Wood Thrush (Hylocichla mustelina): revise global status?

BirdLife species factsheet for Wood Thrush

Wood Thrush (Hylocichla mustelina) is a widespread breeding visitor to the eastern USA and south-eastern Canada, wintering in Central America from southern Mexico to Panama. It breeds in the interior and along the edges of deciduous and mixed forest communities (del Hoyo et al. 2005). In its non-breeding range, it is found mainly in humid to semi-humid evergreen and deciduous forests or palm stands, but also in secondary growth, thickets and plantations (del Hoyo et al. 2005).

The population is estimated to number 12,000,000 mature individuals (Partners in Flight 2019). The species has undergone rapid declines in the past, with an estimated 15,000,000 mature individuals lost since 2017 (Partners in Flight 2019, Rosenberg et al. 2019). Wood Thrush is threatened by the loss and fragmentation of its forest habitat both within the breeding and non-breeding ranges (Taylor and Stutchbury 2016, Evans et al. 2020). In fragmented habitats, breeding pairs suffer from lower reproductive success and higher prevalence of cowbird parasitism (Evans et al. 2020). Further threats include contamination by acid rain, disturbance at nest sites, pollution and window collisions (Evans et al. 2020).

Wood Thrush is currently listed as Near Threatened, approaching the threshold for listing as threatened under Criterion A2a (BirdLife International 2020). However, rates of population decline seem to have slowed down in recent years, suggesting that the species may warrant a change in Red List status. Therefore, it will be re-assessed against all Red List criteria:

Criterion A – The species has been undergoing a large, significant decline between 1970 and 2017, with an average rate of 1.92% per year (Partners in Flight 2019). This amounts to a decline of 18% over ten years (one generation length being estimated at 2.8 years; Bird et al. 2020*). However, investigating short-term trends from the North American Breeding Bird Survey (BBS; Sauer et al. 2017), it seems that declines have slowed down in recent years: For the decade 2002-2012, declines amounted to 21%; but since then the rate of decline has continuously been below 20%. For the period 2005-2015, the species declined by 11%. To calculate the rate of population change for the past ten years, we can assess the trend for 2010-2015 and extrapolate this to 2020, which results in a rate of decline of 11% over the past ten years. This value is statistically significant. Therefore, Wood Thrush warrants listing as Least Concern under Criterion A.

Criterion B – The species’s range is too large to warrant listing as threatened under Criterion B (EOO breeding = 4,870,000 km2, EOO non-breeding = 1,710,000 km2) and thus Wood Thrush qualifies as Least Concern under this criterion.

Criterion C – The global population numbers 12,000,000 mature individuals (Partners in Flight 2019). As such, the species warrants listing as Least Concern under Criterion C.

Criterion D – The population size and range are too large to warrant listing as threatened under Criterion D and thus Wood Thrush qualifies as Least Concern under this criterion.

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

Therefore, it is suggested that Wood Thrush (Hylocichla mustelina) be listed as Least Concern. We welcome any comments on the proposed listing.

Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of its Red List status. Therefore, please make sure your comments are relevant to the discussion outlined in the topic. By submitting a comment, you confirm that you agree to the Comment Policy.

*Bird generation lengths are estimated using the methodology of Bird et al. (2020), as applied to parameter values updated for use in each IUCN Red List for birds reassessment cycle. Values used for the current assessment are available on request. We encourage people to contact us with additional or improved values for the following parameters; adult survival (true survival accounting for dispersal derived from an apparently stable population); mean age at first breeding; and maximum longevity (i.e. the biological maximum, hence values from captive individuals are acceptable).

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

Bird, J. P.; Martin, R.; Akçakaya, H. R.; Gilroy, J.; Burfield, I. J.; Garnett, S.; Symes, A.; Taylor, J.; Šekercioğlu, Ç.; Butchart, S. H. M. (2020). Generation lengths of the world’s birds and their implications for extinction risk. Conservation Biology online first view.

BirdLife International. 2020. Species factsheet: Hylocichla mustelina. http://www.birdlife.org (Accessed 23 April 2020).

del Hoyo, J.; Elliott, A.; Christie, D. 2005. Handbook of the Birds of the World, vol. 10: Cuckoo-shrikes to Thrushes. Lynx Edicions, Barcelona, Spain.

Evans, M.; Gow, E.; Roth, R. R.; Johnson, M. S.; Underwood, T. J. 2020. Wood Thrush (Hylocichla mustelina), version 1.0. In Poole, A. F. (ed.). Birds of the World. Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.woothr.01 (Accessed 23 April 2020).

Partners in Flight. 2019. Avian Conservation Assessment Database, version 2019. http://pif.birdconservancy.org/ACAD.

Rosenberg, K. V.; Dokter, A, M.; Blancher, P. J.; Sauer, J. R.; Smith, A. C.; Smith, P. A.; Stanton, J. C.; Panjabi, A.; Helft, L.; Parr, M.; Marra, P. P. 2019. Decline of the North American avifauna. Science 366(6461): 120-124.

Sauer, J. R.; Niven, D. K.; Hines, J. E.; Ziolkowski Jr., D. J.; Pardieck, K. L.; Fallon, J. E.; Link, W. A. 2017. The North American Breeding Bird Survey, Results and Analysis 1966-2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

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2 Responses to Wood Thrush (Hylocichla mustelina): revise global status?

  1. Red List Team (BirdLife International) says:

    The window for consultation is now closed. We will analyse and interpret the new information and post a preliminary decision on this species’s Red List status on this page in early July.

    Thank you,
    BirdLife Red List Team

  2. Red List Team (BirdLife International) says:

    Preliminary proposal

    Based on available information, our preliminary proposal for the 2020 Red List would be to adopt the proposed classification outlined in the initial forum discussion.

    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 2020 Red List categories will be published on the BirdLife and IUCN websites in December 2020/January 2021, following further checking of information relevant to the assessments by both BirdLife and IUCN.

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