Frogmouths

(Podargidae)

Class: Aves

Order: Caprimulgiformes

Suborder: Caprimulgi

Family: Podargidae

Thumbnail description
Small to rather large nocturnal birds with strong wide beak, hooked at tip, large gap, cryptic coloration, rounded wings, and short legs

Size
7.5–24 in (19–60 cm); 1.5–23.6 oz (43–670 g)

Number of genera, species
2 genera; 13 species

Habitat
Forests, woodlands, scrub, plantations

Conservation status
Near Threatened: 5 species

Distribution
South Asia from India to Vietnam, Java, and Philippines; New Guinea and Solomon Islands to Australia and Tasmania

Evolution and systematics

The frogmouths form a well-defined group that clearly belongs within the Caprimulgiformes. The large strong beak with a very wide gape is the basis of the frogmouth name and provides an immediate distinction from the weaker-beaked owlet-nightjars (Aegothelidae), potoos (Nyctibiidae), and true nightjars (Caprimulgidae) that are their closest relatives. The earliest fossil frogmouths are known from the Eocene of France and Germany, providing evidence of the family's antiquity and adding to other information that implies they diverged from their closest relatives during the early Tertiary. Fossils from Europe also suggest that the modern range, from south Asia to Australia, is a relic of a formerly more extensive Tertiary range, presumably developed when subtropical forests covered much more of the Northern Hemisphere.

Asian frogmouths of the genus Batrachostomus are mainly smaller than the Australasian species of the genus Podargus, and differ somewhat in color and nest structure. But these groups have always seemed sufficiently similar to be regarded as closely related. It came as a surprise when DNA hybridization studies by Charles Sibley and Jon Ahlquist suggested the two genera are so distantly related that they should be placed in separate families. Some recent publications have compromised by placing them in separate subfamilies, Batrachostominae for the Asian species and Podarginae for the Australasian species, and that arrangement is adopted here.

There is now little doubt that three species should be recognized in the Australasian Podargus: the marbled frogmouth (P. ocellatus), the tawny frogmouth (P. strigoides), and the Papuan frogmouth (P. papuensis). However, there is more doubt about how many species are represented among the less well-known Asian frogmouths of the genus Batrachostomus, particularly whether the Sumatran and Bornean populations of short-tailed frogmouth (B. poliolophus) are distinct species, and whether some of the more distinctive subspecies of Javan frogmouth (B. javensis) might be better treated as species than subspecies.

Physical characteristics

Frogmouths are rather stocky, compact birds that resemble other Caprimulgiformes in having a relatively large head with large eyes. The strong wide bill is the family's hallmark and is relatively even wider in the Asian than in the Australasian species. The base of the bill has strong rictal bristles; some Asian species also have other well-developed facial bristles, providing an assemblage of whiskers that is presumed to help direct prey into the gape and protect the eyes from impact with hard prey. Wings and tail are rather short to moderately long, but both are always rounded. Very short legs allow only a restricted shuffling gait. The claw of the middle toe lacks the serrated comb found in other Caprimulgiformes.

The Australasian species have rather variable brown, buff, or grayish coloration with predominantly streaked patterns, providing excellent camouflage at daytime roosts. Sexual differences in color are not large, although the female may be brighter than the male. In several Asian species the "reversed" sexual dichromatism (difference between sexes in coloration) is much greater, with mainly bright-rufous females having bold white signal markings on their underparts and wings, whereas males are mainly brown with intricately barred and streaked patterns. Some Asian frogmouth males incubate on the exposed nest during the daytime when concealment is most needed, whereas the female is active in nocturnal territorial advertisement. These reversed roles may explain the apparent reversal of the usual pattern of sexual dichromatism in birds.

Distribution

Asian frogmouths occur in forested regions from southern India and Sri Lanka and the southern slopes of the Himalayas east to Vietnam and the Philippines, and through the Indonesian islands to Java. They reach greatest diversity in Sumatra and Borneo, which have up to five species in lowland forests and two more in lower montane forests.

Three species of Australasian frogmouths range from New Guinea and its satellite islands and the northern Solomon Islands to Australia and Tasmania. Only the tawny frogmouth occurs in Tasmania and over most of Australia, but all three species are present in Queensland and two are widespread in New Guinea.

Although they can fly strongly, frogmouths are among the most sedentary birds. There are no migratory species, and there are no records of extralimital vagrancy for any species. The tawny frogmouth apparently remains on its territory throughout the year, with no seasonal wandering even in the arid regions of Australia. While there must undoubtedly be some dispersal of young birds, even this remains to be documented.

Habitat

Forests are the principal habitat of most frogmouths and the only habitat of many species. The tawny frogmouth alone has adapted to life in more open woodlands and scrub, allowing it to colonize large unforested areas of Australia that offer the minimum requirement of groves of trees or bushes in which to nest and roost, and perches from which to hunt.

Detailed differences in habitat preferences and body size among Asian frogmouths may contribute to the coexistence of several species in regions such as Borneo. There, montane forests are shared between the large Dulit frogmouth (Batrachostomus harterti) and the smaller short-tailed frogmouth; the primary lowland forests support the large frogmouth (B. auritus), the rather small Gould's frogmouth (B. stellatus), and the very small Javan frogmouth; and disturbed secondary forests in the lowlands have only the Sunda frogmouth (B. cornutus).

Behavior

All frogmouths roost during the day and are active from dusk through part or all of the night until around dawn. Daytime roosts of several species including the tawny frogmouth and Sri Lanka frogmouth (Batrachostomus moniliger) are on branches in trees or bushes, where the birds remain quiet and still unless approached so closely that they suddenly fly away to escape. They give an impression of remaining immobile if not asleep when approached, with the beautifully camouflaged plumage sleeked to aid concealment, but in fact they watch intruders through nearly closed eyes and keep them in view with very slow head movements.

There are a few reports of daytime roosting in tree holes by the marbled frogmouth (Podargus ocellatus) and large frog-mouth, which, if observations are typical, may explain why these species are rarely encountered during the day. However, there have been other daytime sightings of Asian frog-mouths in thick tangles of vines in forests, where a small motionless and well-camouflaged bird is remarkably inconspicuous.

Nocturnal activity consists largely of movements around the territory to hunt from different perches, and singing or calling to defend the territory. The Australasian species have sustained cooing or hooting songs that may be given as duets by paired birds. These are backed up by aggressive calls and loud bill-snapping if another bird of the same species enters the territory. Asian frogmouths are less well known, but females of several species give loud territorial whistling, mewing, or trilling calls, while the males have different and often more subdued calls.

Feeding ecology and diet

Insects and other active invertebrates caught on sallying flights from a perch form the main prey of frogmouths, but the larger Australian species also take small vertebrates. Predominant insect prey are beetles, moths, orthopterans, stick insects, cicadas, bugs, and many other types, commonly supplemented with other arthropods such as spiders and millipedes. Recorded vertebrate prey include frogs and mice. Details of hunting behavior are unknown or poorly known for many species. However, the tawny frogmouth feeds mainly by swooping to the ground from a prominent perch to pick up moving prey in the beak, then shakes or beats large or tough prey against the perch to immobilize it. The small Javan frogmouth is thought to catch much of its insect food by sallying to flutter at leaves and trunks, a smaller proportion perhaps being obtained flycatcher-like in sustained flight or from the ground.

Several frogmouth species use prolonged gaping in threat display, but it is also possible that similar actions are used in passive fly-trapping. It is suggested that sticky or smelly saliva facilitates this feeding method, but there are few detailed observations and more study is needed.

Reproductive biology

Unlike most Caprimulgiformes, frogmouths build well-formed nests. Those of Australian frogmouths are small flat pads of twigs or tendrils placed among forking branches or on a flat branch, the structure reminiscent of a pigeon's nest and built by both birds of the pair. Asian frogmouths build relatively tiny nests on a branch or stem, the tiny cup made of moss or lichens interwoven with feathers plucked from the birds' own underparts.

Clutches are one or two eggs in most species, but three or sometimes four in the tawny frogmouth. The eggs are unmarked white, but camouflaged eggs are not needed because a beautifully concealed adult bird invariably covers the egg or eggs. So far as is known, daytime incubation is carried out exclusively by the male, although the female is known to take some part in nocturnal incubation in three species. The incubation period (around 30 days) is known only for the rather large tawny frogmouth.

At least in the Australian species, nestlings hatch with a coat of white down and later acquire a second coat of gray down before juvenal feathers develop. Small young are continuously brooded by the male in daytime, but fed by both parents at night on regurgitated food. The fledging period is known only for the tawny frogmouth (25–35 days), the young of which are fed for another one to two weeks after fledging, then remain in the company of their parents for several months.

Conservation status

No frogmouth species is listed as globally threatened, but undoubtedly there have been large declines in many species due to deforestation. Five species are considered Near Threatened—the large frogmouth, Dulit frogmouth, Bornean frogmouth (B. mixtus), short-tailed frogmouth, and Gould's frogmouth.

Significance to humans

Frogmouths are mainly unobtrusive forest birds commonly overlooked even by local people. Early European settlers in Australia were familiar with nocturnal calls of the tawny frogmouth, but it is evident from the given common name "Mopoke" or "Morepork" that there was confusion with the common small owl, the southern boobook (Ninox boobook).

Species accounts

Resources

Holyoak, D. T. "Frogmouths." In Handbook of the Birds of the World, Vol.5, Barn-owls to Hummingbirds, edited by Josep del Hoyo, Andrew Elliott, and Jordi Sargatal. Barcelona: Lynx Edicions, 1999.

Holyoak, D. T. Nightjars and their Allies. Oxford: Oxford University Press, 2001.

[Article by: David T. Holyoak, PhD]