. Of the initial number of occupied burrows, there were 4,543 (about 12% of the burrows occupied) which were initially occupied but later abandoned by the birds (Albores-Barajas et al. in press).

Counts produced much lower numbers than previous estimates. It is likely that the methods used in 1998-99 overestimated the population size. The author extrapolated counts of nests in selected areas to the whole surface of the island used by birds without considering that there were large areas without any burrow. During the last survey, thanks to aerial photography, it was possible to precisely count all the burrows which were actually present (Albores-Barajas et al. in press). The total number of burrows counted, including occupied and abandoned burrows, was 56,395 with a relatively low occupancy (between 75% and 50% depending on the area).

Between 2014 and early 2016 the birds on Natividad were also affected by the presence of a wire fence next to the local rubbish dump and on the trajectory used by bird when moving from colony to sea. Several carcasses were found during the survey in February 2016. When the situation was analysed using thermal cameras, it was observed that at least 20 individuals collided with the fence every day, resulting in seven dead individuals/day. This mortality rate was estimated equivalent to the predation of five cats (Albores-Barajas et al. in press). The fence was immediately removed after the observations when it was communicated to the local community and to the Biosphere Reserve.

Trend justification
The species declined dramatically owing to predation, in particular by introduced cats. However, eradication of these from its principal breeding island, Natividad, suggests that immediate threats to the species have now been significantly reduced and recruitment to the population may increase. Nevertheless, the current trend still remain unknown. Although the counts from 2016 are much lowers than in 1998-1999, data from 1998-1999 has overestimated the population and thus, the data doesn't seem to be comparable. Bycatch in fisheries remains a threat but its impacts are unknown.

Distribution and population

Puffinus opisthomelas breeds on six islands or small islets (including Guadalupe -and its islets-, San Benito- East, Middle and West- and Natividad), off the Pacific coast of Mexico. The current world population was estimated between 55,000 and 95,000 pairs, with the vast majority (>95%) occurring on the island Natividad (Keitt 1998). On San Benito, there were 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). On Guadalupe, the population was estimated at 2,500 pairs in 1991 (Everett and Anderson 1991). Not much is known for the other three islands.

In 1927, nesting was reported in Isla Rasa, Gulf of California, but was believed to be extirpated from that island by introduced rodents (Velarde et al. 2015). Nesting has been reported again since 2010, however the nesting population has not been determined (Velarde et al. 2015). Individuals have been recorded at sea in the area between the Guaymas Basin and Midriff Island Region individually or in groups of up to 15 individuals (Velarde et al. 2015).  

On Natividad, there was a 15% decrease in habitat and a 13-20% loss in burrows between c.1970 and the mid-1990s, and the estimated population decline was 4% per annum (Keitt 1998). Where surveys in the late 1990s estimated 114,000 burrows (Keitt et al. 2003), only 56,000 burrows were recorded in 2016, however it is considered that the methods used in 1998-1999 overestimated the population size (Albores-Barajas et al. in press). Birds disperse mainly to the North reaching central California, USA, and rarely British Columbia (Carboneras 1992d). 


Ecology

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).

Threats

On Natividad, in the past, predation by c.20 feral cats reduced the population causing >1,000 recorded deaths per month prior to their eradication which reduced mortality to less than 100 per month (Keitt et al. undated, 2002). Road construction and the establishment of a small fishing community have decreased breeding habitat and burrows (Keitt 1998), and trampled burrows and lights continue to cause some mortality (Keitt and Albores-Barajas unpublished data). 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). Predation by Western gulls and common raven can be easily observed both on chicks and adults, while eggs and young chicks are occasionally predated by white-tail antelope squirrel, Ammospermophilus leucurus (Albores-Barajas and Garcia unpublished data). The Guadalupe population is thought to be predated by cats (B. Tershy and B. Keitt in litt. 1999). It is possible that cat predation has caused the extirpation, or at the very least, significantly reduced populations of this species from the main island of Guadalupe (Keitt et al. 2006). In 2003, remains were found on cliffs at the southwest edge of Guadalupe in excellent potential nesting habitat. It is not known if these observations indicate breeding or prospecting birds. The presence of habitat inaccessible to cats provides hope that this species may still breed in small remnant populations on the main island of Guadalupe (Keitt et al. 2006). Calls from prospecting adults in the cliffs of Melpomene Arroyo, indicate birds are readily available to recolonize the main island in the absence of cat predation (Keitt et al. 2006). On all islands, introduced herbivores (donkeys, goats, sheep and rabbits) have caused erosion (reducing soil for burrows), trampled burrows, and destroyed vegetation, and rabbits may have displaced birds from burrows (Keitt 1998, B. Tershy and B. Keitt in litt. 1999). Gill-net fisheries may cause some mortality (Carboneras 1992d). The human population at Natividad Island has continued to increase, with more houses built over the last few years. Although the houses are concentrated on the town area, there is more human presence on the island. Also, from time to time there are mountain biking races organized on the island and the track runs through the colony. Although the bikes tend to stay within the race track, it might be that this causes some human disturbance to the burrowing seabirds. 

Conservation actions

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. in press Conservation Evidence). In 1998-1999, introduced herbivores were eradicated from San Benito (B. Tershy and B. Keitt in litt. 1999). Guadalupe is designated as a biosphere reserve (S. N. G. Howell in litt. 1998), and the process of eradicating the cats is under way.

Conservation Actions Proposed

Monitor the population. Prohibit future road construction on Natividad and remove rubbish or manage in a more efficient way (B. Tershy and B. Keitt in litt. 1999, Albores-Barajas et al. in press Conservation Evidence). Control the middle and west San Benitos Islands to visitors (B. Tershy and B. Keitt in litt. 1999). Prevent future introductions of native (gulls and ravens) and non-native predators and ensure all breeding islands remain cat-free (B. Tershy and B. Keitt in litt. 1999) and exotic rodent-free (Velarde et al. 2015). 


Identification

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.

Acknowledgements

Text account compilers
Capper, D., Isherwood, I., Calvert, R., Bird, J., Benstead, P., Anderson, O., Moreno, R.

Contributors
Soldatini, C., Keitt, B., Howell, S., Velarde, E., Dell’Omo, G., Albores-Barajas, Y.


Recommended citation
BirdLife International (2017) Species factsheet: Puffinus opisthomelas. Downloaded from http://www.birdlife.org on 24/11/2017. Recommended citation for factsheets for more than one species: BirdLife International (2017) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 24/11/2017.