Abstract
We review existing data on the distribution of the Least Tern, Sternula antillarum along the Brazilian coast, based on the literature and museum specimens, and present results of a year-long study (October 2008 to September 2009) on the seasonal abundance of this species on a large tidal flat area, Cajuais Bank, in the State of Ceara, north-eastern Brazil. We evaluate whether the observed variation in the abundance of terns is due to the occurrence of an undocumented breeding colony, or alternatively, whether it results from an influx of migrants from the Northern Hemisphere. The recovery of historical data revealed that all literature references on the distribution of birds in the Americas, published up to the late 1990s include Brazil in the non-breeding range of the Least Tern. This inclusion is based on a few, old (late 19th and early 20th centuries) museum specimens, all of which have been collected on the northern and north-eastern coasts of this country'. From the late 1980s, birds continued to be occasionally recorded along the coastline, running from the State of Amapa (01°N) up to the State of Bahia (10°S), with records of single individuals in south-eastern and southern Brazil. An alleged record from Rocas Atoll, 260 km off the Brazilian mainland, might tentatively refer to the Old World Little Tern S. albifrons. At Cajuais Bank, Least Terns occurred from October 2008 to April 2009, and in September 2009. The highest numbers (> 800 individuals) were recorded in January-February (Southern Hemisphere's summer). The species was observed in rather small numbers (< 30) in March-April (early-mid Southern Hemisphere fall) and in September (early Southern Hemisphere spring), being absent from the area in May-August (Southern Hemisphere's fall and winter). In October-December 2008 (Southern Hemisphere's spring), and September 2009 (early Southern Hemisphere's summer), no birds were in breeding plumage. In January, about 32% of the birds were in breeding plumage, while from February-April (Southern Hemisphere's summer to mid fall) all were in breeding plumage. This pattern is consistent with what would be expected for migratory birds. The Cajuais Bank itself (and adjacent beaches and inland environments) holds the largest numbers of Least Terns recorded in Brazil, thus being one of the most significant sites along the migratory route of the species.
Highlights
The Least Tern, Sternula antillarum Lesson, 1847, is a small seabird that breeds along the southern coasts and major interior rivers of the USA, coasts of Mexico, the Caribbean and the islands off northern South America
The Least Tern has been well-studied on its breeding grounds in the USA, little information other than distribution records is available for this species in Brazil
Reference works on the distribution of birds in the Americas published up to the late 1990s (e.g., MURPHY 1936, HELLMAYR & CONOVER 1948, SICK, 1985, 1997) all included Brazil in the non-breeding range of the Least Tern
Summary
The Least Tern, Sternula antillarum Lesson, 1847, is a small seabird (total length: 210-230 mm; body mass: 30-45 g) that breeds along the southern coasts and major interior rivers of the USA, coasts of Mexico, the Caribbean and the islands off northern South America. A colony of 60-70 pairs was found breeding on an estuarine island in the State of Maranhão, northern Brazil, during May-July 2008 (RODRIGUES et al 2010) This information promptly led to changing the status of the Least Tern in the most recent check-list of Brazilian birds, from ‘visitor’ to ‘resident’ species (CBRO 2010). The Least Tern has been well-studied on its breeding grounds in the USA (see GOCHFELD & BURGER 1996, THOMPSON et al 1997), little information other than distribution records is available for this species in Brazil. We were interested in evaluating whether the observed variation in the abundance of terns was consistent with the occurrence of a breeding colony nearby, or whether it resulted from movements of migrants from the Northern Hemisphere
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