LARIDAE Gulls and Terns FAUNA PARAGUAY

LARIDAE - GULLS AND TERNS

Ten species in this global family reach Paraguay. Though generally marine in habits, some species have adapted to freshwater habitats. Two species of tern occur regularly in Paraguay, the remaining eight species in the family, including all the gulls, are considered vagrant and known from very few records. The Gulls and Terns have sometimes been separated into two families Larinae and Sterninae respectively, but here they are considered subfamilies. All species are to some degree diurnal and colonial with monogamous pair bonds. They are long-lived. Ageing and moult of species in this family is a complex issue, with many species taking several years to attain full adult plumage. An excellent introduction to the subject is given un Jaramillo A (2003) Birds of Chile p124-125. The characteristics of the subfamilies are summarised below.

Subfamily Larinae "Gulls"
The three Paraguayan species are all considered vagrant and belong to the "Masked Gull" subgroup. Gulls in this subgroup are characterised by their dark hoods, dark underside of the primaries and a conspicuous white triangle on the leading edge of the wing. They are medium-sized with long, narrow wings. Like all gulls they have 10 functional primaries and an 11th vestigial primary. Flight is buoyant with shallow wing beats and much soaring and gliding. The tail is square with 12 rectrices and about 40% of the wing length. Gulls have strong bills, slightly-hooked with a tapering nail and steep gonys. The Paraguayan species have comparatively slender bills compared with many others in the family. The tarsus is long and slender, with three webbed front toes and a much reduced, raised hallux. Gulls frequently swim. In general plumage consists of a combination of greys, black and white and with brightly-coloured bill and legs. Wing pattern is frequently important for identification purposes. Adults undergo a complete annual moult after breeding and a partial moult of head and body feathers prior to breeding. The three Paraguayan species are two-year gulls, reaching adult plumage in two years. Females are smaller than males.

Subfamily Sterninae "Terns"
Seven species of superficially gull-like waterbirds. They can be distinguished by their more lightweight, slender and graceful jizz. Terns have long, pointed wings, short legs and a deeply forked tail. The tarsus is weak, the feet are webbed and the reduced hallux is raised. Terns rarely or never swim. Flight is strong, elegant and buoyant with rapid, deep wing beats. Terns never glide or soar. Foraging technique generally consists of diving from the air, some species hovering briefly before doing so. In most species the bill is slender and tapering, but it is thicker in Phaetusa and Gelochelidon (the latter aptly-named "Gull-billed Tern"). Two species are resident but some species are capable or remarkable migrations, that of the Arctic Tern taking it from pole to pole, a round trip of almost 25,000km - the longest of any bird. Typical tern plumage consists of grey wings, white underparts and a black hood, that is shed in many species during the non-breeding season. They nest in unlined hollows on sandbanks. Young are precocial.

REFERENCES
Campbell B & Lack E 1985 - A Dictionary of Birds - T & AD Poyser.
Jaramillo A 2003 - Birds of Chile - Princeton University Press.
Snow D & Perrins C (eds.) 1998 - The Birds of the Western Palearctic Concise Edition: Volume 1 Non-Passerines - Oxford University Press.