On 7 August 2012 two equivocal terns were seen and snapped at Llerena Reservoir, also called Arroyoconejos Reservoir, in Badajoz, by Francisco Montaño, Joaquín Vázquez and Benjamín Muñoz. The on-the-spot observers identified them originally as Royal Terns (Sterna maxima) but many pundits, after chewing over the photos, now plump for Lesser Crested Terns (Sterna bengalensis). Whichever, it's a first for Extremadura, since neither of the two species has ever been seen here before.
The support for Lesser Crested Tern are: size similar to neighbouring Black Headed Gulls (Larus ridibundus), the size of the terns and their wings in relation to the background Cattle Egrets and one of the birds has a completely back cap goes against the Royal Tern option. Apparently they tend to lose this feature very early in spring and very few birds hang onto the black cap up to June, never mind August when the photo was taken. Neither are the subtle colour differences between the two species sufficiently clear in the photograph.
It wouldn't be amiss now to fill in a few details about both species.
The Royal Tern is a tropical species that breeds in America (maxima subspecies) and Western Africa (albididorsalis subspecies), in the latter case in colonies in Mauritania, Senegal and Gambia. These birds tend to spread north as far as Morocco in post-breeding dispersal and it is thought to be this population that occasionally overshoots and turns up in Spain, where there have been 21 accepted records involving 26 birds up to 2009 (though there were none from 2007 to 2009). The bulk are seen in Andalusia, in provinces close to the Strait of Gibraltar, from July to November (although sightings range from April to December). In Europe, however, there have been at least two records of birds ringed in the US, one in the UK and another in Catalunya (in an unusual month: December).
For its part the Lesser Crested Tern breeds on the Red Sea, Indian Ocean and Oceania, plus a small Mediterranean population (Libya), which migrates through the Strait of Gibraltar. It turns up in Spain more regularly than the Royal Tern. Since it has been classed as a rarity only since 2006, the species has not been analysed by De Juana (2006). Furthermore it is a rarity only in mainland Spain. On the other side of the Strait in Ceuta it is considered to be habitual. Even so 15 records of 21 birds have been accepted in four years (2006-2009), mostly in Andalusia around the Strait. The actual number is likely to be much higher, especially in recent years when more people have been on the lookout for the species.
Sources:
- De Juana, E. 2006. Aves raras de España. Lynx Edicions. Barcelona.
- CR-SEO (Comité de Rarezas de SEO). 2011. Observaciones de aves raras en España, 2008. Ardeola 58(2).
