About Picathartes gymnocephalus (Temminck, 1825)
Picathartes gymnocephalus, the white-necked rockfowl, measures 38 to 41 cm (15 to 16 in) in total length, with its notably long tail making up around 18 cm (7.1 in). Adults show very little sexual dimorphism in plumage, and cannot be distinguished by appearance between sexes. On an adult, the entire head is bare of feathers except the chin and throat, with only a thin layer of fuzz on the forehead. The head skin is bright yellow, with two large circular black patches behind the eyes that hold the ears; a thin 2 mm (0.079 in) wide strip of yellow skin on the crown keeps the two patches from connecting. The black patches have a raised edge and look like a distinct facial feature. A thin black line forms the eyelid and eyering around the bird's large, dark brown eyes. The beak is robust, disproportionately large, black, crow-like, noticeably curved downward on the upper mandible, and around 30 mm (1.2 in) long. The chin and throat are covered in thin white feathers, and the neck is long and slender. The nape has white feathers, while the hindneck is nearly bare, showing off orange-yellow skin. The upper mantle is solid black, which blends into a greyish-black lower mantle. The species has very muscular thighs that support its partially terrestrial lifestyle. The back, rump, and undertail are bluish grey, the dark brown tail is shaped like a tent, the moderately sized wings are also dark brown, and the underparts are creamy white, with the richest cream color on the upper breast. In dim light, the white-necked rockfowl can look entirely black above and white below. Its legs are blue and relatively long, its plumage is overall smooth with long feathers, and adults weigh 200 to 250 g (7.1 to 8.8 oz). Newly hatched nestlings are born naked, with dark-brown skin on their upper side, translucent pink skin on their underside, are blind, and have an orange-red gape. After a few days, the gape shifts to bright yellow-orange. The head skin of newly hatched birds is entirely yellow, lacking the adult's black patches; these patches develop around one week before the young leave the nest. Immature birds after four weeks of age are very similar to adults, but have creamier, silkier underparts, fewer feathers on the neck, paler yellow on the head, and most noticeably a much shorter tail than full adults. While multiple calls have been documented, the white-necked rockfowl is usually silent. Its call has been compared to a chicken's clucking, with repeated "chuk-chuk-chuk" or "choop-choop-choop" notes produced at a steady rate of eight notes every five seconds, and this call typically lasts for at least a minute. It has been proposed that this call may be a proper song, but more research is needed to confirm this. Its alarm call, one of its most frequent vocalizations, is described as a continuous, low-pitched, guttural chatter similar to "ow, ow, ow". Adults and juveniles also produce a drawn-out "owooh" call note, and fledglings can give a loud, quavering one-second whistle as a contact call. This species is endemic to West Africa, found from Guinea to Ghana. It is locally common in southeastern Guinea, widespread across Sierra Leone outside the north and northwest, and widespread across Liberia outside the north and northeast. In Côte d'Ivoire, it is currently restricted to areas near the Guinean and Liberian border. It also occurs in Ghana, where it is local and uncommon in south-central parts of the country. The species' total range covers approximately 391,000 km2 (151,000 sq mi), but it occurs in low-density patches across this large area. It does not migrate, though it does disperse widely after the breeding season. The white-necked rockfowl inhabits steep slopes in both primary and mature secondary forests. These forested areas are typically rocky and found in hilly lowland areas up to 800 m (2,600 ft) in elevation. The species often lives close to flowing streams and rivers, to access wet mud for nest construction, and colonies are usually found within 100 m (330 ft) of a stream. It also appears to prefer living near inselbergs. It is occasionally seen in partially cleared forest and near human settlements, but this is considered abnormal. Recent surveys found that abandoned rockfowl nests are more likely to be located in or near secondary forest. This species typically stays low in vegetation or on the ground near its nesting grounds. It moves quickly through its forest habitat primarily through hops and bounds, pausing before resuming movement, and uses its tail for balance while moving through the forest. It also flies at low altitude for short distances between vines and trees, and rarely flies long distances. It can hide quickly from sight in thick creepers or rocky areas, and is capable of high jumps – it can sometimes jump 6 m (20 ft) from a cave floor up to its nest, using its wings only partially to assist. It was once thought that the rockfowl rarely travels far from its breeding grounds, but new data shows the species has a much larger range than previously believed. Rockfowl will continue to roost on their nests for some time after the breeding season ends. These birds are normally solitary or found in pairs, though sometimes they occur in groups of three to five birds. They typically silently move away from any unusual activity in the forest, but if they know they have been spotted, they can become quite inquisitive and occasionally approach observers. During one display behavior, multiple birds in a colony form a loose circle. Individual birds run toward each other, forcing the approached bird to retreat slightly before chasing either the bird that charged it or another bird in the circle. At points during this display, a bird will lean forward on a branch, tuck its head between its legs, and half-spread its wings to show its crown to other birds. This behavior is thought to signal an intent to roost in a group, though recent evidence suggests the display may also be related to breeding. To scratch its head, the bird lifts its foot over its wing. This species is long-lived. The white-necked rockfowl breeds primarily in caves, and breeding pairs nest either alone or as part of a small colony. Though its courtship behavior is not documented, the species is monogamous, meaning it only breeds with its mate, despite earlier claims that it bred cooperatively. It has two breeding seasons each year, with timing tied to the birds' location and the local wet season; nesting occurs just before and after the wet season, and the two seasons average 127 days apart. The species reuses its nest, and typically repairs it two to eight weeks before laying eggs. Birds in Guinea breed from July to January, while birds in Sierra Leone breed from November to February and from April to October. In Liberia, breeding occurs from September to December and from March to July, and in Ghana breeding happens from March to June and September to November. Breeding caves are usually deserted when the birds are not breeding, so increased use of the caves by rockfowl is the first sign of approaching breeding. Nesting colonies average two to five nests, though one colony held forty nests. Non-breeding rockfowl may sometimes be present in colonies alongside breeding birds, and these non-breeding birds sometimes commit infanticide to gain access to high-quality nesting sites or mates. Birds in these colonies often chase each other in circles, even through the treetops, which is a very rare location for this species to visit. Unusually for a rainforest-dwelling bird, the white-necked rockfowl builds its nest from mud mixed with varying amounts of plant fibers. Mud is collected from nearby rivers and streams, and shaped into a strong, thick-walled, deep cup that attaches to a cave wall or roof, a cliff, or a large boulder around 2 to 4 m (6.6 to 13.1 ft) above the ground. The rock surface must slope inward to protect the nest from rain. The inside of the cup is lined with plant phloem fibers and roots. White-necked rockfowl nesting caves usually host wasp populations, and wasp nests 2 to 3 cm (0.79 to 1.18 in) long are often embedded in the rockfowl nests. It is thought that rockfowl use existing wasp nests as a nucleus to build around, to allow nest construction on smooth cave walls. Cliff nests are always built a distance away from nearby plants. Both members of a breeding pair work on nest construction, alternating roles: one collects materials while the other shapes them into the nest. The mud is sometimes swallowed and regurgitated before use. While nearly all rockfowl nests are found in caves or on cliffs, there are records of nesting on a riverbank and on a fallen tree trunk. Nests are usually built at least 1 m (3.3 ft) apart, but one colony had six nests positioned next to each other. Nests vary widely in size, but average 108 mm (4.3 in) long, 172 mm (6.8 in) wide, and 129 mm (5.1 in) deep. It was once thought that each breeding pair builds two nests, one for breeding and one for roosting, but recent surveys found no evidence to support this – all nests in a colony are used for breeding. Each nest holds one to two eggs, most commonly two, laid one or two days apart. The eggs weigh around 14.5 g (0.51 oz), and average 25.8 by 38.3 mm (1.02 by 1.51 in) in size. They are creamy white with dark brown blotches, which are more densely concentrated near the larger end of the egg. Incubation begins after the first egg is laid, and lasts 23 to 28 days. The parents take turns incubating the eggs, and hatching typically takes at least 12 hours. The eyes of infant rockfowl open after nine to eleven days, and feathers begin to grow seven days after hatching. After hatching, nestlings are fed almost four times per hour. To feed the young, an adult clings to the side of the nest while fluttering its wings, and some birds use their tail as a prop under the nest to support their body. Rockfowl have been documented killing other rockfowl's young, and nest predation is carried out by cobras of the genus Naja, the Nile monitor, sun squirrels, red-chested goshawks, and Procolobus monkeys. On average, this leads to only 0.44 nestlings surviving per breeding pair. Due to the risk of infanticide from other rockfowl, parents guard their nests and vicious fights between birds often occur. This infanticide behavior is common in white-necked rockfowl but rare among other bird species. It is not fully understood, but is thought to occur due to resource competition or sexual selection. Young rockfowl leave the nest 23 to 27 days after hatching, and at this point they resemble adults but have much shorter tails. The young leave the nest by standing on the nest edge, giving a piercing whistle, then gliding down to the ground on outstretched wings, where an adult carrying insects meets them. Even after leaving the nest, the young return to roost on it with their parents.
