This year-long study (October 2002–September 2003) documented the seasonal occurrence of Common Terns Sterna hirundo and ‘Cayenne Terns’ S. sandvicensis eurygnathus at the Coroa do Avião, a barrier islet located in an estuarine complex of the State of Pernambuco in northeast Brazil. Common Terns were present in the area almost all year round, except in December, February and March, with peak abundance in May–July. The islet is used by Common Terns as a stopover area during migration to and from their main wintering grounds in southern South America. ‘Cayenne Terns’ occurred mainly from August to October, being absent from December to April. In Brazil the ‘Cayenne Tern’ breeds from April to October and most birds at the islet were likely to have been individuals dispersing from colonies in southeast Brazil.

In the New World, the Common Tern Sterna hirundo breeds in central and eastern Canada and northern USA but spends the non-breeding period in South America (Gochfeld & Burger 1996). The ‘Cayenne Tern’ S. sandvicensis eurygnathus breeds in the southern Caribbean Sea and along the Atlantic coast of South America (Gochfeld & Burger 1996; Yorio & Efe 2008). The ‘Cayenne Tern’ was formerly treated as a distinct species, but it is now generally regarded as a subspecies of the Sandwich Tern S. sandvicensis (Gochfeld & Burger 1996), because hybridisation between both taxa has been recorded in the Caribbean.

A recent study by Efe et al. (2009) based on genetic evidence, split ‘Eurasian’ and ‘American’ Sandwich Terns into two separate species, S. sandivicensis and S. acuflavidus respectively, and synonymised the ‘Cayenne Tern’ with the ‘North American Sandwich Tern’ because they are genetically very similar, even though the two taxa are morpho- logically distinct. A detailed comment on the taxonomy proposed by Efe et al. (2009) is currently in preparation and for the purposes of this paper ‘Cayenne Tern’ continues to be treated as subspecifically distinct from the ‘American Sandwich Tern’.

Few studies focused on seabirds have been conducted on the coast of northeast Brazil between the States of Ceará and Sergipe (Lat. 2–10oS; Figure 1). Available data are mainly from ring recoveries, opportunistic non-systematic observations, and lists of species (e.g. Azevedo-Júnior et al. 2001; Sousa et al. 2005; Girão et al. 2008). In a three-year study of migratory birds on the northern coast of Pernambuco, Telino-Júnior et al. (2003) reported the occurrence of Common and ‘Cayenne’ Terns but not their seasonal numbers. In this paper we present the results of a year’s study of the seasonal occurrence of Common and ‘Cayenne’ Terns in a tropical estuarine complex of the State of Pernambuco in northeast Brazil.

The authors thank two anonymous referees and the Editor for commenting constructively on earlier versions of the manuscript. In 2002–03 CEF was supported by a scholarship of the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil.

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