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Range DescriptionThe species breeds in the Balearic Islands, Spain. The breeding populationwas most recently estimated at 3,200 pairs which equates to 6,400 mature individuals (BirdLife International 2015). This figure is significantly larger than previous estimates of 2,000-2,400 pairs recorded in 2005 (Jones et al. 2008) but this is primarily due to increased survey effort (better prospecting of known breeding sites plus discovery of new sites) and does not reflect a genuine increase of the population. The islands of Mallorca have 900 pairs; Cabrera 449 pairs; Menorca 405 pairs; Ibiza 747 pairs and Formentera 692 pairs (Arcos 2011a). The world population was until recently believed to number 8,000-10,000 individuals (Louzao 2006a, Wynn and Ysou 2007), however recent winter at-sea surveys and counts from Gibraltar of post-breeding birds leaving the Mediterranean suggest the total population may in fact lie within in the range 20,000-30,000 individuals (Arcos 2011b, Arroyo et al. 2015). This is supported by a count of at least 16,400 individuals off Valencia in December 2009 (Aleixos 2012). Reasons for the discrepancy between breeding and non-breeding population estimates are unclear, but it is most likely that this species has a particularly large floating population of immatures and non-breeders.
On Cabrera Island, 60% of the colonies have disappeared in the last few decades, while colonies on Formentera have experienced a strong decline in recent years, from more than 1,500 breeding pairs in the early 1990s to less than 1000 pairs in 2001 (Ruiz and Mart 2004) and 692 pairs in 2003-2006 (Arcos 2011a). Population viability analysis has shown that in the presence of environmental and demographic stochasticities, mean extinction time for the world population was estimated at 40.4 years, and mean growth rate showed a 7.4% decrease each year (Oro et al. 2004). However, this model was run using the earlier population estimate of c. 2,000 breeding pairs and so new analysis of extinction risk using the updated population estimate and improved demographic data is required. At the end of the breeding season, birds may forage in areas off the coast of north-west Africa (Louzao et al. 2012). In winter, it occurs in the Balearic Sea and off the north-east Spanish coast with most of the population traditionally concentrated between Valencia and Catalonia from November to February, although recent data suggest that some birds remain in the Atlantic. For instance, in winter 2007/2008 significant numbers (with a peak count of 710 birds) remained off the coast of Brittany (France), perhaps in response to unusual sea surface temperatures (Plestan et al. 2009). Some birds migrate north in summer to seas off the British Isles and the south of the Scandinavian Peninsula (Wynn et al. 2007). Numbers recorded in the traditional post-breeding quarters have declined since the mid-1990s, with a corresponding increase in numbers along the coasts of northern France and south-west U.K. (Wynn and Ysou 2007, Wynn et al. 2007) (including an exceptional gathering of 4,600 in the Baie de Lannion, Brittany in August 2010) (D. Andrews in litt. 2010).