Svasso cornuto

Identificazione

Horned grebes can be easily recognised by its red-and-black alternate (breeding) plumage, its black-and-white basic (non-breeding) plumage, and its characteristic "horns". It is 31–38 cm (12–15 in) long, has a wingspan 55–74 cm (22–29 in) wide and weighs 300–570 g (11–20 oz). It has a moderately long neck, flat forehead and a rear crown of black feathers. Its beak is straight and pointy with a white tip. Both subspecies are physically similar, with P. a. auritus (Eurasia) appearing darker than P. a. cornutus (North America), which has light grey feathers on its back, which are inconspicuous or absent in P. a. auritus. The horned grebe is often confused with the black-necked grebe, which is similar in size and colouring but differentiates by a steeper forehead, a more slender bill and a fluffier rump.<p>The alternate (breeding) plumage of the horned grebe has bright erectable "horns", black fan-shaped cheek feathers and an overall red-and-black colour. The neck, flanks, lores and upper-chest are chestnut brown, while the crown and back are black. The belly is a dull white. Males are slightly larger and brighter than females but are generally indistinguishable.<p>The basic (non-breeding) plumage is overall black and white. The neck, chest and cheeks are white, while the back and crown are a dull black-grey. The border between the crown and the cheeks extend in a straight line behind the eyes. The basic plumage does not have the "horns".<p>Juveniles appear similar to a non-breeding adult except they are a slightly duller shade of white and their back is tinged with brown. The line separating the cheeks and crown is less distinct and their beak is paler. The chicks are fluffy, with a dull grey back, a white belly and distinct black-and-white facial and neck striping.

Tassonomia

A handful of fossil species, such as the Pliocene aged P. howardae, P. pisanus and P. solidus, and the Pleistocene aged P. dixi have been described to be related to or perhaps some of them might be pre-Holocene material of the horned grebe. However while P. howardae and P. dixi are regularly recognized as fossil material of the horned grebe by some authors, P. pisanus and P. solidus are argued to be valid species that are close to the ancestry of the horned grebe.

Nome scientifico

Podiceps auritus

Autorità

(Linnaeus, 1758)

Ordine

Podicipediformes

Famiglia

Podicipedidi

Genere

Podiceps

Areale e habitat

The horned grebe is distributed in Eurasia and North America. In Eurasia, it breeds in a few isolated locations in Greenland (rare), Iceland, Scotland and Norway, while extensively from Sweden to the Russian Far East. In Europe, it winters along the coasts of Iceland, Norway and the British Isles down towards the Mediterranean, the Black Sea and the Caspian Sea. In eastern Asia, the horned grebe winters along the coasts of China, Korea and Japan.<p>In North America, its breeding distribution is restricted to the northwestern region of the continent, with 92% located in Canada. The total North American breeding range spans from south-central Alaska to northwestern Ontario. It breeds as far north as the Yukon and southern Nunavut to northwestern states, from Washington to Minnesota. Additionally, there is a small population who breed annually on the Magdalene Islands of Quebec. Its wintering range is also primarily coastal from southern Alaska down to the northern Gulf of California. Its eastern overwintering range is from southern Nova Scotia, down to the Florida Keys and sometimes west to Texas.<p>Horned grebes breed primarily in temperate zones, including prairies and parklands, but are also seen in boreal and subarctic regions. They breed in small to moderately sized (0.5-10 ha) shallow freshwater ponds, marshes and shallow bays on lake edges with beds of emergent vegetation. They prefer areas with sedgess, rushes, and cattails along with large areas of open water. This habitat provides a suitable site for nest material, anchorage, concealment and protection for young.<p>During migration, they will stop along lakes, rivers and marshes. Following migration, they winter in marine environments of estuaries and bays or inland on large lakes, although in some places, for example in Norway, large concentrations congregate on inland lakes.

Areale riproduttivo

NA, PAL : Iceland, Scandinavia to e Russia; s, c Alaska, w, c Canada and n tier of w, c contiguous USA states

Areale non riproduttivo

to coasts of North Sea, n Mediterranean and w Asian seas; Korean Pen. and Japan; Aleutian Is. to n Baja California (nw Mexico); w North Atlantic and Gulf of Mexico coasts to c Florida

Migrazione

Complete migrants

Comportamento

Young begin calling for begging purposes in a slightly trilled peeping noise, similar to that of a domestic chick. As they mature, their song changes to a more adult-like chittering. Their typical advertising call is loud and nasally "aaarrh" descending in a pitch and ending in a trill. They use other calls during copulation, alarm and breeding ceremonies that are slightly variable from the advertising call. Horned grebes are extremely vocal during breeding, territory establishment and defence. Their song is subdued during autumn migration and at wintering sites.<p>Horned grebes dive underwater using their large feet for agile manoeuvrability to feed on aquatic arthropods, fish and crustaceans. They will also catch airborne insects on the water's surface. Underwater they swallow or capture large prey and re-emerging at the surface to manipulate the fish headfirst. They usually feed solitarily or in small groups of up to five individuals. During the summer, aquatic and airborne arthropods are preferred, whereas winter selection favours fish and crustaceans.<p>The horned grebe has a unique adaptation for swallowing fish whole. They will eat their own feathers from a young age, so that their stomach has a matted plug that functions as a filter to hold fish bones until digestion.<p>Horned grebes are monogamous and develop their relationship through elaborate mating routines. There are four pair bonding ceremonies; discovery ceremony, weed ceremony, head-shaking ceremony and triumph ceremony. The discovery ceremony begins with advertising displays, which include an upright posture, erect "horns" and sounding of their advertising call. Then, both male and female engage in bouts of penguin dance and preening. This initial pair bonding ceremony is to ensure correct species identification, sex and compatibility. The weed ceremony follows the completion of a successful discovery ceremony. The male and female will dive, retrieve weeds and rise in synchronisation. The pair will come breast-to-breast with their weeds then turn side by side to continue swimming. This weed-rush can continue multiple times until both individuals are satisfied. Finally, the head-shaking ceremony and triumph ceremony are performed for primarily established pairs. Once copulation is to take place, it always occurs on a platform nest built by the pair.<p>Horned grebes usually arrive at breeding grounds in pairs or solitarily to seek out a mate in the spring or early summer. A pair may nest entirely alone or in a loose colony that typically contains approximately 20 breeding pairs, each pair nesting some distance from the others. During nesting, horned grebes are known to defend their nests very aggressively. Nests are built from plant matter and are most commonly affixed to emergent vegetation otherwise built on land or in shallow open water. Depending on location the eggs are laid between April and September, with June being the most common month. The females lays a single clutch of three to eight eggs, which are coloured white, brownish or blueish green. These eggs measure 58 by 39 millimetres (2.3 by 1.5 in) on average. Both males and females share incubation time for 22 to 25 days. When the young hatch, they can swim and dive within the first few days, although they must be kept warm by their parents for up to 14 days. During this time, the juvenile chicks can often be seen riding on the backs of their swimming parents right in-between the wing and the back. Later the horned grebe will take their first flight at 55–60 days old. The species finally reaches sexual maturity at 2 years old.

Conservazione

Tendenza della popolazione

declining