Mascarene Swiftlet (Aerodramus francicus): revise global status?

BirdLife species factsheet for Mascarene Swiftlet: http://datazone.birdlife.org/species/factsheet/mascarene-swiftlet-aerodramus-francicus

Mascarene Swiftlet (Aerodramus francicus) has recently been split into two subspecies (Kirwan et al. 2018). The nominate subspecies is found on Mauritius, where the species has, at the very least, undergone historical declines (Safford 2013). The newly recognised subspecies saffordi is endemic to La Réunion (to France), where the population size appears to be considerably larger than on Mauritius, but a succession of increased population estimates for the island may have been a result of more comprehensive surveying assessments (V. Tatyah, N. Laurent and F.-X. Couzi in litt. 2018).

Mascarene Swiftlet occurs in a range of habitats, but breeds colonially in caves and man-made structures like dams (V. Tatayah, N. Laurent and C.-X. Couzi in litt. 2018, see also Safford 2013). Vandalism and development of these caves, and collection of the species’s nests are the principle threat to the species in Mauritius (A. Cheke pers. comm. 2000; C. Jones in litt. 2000; V. Tatayah, N. Laurent and F.-X. Couzi in litt. 2018), and such threats are also present on La Réunion (V. Tatayah, N. Laurent and F.-X. Couzi in litt. 2018). Some caves have also been completely closed up such that the swiftlets cannot reach their breeding sites any more, and the species may be considered undesirable at some breeding localities (e.g. Bras de la Paine) as the droppings may end up in water intended for human consumption (V. Tatayah, N. Laurent and F.-X. Couzi in litt. 2018). The use of pesticides within its range is also likely to be having a large impact on the availability of food resources for this species (V. Tatayah, N. Laurent and F.-X. Couzi in litt. 2018).

The species is currently considered to be Near Threatened (see BirdLife International 2018), but has been reassessed here against all criteria.

Criterion A – While it may be possible to infer that the species is in decline as a result of the ongoing threats within its range, it is currently not possible to clearly quantify the rate of decline. Therefore, it cannot be accurately assessed against this criterion.

Criterion B – The species’s Area of Occupancy has not been calculated per IUCN requirements (see IUCN Standards and Petitions subcommittee 2017), but it likely exceeds the threshold for Vulnerable (2,000 km2). Its Extent of Occurrence (16,200 km2), however, does meet the threshold for Vulnerable under Criterion B1 (20,000 km2). To warrant listing as such does still require at least two other conditions to be met.

The species is not known to undergo extreme fluctuations, and so condition c) is not met. Yet, the ongoing threats outlined above mean that it is highly likely that there is an ongoing decline in the quality of the species’s habitat, and we can infer an ongoing decline in the population. Therefore, conditions b(iii,v) are met. Ongoing closure of caves and the destruction of colonies could mean that condition b(iv) is met, but as each island may represent a subpopulation, this condition is dependent on the application of the term location*.

Condition a) is definitely dependent on how we look at the number of locations too, as the species does not fulfil the definition of severely fragmented (per IUCN Standards and Petitions Subcommittee 2017). As nest collectors may be able to travel across each island, it could be said that the number of locations where this species occurs is two. In this case, there would be no continuing decline in the number of locations, and the species would trigger a listing as Vulnerable under Criterion B1ab(iii,v). If, however, the most severe threat is deemed to be individual closures of caves, or the use of pesticides on the islands, then the area of impact of each threatening event (i.e. a single breeding site, or a single farmer using pesticides) would be far smaller and the number of locations would likely be >10; but there would likely be an ongoing decline in the number of locations. Thus, in this case the species would at worst be Near Threatened under Criterion B1ab(iii,iv,v). Therefore, further comment is requested in regards to the application of the term location* per the IUCN definition (see below).

Criterion C – The population on Mauritius was estimated at 2,244-2,610 individuals (G. Middlteon per C. Jones in litt. 2000), and the population on La Réunion is likely to be far larger, with some of the largest colonies thought to contain 5,000 pairs (St Denis à La Possession) and 1,000 pairs (Bras de la Plaine) (V. Tatayah, N. Laurent and F.-X. Couzi in litt. 2018). The overall population size has not been estimated, but based on these figures, it may be precautionarily placed in the range 10,000-19,999 mature individuals. This is too large to trigger the threshold for Vulnerable under this criterion, but it does approach the threshold, and so it could warrant listing as Near Threatened. However, further conditions would also have to be met.

The species is not known to undergo extreme fluctuations. It occurs in several subpopulations, and the largest of these subpopulations contains significantly more than 1,000 mature individuals. Therefore, the species cannot be listed under Criteria C2b, C2a(ii) and C2a(i). Also, with no clear estimation of the overall rate of population change it is not possible to use criterion C1. Therefore, the species would not warrant listing under Criterion C and may be considered Least Concern under this criterion.

Criterion D – The species’s population size is too large to warrant listing under this criterion; and while it is restricted to a small number of locations, ongoing threats are unlikely to drive the species Extinct or to Critically Endangered within a very short period of time (per IUCN Standards and Petitions Subcommittee 2017). Therefore, at worst it would warrant listing as Near Threatened under criterion D2.

Criterion E – To the best of our knowledge, there has been no quantitative analysis of extinction risk conducted for this species. Therefore, it cannot be assessed against this criterion.

Therefore, the species at least warrants listing as Near Threatened under Criterion D2 (possibly B1ab(iii,iv,v) too), and potentially as Vulnerable under Criterion B1ab(iii,v). We welcome comments, particularly regarding the number of locations* where the species occurs, but please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’ Red List status. Therefore, please make sure your comments are relevant to the information that is sought, or about the species’ Red List status.

*The term ‘location’ refers to a distinct area in which a single threatening event can rapidly affect all individuals of the taxon present, with the size of the location depending on the area covered by the threatening event. Where a taxon is affected by more than one threatening event, location should be defined by considering the most serious plausible threat (IUCN 2001, 2012).

An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.

References

BirdLife International. 2018. Species factsheet: Aerodramus francicus. Downloaded from http://www.birdlife.org on 08/10/2018.

IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K.

IUCN. 2012. IUCN Red List Categories and Criteria: Version 3.1. Second edition. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K. Available at: www.iucnredlist.org/technical-documents/categories-and-criteria.

IUCN Standards and Petitions Subcommittee. 2017. Guidelines for Using the IUCN Red List Categories and Criteria. Version 13. Prepared by the Standards and Petitions Subcommittee. Downloadable from http://www.iucnredlist.org/documents/RedListGuidelines.pdf.

Kirwan, G. M.; Shirihai, H.; Schweizer, M. 2018. A morphological revision of Mascarene Swiftlet Aerodramus fancicus, with the description of a new subspecies from Reunion. Bull. B.O.C. 138(2): 117-130.

Safford, R. J. 2013. Mascarene Swiftlet Aerodramus francicus; pp 593-595 in Safford, R. J. and Hawkins, A. F. A. (eds) The Birds of Africa. Volume VIII: The Malagasy Region. Christopher Helm, London.

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2 Responses to Mascarene Swiftlet (Aerodramus francicus): revise global status?

  1. Rob Martin (BirdLife International) says:

    Given the observed potential extent of the threat of nest collection does not seem to operate at a whole island scale within a rapid timescale (e.g. one generation), the appropriate number of locations here appears to be greater than 10, and an ongoing decline is inferred through the loss of individual breeding sites and the impact of pesticides.

  2. Rob Martin (BirdLife International) says:

    Preliminary proposal

    Based on available information, our preliminary proposal for the 2019 Red List would be to list Mascarene Swiftlet as Near Threatened under criterion B1ab(iii,iv,v).

    There is now a period for further comments until the final deadline in mid-July, after which the recommended categorisations will be put forward to IUCN.

    Please note that we will then only post final recommended categorisations on forum discussions where these differ from those in the initial proposal.

    The final 2019 Red List categories will be published on the BirdLife and IUCN websites in December, following further checking of information relevant to the assessments by both BirdLife and IUCN.

Comments are closed.