Are there 25 000 Critically Endangered Balearic Shearwaters?

Gonzalo Arroyo (Fundación Migres, Cadiz, Spain) and colleagues have come up with a new estimate of the numbers of ACAP-listed  Shearwaters Puffinus mauretanicus based on counts of migrating birds, publishing in the journal Bird Conservation International.

“The Balearic Shearwater Puffinus mauretanicus is considered one of the most threatened seabirds in the world, with the breeding population thought to be in the range of 2,000–3,200 breeding pairs, from which global population has been inferred as 10,000 to 15,000 birds.  To test whether the actual population of Balearic Shearwaters is larger than presently thought, we analysed the data from four land-based census campaigns of Balearic Shearwater post-breeding migration through the Strait of Gibraltar (mid-May to mid-July 2007–2010).  The raw results of the counts, covering from 37% to 67% of the daylight time throughout the migratory period, all revealed figures in excess of 12,000 birds, and went up to almost 18,000 in two years.  Generalised Additive Models were used to estimate the numbers of birds passing during the time periods in which counts were not undertaken (count gaps), and their associated error.  The addition of both counted and estimated birds reveals figures of between 23,780 and 26,535 Balearic Shearwaters migrating along the north coast of the Strait of Gibraltar in each of the four years of our study.  The effects of several sources of bias suggest a slight potential underestimation in our results.  These figures reveal the urgent need to reformulate the population viability analysis for the species, and then if necessary reconsider its conservation status.”

Balearic Shearwater at sea


Arroyo, G.M., Mateos-Rodríguez, M., Muñoz, R., De La Cruz, A., Cuenca, D.& Onrubia, A. 2014.  New population estimates of a critically endangered species, the Balearic Shearwater Puffinus mauretanicus, based on coastal migration counts.  Bird Conservation International  DOI:

John Cooper, ACAP Information Officer, 27 December 2014

The Agreement on the
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ACAP is a multilateral agreement which seeks to conserve listed albatrosses, petrels and shearwaters by coordinating international activity to mitigate known threats to their populations.

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