NOTES 
SANDWICH TERNS ON ISLA RASA, 
GULF OF CALIFORNIA, MEXICO 
ENRIQUETA VELARDE, Instituto de Ciencias Marinas y Pesquerias, Universidad 
Veracruzana, Hidalgo 617, Col. Rio Jamapa, Boca del Rio, Veracruz, Mexico, C.P. 
94290 ; enriqueta_velarde@yahoo . com . mx 
MARISOL TORDESILLAS, Alvaro Obregon 549, Col. Poblado Alfredo V. Bonfil, 
Cancun, Quintana Roo, Mexico, C.P. 77560; matordesillas@prodigy.net.mx 
In North America the Sandwich Tern ( Thalasseus sandvicensis) breeds locally 
along marine coasts and offshore islands primarily of the southeastern United States 
and Caribbean (AOU 1998). In these areas it commonly nests in dense colonies of 
the Royal Tern (T. maximus) and Laughing Gull ( Leucophaeus atricilla-, Shealer 
1999). It winters along the coasts of the Atlantic Ocean and Gulf of Mexico from 
Florida to the West Indies, more rarely as far south as southern Brazil and Uruguay. 
It also winters on the Pacific coast, mainly from Oaxaca, Mexico, south to Panama 
(Howell and Webb 1995), occasionally to Colombia, Ecuador, and Peru (AOU 1998). 
As there are no breeding colonies on the Pacific coast, all birds wintering there are 
believed to represent migrants from Atlantic and Caribbean colonies (Collins 1997, 
Hilty and Brown 1986, Ridgely 1981, Ridgely and Greenfield 2001). Sandwich Terns 
have occasionally wandered as far north as eastern Canada (New Brunswick, Nova 
Scotia, and Newfoundland) and inland to Minnesota, Michigan, and Illinois (AOU 
1998, Clapp et al. 1983). In the Pacific vagrants are known from California and the 
Hawaiian Islands (Hamilton et al. 2007, AOU 1998). 
Here we report vagrancy of Sandwich Terns to Isla Rasa, Gulf of California, Mexico, 
with our observations of single individuals in 1986 and again in 2008. Tordesillas 
noted the first on 17 May 1986, during her behavioral observations of nesting Elegant 
Terns. The Sandwich Tern was identified by being close in size to the Elegant Terns 
but having a black bill tipped with yellow. At that time, it appeared to be attending a 
nest. About an hour later we returned to the site but could not get sufficiently close 
to isolate its nest in a photograph without severely disturbing the dense Elegant Tern 
nesting colony surrounding it. On the following day we returned to the location of 
the Sandwich Tern and found only an Elegant Tern occupying the nest. The large 
size of this Elegant Tern colony, with 45,000 individuals (Tobon 1992), and the high 
densities of both the Royal and Elegant (9 nests/m 2 and 15 nests/m 2 , respectively), 
precluded us from following the nest of this apparent Sandwich Tern/Elegant Tern 
mixed pair further, to determine its eventual outcome. If the colony is approached 
terns leave the nest and the eggs are exposed to severe predation by Yellow-footed 
Gulls ( Larus livens). 
In 2008, the second Sandwich Tern at Isla Rasa was not observed directly but 
inadvertently documented, this time among the densely nesting Royal Terns (which 
numbered near 14,000 individuals, the average annual population size for the island; 
Velarde and Anderson 1993, Velarde et al. 2005). While photographing newly 
hatched Royal Tern chicks within one of the many small subcolonies on the island 
on 4 May, Velarde captured a partial image of a Sandwich Tern (Figure 1). It was 
only later that evening, while reviewing her photos, that she discovered one of her 
images included the yellow-tipped black bill, breast, and folded wing of a Sandwich 
Tern among the many Royal Terns. 
Along the Pacific coast north of Oaxaca, the Sandwich Tern has been reported 
previously only from California, where there are four records (Hamilton et al. 2007, 
ww.californiabirds.org/cbrc_book/update.pdf), all during the breeding season. It has 
230 
Western Birds 40:230-233, 2009 
