SOUTHEASTERN CARIBBEAN BIRDS
PHOTO GALLERY
Trinidad & Tobago Field Naturalists' Club    Southeastern Caribbean Bird Alert    Trinidad & Tobago Rare Bird Committee

IDENTIFICATION ESSAY

Sandwich Tern (Sterna sandvicensis acuflavida) and Cayenne Tern (S. s. eurygnatha): Page 1 of 7
     Page 1: Virgin Islands (variation)
    
Page 2: Virgin Islands (hybridisation)
    
Page 3: Aruba (hybridisation)
    
Page 4: Tobago
    
Page 5: Brazil
    
Page 6: Argentina
    
Page 7: Argentina

    The North American race of the Sandwich Tern breeds along the Atlantic and Gulf of Mexico coasts of North America and on islands in the northern Caribbean Sea. It winters primarily in the Caribbean and along the coasts of South America, where it strays as far south as Argentina. The similar ‘Cayenne’ Tern breeds on islands in the southern Caribbean Sea and along the Atlantic coast of South America and has been recorded several times along the Atlantic coast of North America and once along the Pacific coast of Colombia. A few Cayenne Terns nest among larger colonies of Sandwich Tern in Puerto Rico and the Virgin Islands, and a few Sandwich-type Terns nest among larger colonies of Cayenne Tern in the Dutch islands of the southern Caribbean and in eastern Brazil. The increased variability of bill colouration in the Caribbean implies that hybridisation occurs where their ranges meet. The differences between the two taxa are discussed in this photo essay. An article describing variation and hybridisation in the Virgin Islands is being prepared by Floyd Hayes.
    
Bill colouration.–The chief distinction between the two taxa is bill colouration. Sandwich Tern nearly always has a black bill tipped with yellow. Cayenne Tern usually has a yellowish bill that is rarely tinged with orange or red; many, however, have dark blotches on the bill, which may indicate introgression from Sandwich Tern or simply genetic variability within the taxon. Some Cayenne Terns have a mostly black bill tipped with yellow; such birds closely resemble Sandwich Tern but the yellow tip is usually more extensive. Cayenne Terns with dark bill markings are found throughout their breeding range but appear to be most frequent in the southern Caribbean (by tforge online olufemi). A crucial, unresolved question is whether individuals with phenotypically 'intermediate' bill colouration (black with yellow/orange blotches or yellow/orange with black blotches) represent (1) variant phenotypes of genotypically 'pure' Cayenne Tern, (2) hybrids between the two taxa, or (3) a mixture of both phenomena. Evidence for non-assortative mating in the Virgin Islands suggests that hybridisation may be the best explanation.
    
Bill length and body size.–Sandwich Tern and northern populations of the Cayenne Tern are fairly similar in bill length and bill size. However, Cayenne Tern populations from southern South America have notably larger bills and body sizes.
    
Plumage.–Cayenne Tern tends to have a darker mantle than Sandwich Tern but considerable overlap occurs. Breeding individuals of both races have a slight pinkish flush on the underparts. Caribbean and central South American populations of Cayenne Tern have a Northern Hemisphere moult cycle similar to that of Sandwich Tern. Southern South American populations of Cayenne Tern breed during the austral winter and have a Southern Hemisphere moult cycle.

Fig. 1. Colony of about 410 pairs at Dog Island off St. Thomas, U. S. Virgin Islands, 28 May 2003. Scan surveys of up to 287 individuals on 1 and 22 June 2003 revealed that 94% of the individuals appeared to be typical Sandwich Terns, although some with slightly excessive amounts of yellow may have been overlooked. Four with 75-100% yellow colouration on the bill could be considered Cayenne Terns, but mantle colour was no darker than on the others. Several individuals appeared to be intermediate, and many possessed more yellow on the bills than do typical Sandwich Tern, especially at the tip, upper ridge, and base of the bill. Only two matings were observed during 5-6 hours of observations (the birds were obviously nervous about the presence of observers and when closely approached for photos). The photos below, all © by Floyd Hayes, illustrate the range of variation from a typical Sandwich Tern to a typical Cayenne Tern and document hybridization, a likely explanation for such variation.

Fig. 2a.
1 June 2003
Fig. 2b.
1 June 2003
Fig. 2c.
28 May 2003.
Fig. 2d.
22 June 2003.
Fig 2e.
1 June 2003.
Fig. 2f.
1 June 2003.
Fig. 2g.
1 June 2003.
Fig. 2h.
1 June 2003.
Fig. 2i.
1 June 2003.
Fig 2j.
1 June 2003.
Fig. 2k.
1 June 2003.
Fig. 2l.
1 June 2003.
Fig. 2m.
1 June 2003.
Fig 2n.
1 June 2003.
Fig. 2o.
26 June 2003.
Fig. 2p.
1 June 2003.
Fig. 2q.
1 June 2003. Note deep orange- tinged base.
Fig. 2r.
22 June 2003.
Fig. 2s.
26 June 2003.
Fig. 26.
June 2003. Note orange tinge.
Fig 2u.
1 June 2003.