Great Northern Divers Gavia immer collected during the impact assessment of the 2002/03 Prestige oil spill were studied and compared with similar casualties collected during three other major oil spills. It was concluded that adults tend to winter further to the north in Western Europe than juveniles, and that the sexes were evenly represented in all spills. Wing and bill lengths of adults were also compared with those of museum specimens collected during the breeding season in Canada, Greenland and Iceland in an attempt to determine the possible breeding origin of birds wintering in northwest Spain. Wing lengths suggested that Iceland and Greenland were possible breeding grounds, while the incidence of embedded gunshot (18% of adults) further suggested that at least some birds originated from Greenland, where hunting is still legal. However, biometrics and gunshot incidence alone were inconclusive in determining a possible breeding origin. DNA analyses are being considered, including comparisons with gene sequences for this species already available in the GenBank. Mortality during the Prestige oil spill may have affected 18–22% of the total wintering population in Spain. Besides oil-induced mortality, drowning in fishing gear is identified as a real threat in Galicia. A systematic beached bird survey in the region would help determine the relative importance of different causes of mortality in this species.

Several major oil spills in Europe have affected wintering Great Northern Divers Gavia immer (Hope Jones et al. 1978; Heubeck & Richardson 1980; Heubeck et al. 1993; Weir et al. 1996; Heubeck 1997; SEEEC 1998; Cadiou et al. 2003b), and the Prestige incident off Spain in 2002 was no exception. The oil tanker Prestige, carrying a cargo of 77,000 tonnes of heavy bunker oil, sank off the coast of Galicia, northwest Spain on 19 November 2002. Most of the Galician coast was severely polluted (González et al. 2006) and thousands of oiled seabirds were retrieved from beaches between November 2002 and summer 2003 (García et al. 2003). Pollution was not confined to Galicia, and other parts of the northern Spanish, the Portuguese and the French coastlines were repeatedly contaminated with oil. Statistics released by SEO/Birdlife indicated that 65 Great Northern Divers were retrieved from beaches, three of which were found in Portugal, two in France and the remainder in Spain (García et al. 2003). Great Northern Divers are scarce wintering birds in Atlantic Iberia, with a conser- vative estimate of 270–340 individuals for its Spanish sector (Sandoval & De Souza 2005). Most occur in Galicia and Asturias, where they frequent shallow open coasts, small embayments, rías (river valleys flooded by the sea) and estuaries, and the Galician population has recently been estimated at 123 (95% CI 76–166) individuals (De Souza et al. 2010). Regular beached bird surveys in the 1980s and 1990s on the Atlantic coast of Iberia found only occasional strandings of Great Northern Divers (Arcos & Solís 1997; Granadeiro et al. 1997; Arcos et al. 2005). In the absence of current regular beached bird surveys in the area, there is little knowledge of the age- and sex-composition of the wintering population, and their breeding origin is unknown. Therefore, during the routine impact assessment organised in response to the Prestige spill (Heubeck et al. 2003), it was decided to pay special attention to this species.

Postmortem examinations were part of the impact assessment operation organized by SEO/BirdLife in collaboration with the Faculty of Sciences of A Coruña University. Special thanks are given to the volunteer students that kept the operation working on a daily basis. We also thank the Dirección Xeral de Conservación da Natureza (Xunta de Galicia), and especially Miguel Lorenzo, Vicente Piorno and Marta Prieto, for giving us access to the bodies deposited in rehabilitation centres. Biometrics of breeding populations were derived from skin collections in the American Museum of Natural History (courtesy of Allison V. Andors), Museum of Natural Sciences, National Museums of Canada (Henri Ouellet), Reykjavik Museum (Aevar Petersen) and Zoologisk Museum, Copenhagen (Kaj Kampp). Andrés Barreiro Lois and Ana López Beceiro from the Rof Codina Clinical Hospital kindly gave us access to their X-ray facilities. María Isabel Fraga, from the Natural History Museum Luis Iglesias of the University of Santiago de Compostela, ensured that all birds examined were retained in Galicia for further study by the scientific community. Bob McGowan and Aevar Petersen kindly reviewed the manuscript, resulting in substantial improvements.

Álvarez Laó, C. M. 1993. Censo de aves marinas invernantes en la Península Ibérica 1992. In: Gorospe, G. (ed.) Aves Marinas Ibéricas ‘92. Actas IV Congreso del GIAM 1992: 17–29. Itsas Enara Ornitologi Elkartea, Donosti.

Anonymous 2009. Piniarneq 2010; aallaaniartunut atassuteqaat pisanillu nalunaarsuiffiit. Kalaallit Nunaanni Namminersorlutik, Aalisarnermut, Piniarnermut Nunalerinermullu Aqutsisoqarfik, Nuuk. Available online http://dk.nanoq.gl/Emner/Erhverv/Erhvervsomraader/Fangst_og_Jagt/~/media/B956B6000EB34785A567EA688ACF4A96.ashx

Arcos, F. & Solís, L. 1997. 16 años de inspecciones costeras de aves orilladas en Galicia (1980–1996). Actas Primeras Jornadas Ornitológicas Cantábricas.

Arcos, F., César, X., Salvadores, R. & Mouriño, J. 2005. Plan de vigilancia ambiental de detección y evaluación de los efectos del vertido de petróleo sobre aves marinas en Galicia. Informe final, Convenio de colaboración entre la Fundación Arao y ARCEA Xestión de Recursos Naturais S.L., Vigo.

Bourne, W. R. P., Parrack, J. D. & Potts, G. R. 1967. Birds killed in the Torrey Canyon Disaster. Nature 215: 1123–1125. [Crossref]

Cadiou, B., Cam, E., Fortin, M., Monnat, J.-Y., Gélinaud, G., Cabelguen, J. & Le Roch, A. 2003a. Impact de la marée noire de l’Erika sur les oiseaux marins migrateurs : détermination de l’origine et de la structure des populations par la biométrie. Rapport Bretagne Vivante- SEPNB, DIREN Bretagne.

Cadiou, B., Chenesseau, D. & Joslain, H. 2003b. Marée noire de l’Erika - Contribution à l’étude de l’impact sur l’avifaune. Bilan national des échouages et de la mortalité des oiseaux (BNEMO). Rapport Bretagne Vivante-SEPNB, LPO Loire-Atlantique, Observatoire des marées noires, DIREN Bretagne.

Camphuysen, C. J. 2007. Gaviidae: Divers or loons. In: Camphuysen, C. J., Bao, R., Nijkamp, H. & Heubeck, M. (eds.) Handbook on Oil Impact Assessment. Technical document 4.1, Assessing the damage. Report to DG Environment, European Commission, Royal Netherlands Institute for Sea Research, Texel. Available online www.oiledwildlife.eu.

Camphuysen, C. J., Bao, R., Nijkamp, H. & Heubeck, M. (eds.) 2007. Handbook on Oil Impact Assessment. Report to DG Environment, European Commission, Royal Netherlands Institute for Sea Research, Texel. Available online www.oiledwildlife.eu.

Camphuysen, C. J. & van Franeker, J. A. 2007a. Ageing and sexing manual for stranded seabirds. In: Camphuysen, C. J., Bao, R., Nijkamp, H. & Heubeck, M. (eds.) Handbook on Oil Impact Assessment. Technical document 4.1, Assessing the damage. Report to DG Environment, European Commission, Royal Netherlands Institute for Sea Research, Texel. Available online www.oiledwildlife.eu.

Camphuysen, C. J. & van Franeker, J. A. 2007b. Condition manual: the physical condition of stranded seabirds. In: Camphuysen, C. J., Bao, R., Nijkamp, H. & Heubeck, M. (eds.) Handbook on Oil Impact Assessment. Technical document 4.1, Assessing the damage. Report to DG Environment, European Commission, Royal Netherlands Institute for Sea Research, Texel. Available online www.oiledwildlife.eu.

De Souza, J. A., Barros, A., Sandoval, A. & Bao, R. 2010. A review of the status of Great Northern Diver Gavia immer in Galicia, northwest Spain. Seabird 23: 76–90. [Crossref]

Evers, D. C. 2004. Status assessment and conservation plan for the Common Loon (Gavia immer) in North America. U.S. Fish and Wildlife Service, Hadley, MA.

Foxton, P. & Heubeck, M. 1995. Response to the Esso Bernicia oil spill. Proceedings of the Royal Society of Edinburgh 103B: 233–245. [Crossref]

García, L., Viada, C., Moreno-Opo, R., Carboneras, C., Alcalde, A. & González, F. 2003. Impacto de la marea negra del “Prestige” - sobre las aves marinas. SEO/BirdLife, Madrid.

González, J. J., Viñas, L., Franco M. A., Fumega, J., Soriano, J. A., Grueiro, G., Muniategui, S., López-Mahía, P., Prada, D., Bayona, J. M., Alzaga, R. & Albaigés J. 2006. Spatial and temporal distribution of dissolved/dispersed aromatic hydrocarbons in seawater in the area affected by the Prestige oil spill. Marine Pollution Bulletin 53: 250–259. [Crossref]

Granadeiro, J. P., Silva, M. A., Fernandes, C. & Reis, A. 1997. Beached bird surveys in Portugal 1990-1996. Ardeola 44: 9–17.
 Heubeck, M. 1997. The long-term impact of the Esso Bernicia oil spill on numbers of Common Loons Gavia immer wintering in Shetland, Scotland. In: Effects of Oil on Wildlife. Fifth International Conference, November 3-6, Monterey, California: 110–122.

Heubeck, M., Camphuysen, C. J., Bao, R., Humple, D., Sandoval, A., Cadiou, B., Bräger, S. & Thomas, T. 2003. Assessing the impact of major oil spills on seabird populations. Marine Pollution Bulletin 46: 900–902. [Crossref]

Heubeck, M. & Richardson, M. G. 1980. Bird mortality following the Esso Bernicia oil spill, Shetland, December 1978. Scottish Birds 11: 97–108. [Crossref]

Heubeck, M., Richardson, M. G., Lyster, I. H. J. & McGowan, R. Y. 1993. Post-mortem examination of Great Northern Divers Gavia immer killed by oil pollution in Shetland, 1979. Seabird 15: 53–59.

Hope Jones, P., Monnat, J.-Y., Cadbury, C. J. & Stowe, T. J. 1978. Birds oiled during the Amoco Cadiz incident - an interim report. Marine Pollution Bulletin 9: 307–310. [Crossref]

Lyngs, P. 2003. Migration and winter ranges of birds in Greenland. An analysis of ringing recoveries. Dansk Ornitologisk Forenings Tidsskrift 97: 1–168.

Monnat, J.-Y. 1978. Mortalités d’oiseaux a la suite du naufrage du pétrolier Amoco Cadiz. Penn ar Bed 11: 339–360.

Munilla, I., Díez, C. & Velando, A. 2007. Are edge bird populations doomed to extinction? A retrospective analysis of the common guillemot collapse in Iberia. Biological Conservation 137: 359–371. [Crossref]

Pombo, A. 1998. Gavia immer. In: De Souza, J. A., Martínez Lago, M., Monteagudo, A., Pérez Villa, G. & Sandoval, A. (eds.). IV Anuario das Aves de Galicia 1996. Grupo Naturalista Hábitat, A Coruña.

Sandoval, A. & De Souza, J. A. 2005. Colimbo Grande, Gavia immer. In: Madroño, A., González, C. & Atienza, J. C. (eds.). Libro Rojo de las Aves de España: 33–34. Dirección General para la Biodiversidad - SEO/BirdLife, Madrid.

SEEEC. 1998. The Environmental Impact of the Sea Empress Oil Spill. Final Report of the Sea Empress Environmental Evaluation Committee. The Stationary Office, London.

Sociedade Galega de Historia Natural 1979. Informe sobre las aves marinas afectadas por el “Andros Patria” en las costas Gallegas del 3 al 21 de Enero de 1979. Braña, Revista da Sociedade Galega de Historia Natural 1: 15–29.

Storer, R. W. 1988. Variation in the Common Loon (Gavia immer). In: Strong, P. V. (ed.) Papers from the 1987 conference on loon research and management: 54-65. North American Loon Fund, Gifford, NH.

Velando, A. & Freire, J. 2002. Population modelling of European shags (Phalacrocorax aristotelis) at their southern limit: conservation implications. Biological Conservation 107: 59–69. [Crossref]

Weir, D. N., McGowan, R. Y., Kitchener, A. C., McOrist, S. & Heubeck, M. 1996. Effects of oil spills and shooting on Great Northern Divers which winter in Scotland. Dansk Ornitologisk Forenings Tidsskrift 90: 29–33.