Antbird - Morphology

Morphology

The antbirds are a group of small to medium-sized passerines that range in size from the large Giant Antshrike, which measures 45 cm (18 in) and weighs 150 g (5.29 oz), to the tiny 8-cm (3 in) Pygmy Antwren, which weighs 7 g (0.25 oz). In general terms, "antshrikes" are relatively large-bodied birds, "antvireos" are medium-sized and chunky, while "antwrens" include most smaller species; "antbird" genuses can vary greatly in size. Members of this family have short rounded wings that provide good manoeuvrability when flying in dense undergrowth. The legs are large and strong, particularly in species that are obligate ant-followers. These species are well adapted to gripping vertical stems and saplings, which are more common than horizontal branches in the undergrowth, and thus the ability to grip them is an advantage for birds following swarms of army ants. The claws of these antbirds are longer than those of species that do not follow ants, and the soles of some species have projections that are tough and gripping when the foot is clenched. Tarsus length in antbirds is related to foraging strategy. Longer tarsi typically occur in genera such as the Thamnophilus antshrikes that forage by perch-gleaning (sitting and leaning forward to snatch insects from the branch), whereas shorter tarsi typically occur in those that catch prey on the wing, such as the Thamnomanes antshrikes.

Most antbirds have proportionately large, heavy bills. Several genera of antshrike have a strongly hooked tip to the bill, and all antbirds have a notch or 'tooth' at the tip of the bill which helps in holding and crushing insect prey. The two genera of bushbirds have upturned chisel-like bills.

The plumage of antbirds is soft and not brightly coloured, although it is occasionally striking. The colour palette of most species is blackish shades, whitish shades, rufous, chestnut and brown. Plumages can be uniform in colour or patterned with barring or spots. Sexual dimorphism – differences in plumage colour and pattern between males and females – is common in the family. Overall the pattern within the family is for the males to have combinations of grey, black or white plumage and the females having buff, rufous and brown colours. For example, the male Dot-winged Antwren is primarily blackish, whereas the female has rust-coloured underparts. In some genera, such as Myrmotherula, species are better distinguished by female plumage than by male. Many species of antbirds have a contrasting 'patch' of white (sometimes other colours) feathers on the back (known as interscapular patches), shoulder or underwing. This is usually concealed by the darker feathers on the back but when the bird is excited or alarmed these feathers can be raised to flash the white patch. Dot-winged Antwrens puff out white back patches, whereas in Bluish-slate Antshrikes and White-flanked Antwrens the white patch is on the shoulder.