PICIDAE WOODPECKERS FAUNA PARAGUAY

PICIDAE - WOODPECKERS
Twenty-one species occur in Paraguay, classified into two subfamilies and four tribes (see below). Very few fossil woodpeckers are known, but they are clearly a very ancient lineage. The oldest known fossils date from 25 million years ago, but holes drilled in petrified wood suggest that they may have existed in the Americas as long as 45 million years ago. DNA-DNA hybridisation data indicates that woodpeckers may have diverged 50 to 53 million years ago, making them amongst the oldest known avian groups.
Woodpeckers range in size from the tiny Piculets (Picumnus) to the gigantic Ivorybills (Campephilus). Their basic morphology is built around a design for excavating, probing and tree-climbing, though some species have diverged from the stereotypical woodpecker lifestyle. The bill is strong and straight, sometimes with a slightly curved culmen, and the skull is reinforced. The tail is stiffened and foot modified for climbing, whilst the tongue is extremely long, to assist with probing.
The pecking action wears off the horny tip of the bill leaving a chisel-shaped tip. As an adaptation to the incredible force generated by pecking, the skull is highly-specialised where it meets the bill. A hinge between the maxilla and the frontal part of the skull is folded inwards, preventing the bill from violently snapping open, whilst a special shock-absorbing muscle helps absorb the tension. Furthermore the head moves in a straight line as it hammers so that the force is generated in a single, constant plane. Typically the head is small and the ear also shows modifications associated with shock absorption. The window of the inner ear is half the size of that of other birds, the membrane being thickened at one end and the columella with two basal plates instead of the usual one. The neck muscle M. longus colli is associated with hammering and vertebrae 10 to 15 have elongated hypapophyses which act as insertion points for it. Vertebrae 6 to 10 have fused sublateral processes which form guiding channels for the tendons associated with the muscle. In general the nostrils are covered by protective feathers. As a general rule the nostrils are located closer to the culmen in ground-foraging species and further apart in arboreal, hammering species. The tongue is extremely long and can be extended, with movements of the specially modified tip being consciously controlled. Sublingual glands produce a sticky fluid that lubricates the tongue. The head musculature is unique with muscles sheathing the elongated hyoid bone to provide stiffness. The long bones are wound around the back of the skull and meet in the frontal region. Three other large glands are also present in the head, including a large maxillary gland characteristic of the family. The nasal gland is located close to the nasal cavity in most species except the Picumninae (see below). The largest gland of all is the mandibular. Some species have a strong odour of unknown origin, but it may be caused by secretions from the uropygial gland, or else be present on the feathers themselves.
The legs are short with strong feet and powerful, curved claws. The tarsus and toes are covered with overlapping scales. The foot is zygodactyl, but when climbing the fourth toe is extended perpendicular to the rest of the foot, giving it a different arrangement. The hallux (1st toe) is much reduced. The ribs tend to wider in species that hammer. With the exception of the Picumninae the tail is stiffened and helps support the bird when climbing, usually being curved ventrally and barbed at the tip. The pygostyle bone and caudal vertebrae are usually enlarged allowing insertion of the powerful tail muscles. There are 6 pairs of tail feathers, the outermost reduced and the central pair with pointed tips, being reinforced by longitudinal ridges and concave vanes. The feet provide the pull and thrust when climbing, the tail acts to keep the body clear of the substrate. Woodpeckers climb by hopping up vertical trunks. The power stroke of the hop is given with the bill facing upwards and tail lifted only at the very last minute, coupled with the feet being brought very rapidly forwards to grip the trunk and the tail bracing the impact. The bird is momentarily suspended in mid-air. The thickened skin of woodpeckers has been commented upon by taxidermists, but its purpose is uncertain. Typical body feathers do not deviate much from the avian norm, however the scimitar-shaped basal cells of the barbules of the contour feathers are unique to the family and there is a very short aftershaft. Chicks hatch naked and there is no downy phase, the contour feathers begin to grow directly. The pterylosis shows a characteristic naked area along the crest of the head and circular areas of bare skin on the temples. Adult moult occurs after the nesting season. In general there are two centres of secondary moult on the 1st and 8th feathers. Tail specialisation necessitates a variation in the usual moult sequence and the central pair is moulted last. Tail moult begins at the 2nd innermost pair of feathers and proceeds outwards. The Picumninae are unique amongst woodpeckers in that their moult is irregular. Flight is strong and usually undulating, consisting of a series of wing beats interspersed with closed-winged glides.
Woodpeckers are found mainly in forested or wooded environments, though the Colaptes flickers have colonised more open areas. A certain amount of dead wood is usually necessary for habitat to be suitable, especially for larger species. Many species are found in pairs which communicate through drumming, some species are social (Melanerpes, Colaptes camperstris) and a few species are more solitary. They are territorial and show excellent awareness of the distribution of resources within their territory. Piculets and smaller woodpeckers frequently join mixed species flocks, larger species are less prone to do so, and when they do they are generally on the periphery of smaller flocks of larger species. All the Paraguayan species are sedentary residents. Diurnal behaviour follows a well-rehearsed daily routine. Roosting takes place in holes and is in part responsible for the high survival rates of adults. Woodpeckers preen frequently and engage in other plumage maintenance behaviours such as sun-bathing and water bathing. They scratch the head and neck by both the direct and indirect methods and the bill is wiped clean with side to side head movements against a trunk. Resting woodpeckers frequently fluff up their plumage and appear neckless. Experiments have revealed woodpeckers to be highly intelligent and capable of complex learning. Vocalisations are not learned, the usually being simple and harsh, with songs frequently having a rattling quality. Each species possesses a repertoire of five or more call types. Drumming is used as a signalling, not foraging technique and is most often heard during the morning. Such instrumental signalling techniques are unique in the avian world. Social species are most vocal.
Picids feed mainly on insects and their larvae, though some species are specialised sap feeders (Melanerpini) and may take large quantities of fruit. Some genera consume large numbers of ants (Celeus, Colaptes and Dryocopus). The bill is used to hammer holes in wood and to chip off bark to obtain food. Woodpeckers breed in holes carved into wood or termite mounds, the nest chamber being pear-shaped and lined only with wood chippings. Pairs form monogamous bonds that may last for several seasons and it is the male that excavates the nest. Both sexes contribute to incubation and care of young. Pairs copulate frequently during the nesting and laying period. Eggs are white and are laid at daily intervals. Females lay according to a maximum "programmed" clutch size, and can be experimentally induced to lay almost indefinitely by removing eggs. Incubation begins when the final egg is laid and is remarkably short at 9 to 14 days. Unhatched eggs and eggshells are removed from the nest. The nestling period is long at 3 to 4 weeks, all of which contributes to a high nesting success of greater than 70%. Sexual maturity is reached at the end of the 1st year of life, though it is delayed in social species.

The taxonomy of the Paraguayan species is as follows:

Subfamily Picumninae
Represented in Paraguay by three species in the tribe Picumnini.

Tribe Picumnini
Picumnus "Piculets" 3 species - Tiny woodpeckers with a short, soft tail with only slightly stiffened feathers. Three white tail stripes in unusual tail pattern. The plumage is soft and dull, with speckled crown and barring on the underparts. The long tongue has a rounded tip with fine bristles. Bill is short, laterally compressed and pointed with the nostrils covered by feathers. Nasal gland enters the orbit. Moult is irregular. Probably New World origin. Sexual differences defined by crown pattern.

Subfamily Picinae
Eighteen species in three tribes in Paraguay.

Tribe Melanerpini
Melanerpes "Sapsuckers" 3 species - Exclusively New World in distribution. Social woodpeckers with a long, pointed bill. Bold colouration in black, white and yellow. Tongue shortened as a secondary adaptation with a brush-like tip. Strong flyers with only slightly undulating flight. Omnivorous feeding on fruits, invertebrates and sap. Conspicuous and vocal.

Tribe Campephilini
Large, black, crested woodpeckers. Highly specialised for excavating, foraging on large tree trunks. Straight bill with chisel tip. Nostrils covered with feathers (except in Dryocopus galeatus). Tail long, curved forwards, central pair with strongly pointed tips. Red on crown and malar area defines sexual dimorphism.
Dryocopus "Logcocks" 3 species - Front toes and 4th toe of similar length. 1st toe about half the length of the fourth. Tongue with 4 to 6 barbs at tip.
Campephilus "Ivorybills" 3 species - All toes long, the fourth considerably longer than the forward facing toes. Toes directed forwards when climbing and tarsus may touch the substrate, possessing a special tarsal pad as a result. Tongue with groups of randomly-sized barbs at tip

Tribe Colaptini
Exclusively New World in distribution (with the exception of one Asian species of Celeus). Bill well-adapted for excavating, strong and straight with ridges and slit-like, feather-covered nostrils. Sexual dimorphism in red crown and malar colour.
Piculus "Green Woodpeckers" 2 species - Greenish plumage. Bill pointed and slightly curved. Arboreal. Claws well-developed and tail only slightly curved ventrally and moderately stiff.
Colaptes "Flickers" 2 species - Semi-terrestrial and often in open environments. Long, curved and pointed bill. Tail long, but not particularly stiff. Feet robust, well-developed claws and long first toe. Skull lacks the structures commonly associated with hammering. Conspicuous vocal and visual displays. Tongue with 2 or 3 barbs at tip. Sexual dimorphism restricted to malar region.
Celeus "Blonde-crested Woodpeckers" 2 species - Atypical and possibly deserving of higher taxonomic rank. Dark body plumage and pale head. Nostrils not covered by feathers. Bill short and curved. 4th toe shorter or of similar length to front toes. First toe short. Foot morphology not adapted to trunk climbing, though they often do. Sexual dimorphism defined by colour of malar streak and facial area. Tongue with 4 to 6 barbs at tip. Particularly large uropygial gland is present in this genus.
Veniliornis "Little Woodpeckers" 3 species - Greenish or pied plumage. Arboreal behaviour. Lack accessory leg muscle M. semitendinosus. Bill strong and straight, with ridges protecting the nostrils, well-adapted for excavation.

REFERENCES
Campbell B & Lack E 1985 - A Dictionary of Birds - T & AD Poyser.
Winkler H & Christie DA 2002 - Picidae Handbook of the Birds of the World Vol 7 - Lynx Ediciones.
Winkler H, Christie DA & Nurney D 1995 - Woodpeckers: A Guide to the Woodpeckers, Piculets and Wrynecks of the World