Bubù tropicale

Identificazione

The tropical boubou is fairly large for its family (bushshrikes), measuring 23–25 cm in length. Its weight can vary between about 38-70 g, but typically adults weigh between 50 and 60 g. Females are on average a bit smaller than males, but individual variation is so large that for most practical purposes the sexes seem to be of identical size. The wing measures about 98 mm on average (between 85 and 110), the tail is a mere two millimetres longer both on average and as regards variation. The tarsus is 29–37 mm long, 33 mm on average, while the bill's exposed culmen measures a little over 20 mm in some populations and as much as 25 mm in others. Unlike the other measurements where there is mostly individual variation, the bill length might distinctly vary between subspecies.<p>Not only are males and females the same size, they have identical plumage too. The adults' upper parts and tail are glossy blue-black except for concealed white spots on the rump, visible only when the wings are spread and the rump feathers are erected. The underparts are white, in some populations with a buffy or pinkish tinge on the breast and flanks, which is not always noticeable except in good light. The wings of most subspecies have a white stripe on the wing coverts, in some extending onto the secondary remiges. The tips of the outer tail feathers can be white in some subspecies. The bill is black, the legs and feet bluish grey. The irides are dark reddish brown.<p>Nestlings have pinkish-brown skin and are nude after hatching, later growing sparse down; they apparently have spots inside their bills which they show their parents to get fed. Fledglings are similar to adults but duller, with the brownish head and the upper parts appearing mottled due to yellowish-ochre to tawny feather tips, forming a barring on the lower back and rump. The wing-stripe, if present, is dulled down. The undertail coverts are buff, and there is usually dusky barring on the dull white underside, especially on the flanks. The bill is greyish brown above, paler below. Immature birds independent from their parents have lost most of the mottling (except on the wing coverts) and barring, but their wing stripe and underside are still dull white, shading to brown on the flanks; any white tail feather tips appear at this stage.<p>The northwesternmost populations of major might be confused with Turati's boubou (L. turatii), which has no white wing-band and a buff chin and white belly, while major around the southern edge of the Congo Basin look almost identical to the swamp boubou (L. bicolor), but this is pure white below and its females give a characteristic "ratchet" call in duets. Southeastern birds (mossambicus and especially limpopoensis) resemble the southern boubou (L. ferrugineus) but are black above instead of dark brown and grey – though this may be hard to see at a distance and entirely indistinguishable in the occasionally seen hybrid. When the tropical boubou is sympatric with other Laniarius, they are rarely found in the same habitat.<p>This species is heard more often than seen, but its calls often help to locate it visually. To vocalize they move higher off the ground than during their usual activities, and may perch on an exposed site. They also nod their head and bow their body when calling, making them even conspicuous sometimes. Calls like bou, hou or boubou give the bird its name; they may be extended into a bubbling bobobobo and are given loudly and higher-pitched to announce the birds' presence, or lower and softer by foraging groups to maintain contact. Like many bush-shrikes, it has a wide vocal repertoire that includes duets in which two individuals – a male-female pair or two males with adjacent territories – give notes alternately in so rapid a sequence that they sound like one bird. As the voicebox of birds is a syrinx not a larynx, in particular many Passeri can sing in two voices at the same time; tropical boubous that have lost their mate can make the same sequence that a pair would make. Males probably start most duets, and their notes are mostly low-pitched whistles and/or harsh croaks; females' notes are typically higher whistles and/or "harsh tearing or rattling sounds". But although the birds' vocalizations are somewhat harsh, they are still able of a wide range of frequencies and males provide the higher voice in certain duets.<p>Duets usually consist of one exchange, two or three calls in total. But up to seven exchanges have been recorded. A duet may be repeated up to 75 times, and in experiments where the birds were confronted with taped vocalizations, they could be enticed to up to 200 duets. A dozen or more duet types exist, and some seem to confer specific information, forming a Morse code-like sort of language. Examples of typical duet sequences include hoooooo-ho-ho, hoho-u-ho, hoo-hii-hoo, haw-Weeer-haw, hoou-Weer-hoou, houhou-Weeer and bobobobo-Weeer. During territorial contests of subspecies major, a wide range of duet types is sung, in particular 1–3 and 9–11. Playback of duet types 1, 2, 9 and perhaps 6 elicits duets of the same type, as well as of others; types 2 and 6 are typically also answered with types 9 and 11, and type 9 with type 3 and perhaps 11.<p>Duet type 5 of major consists of one to several dozen (typically about 3) short call notes given simultaneously by territorial pairs. Its loud male calls are at about 1.4 kHz, 0.4 seconds and downslurred, while the female calls increase in frequency between 900–1,000 hertz, are less than half as long, and begin about halfway through the males' calls. After a pair has prevailed in a song contest with an intruder, the birds quieten down for some time and ascertain that the contestant has called it quits; then, they usually move to a prominent perch, the male at the very top with his mate below, and sing duet type 5. This vocalization might be considered a sort of "victory chant", conferring information about the events to neighboring pairs.<p>Northern birds typically give resonant hooo or hoou whistles, variously short or drawn-out. Among southern birds, grating Weeer and krzzzz calls, snoring Haaw croaks and rattling Ke-Ke-Ke are more common. Other harsh calls, like a metallic Tschanananana or SCHRANG! SCHRANG! are given by the male in aggression and courtship; eastern birds may also give call lit-tuu-iii at the end of a courtship display. Alarm calls are a variety of chattering outbursts; before roosting, birds often make a series of Tuk calls, and they also give a call when the parents change to brood the eggs or young. During social interactions, the wings are often forcefully shaken, producing a ripping mechanical sound.<p>This species may duet with the slate-colored boubou (L. funebris). Trios are common, especially young birds learning to sing tend to join in when a pair duets. Nestlings that want to be fed give high repeated chirps, as usual among passerines.

Tassonomia

Traditionally, seven subspecies of the tropical boubou were recognized – that is, if L. aethiopicus was considered a distinct species at all. As late as the mid-20th century for example, some authors included the entire tropical boubou complex within L. ferrugineus, which nowadays refers to the southern boubou only.<p>Recent cladistic analysis of nDNA BRM15 intron-15 and mtDNA NADH dehydrogenase subunit 2 and ATP synthase F0 subunit 6 sequence data indicates that the tropical boubou was traditionally recognized is a polyphyletic cryptic species complex, and that three additional species have been recognized, the Ethiopian boubou, the black boubou, and the East Coast boubou.<p>Contrary to what one might expect in a purely phenetic analysis, the conspicuous variation of wing stripes is no good indicator of relationships among these and related boubous. Vocalizations and habitat preferences, on the other hand, allow a good delimitation of the clades conventionally grouped as tropical boubou. The four proposed species are:<p>Current subspecies of the tropical boubou now include:

Nome scientifico

Laniarius major

Autorità

(Hartlaub, 1848)

Ordine

Passeriformi

Famiglia

Malaconotidi

Genere

Laniarius