Caprimulgiformes

(Nightjars)

Class Aves

Order Caprimulgiformes

Number of families 5

Number of genera, species 24 genera, 105 species

Evolution and systematics

Anatomical and biochemical data suggest owls (Strigiformes) are the closest relatives of the Caprimulgiformes, with swifts (Apodiformes) and trogons (Trogoniformes) as more distant relatives. The oldest fossils of Caprimulgiformes are from the Eocene to Oligocene around 55–35 million years ago (mya), by which time representatives of the oilbirds (Steatornithidae), owlet-nightjars (Aegothelidae), frogmouths (Podargidae), potoos (Nyctibiidae), and true nightjars (Caprimulgidae) had emerged, along with several species of the long-extinct caprimulgiform family Archaeotrogonidae.

Inferences from the fossil record coupled with evidence from rates of DNA evolution suggest the initial divergence of these families from a common ancestor of all Caprimulgiformes occurred between the end of the Cretaceous around 65 mya and the middle Eocene (50.5–42.0 mya). Eocene or Oligocene fossils of most of these caprimulgiform families are known from Europe (although owlet nightjars and oilbirds are only tentatively identified), pointing to much wider geographical ranges for many of them in the past than at present because modern oilbirds and potoos are exclusively South and Central American, modern frogmouths are restricted to Asia and Australasia, and modern owlet nightjars are found only in Australasia.

Some doubt remains about the sequence in which the five living families of Caprimulgiformes diverged from each other. Recognition of two suborders, Steatornithi for the oilbird and Caprimulgi for all the others, implies that the oilbirds were the oldest group to diverge and that the ecology (fruiteating) and behavior (echolocation) of the living oilbird are so unique that they easily suggest ancient origins. However, DNA-hybridization evidence and some anatomical features suggest the owlet nightjars diverged from an ancestral caprimulgiform stock earlier than the oilbirds, especially because owlet nightjars have several owl-like features, including a well-developed facial disk. The three remaining families, the frogmouths, potoos, and true nightjars are all well-defined groups but apparently rather closely interrelated. Two of these families are divided into subfamilies, the frogmouths into an Asian group (Batrachostominae) and an Australasian group (Podarginae), the true nightjars into nighthawks (Chordeilinae) and nightjars (Caprimulginae).

Physical characteristics

Caprimulgiformes are small to rather large nocturnal birds (0.7–23.6 oz; 20–670 g), all of which have relatively large heads with large eyes, a wide to very wide gape, and very short legs. The bill is small and weak in nightjars, nighthawks, and potoos; not much stronger in owlet nightjars; relatively strong in the oilbird; but stout and wide in the appropriately named frogmouths. Strong rictal bristles (specialized feathers with elongated, tapering shafts and sometimes short barbs that surround a bird's mouth) occur at the base of the bill in many of the species, often accompanied by other facial bristles. However, within the Caprimulgidae the absence of rictal bristles is the main character distinguishing the nighthawk subfamily Chordeilinae from the nightjars of the Caprimulginae; most potoos also lack rictal bristles. Many forest-living

Caprimulgiformes that feed by sallying from perches have short, rounded wings, but nightjars and nighthawks that feed in more sustained flights bear wings that are long and pointed, as does the strong-flying oilbird. The tail varies from short to long, with a rounded tip in most Caprimulgiformes but square or notched in some Caprimulgidae. Males of a few true nightjar species have wing feathers (innermost secondaries) or tail feathers that have been greatly modified as display plumes. Feet of Caprimulgiformes have four usually slender toes with the hallux (first toe) variously pointing forward, outward, or backward. A pectinate middle claw that is serrated or bears projections like the teeth of a comb occurs only in Caprimulgidae.

Like baby owls, Caprimulgiformes young hatch with a covering of soft down feathers. In the oilbirds, frogmouths, and owlet-nightjars, the first down is pale or white and succeeded by a second down plumage that is usually darker. In potoos and nightjars, however, the first down is often dark or patterned and is replaced directly by juvenal true feathers, which mainly resemble those of the adults in structure and coloration. Adult plumages of most species are marked in subdued shades of brown, gray, buff, rufous (rust-colored), or black, which provide beautifully elaborate concealing coloration. The wing and tail feathers are mainly dark or black, but in nightjars and nighthawks, signal markings are commonly hidden within them and consist of contrasting white, buff, or cinnamon patches that are revealed only when the wings or tail are spread.

Distribution

Caprimulgiformes are absent only from the Arctic, Antarctic, and sub-Antarctic and from most oceanic islands. They are most common in the tropics and subtropics, as expected for birds that mainly rely on large insect prey. Fossil records indicate the modern ranges of most of the families have contracted since the early Tertiary period, when subtropical forests were more widespread than today and included much of Europe and North America.

The oilbird has a modern range largely restricted to tropical South America; the potoos are also Neotropical with northern limits in Mexico, Hispaniola, and Jamaica and southern range limits in Uruguay. Owlet-nightjars have an Australasian range, from the northern Moluccas and New Guinea through Australia to Tasmania. The frogmouths include an Asian subfamily that ranges from southern India and Sri Lanka to Vietnam and southward to Java and the Philippines and an Australasian subfamily that ranges from New Guinea and the northern Solomon Islands through Australia to Tasmania. Only the Caprimulgidae has a nearly worldwide range, which spans from Canada to Argentina and northern Chile in the Americas; and, in the Old World, from Ireland, Scandinavia, and Siberia to South Africa, Sri Lanka, and through the East Indies to Australia and Tasmania, including most large islands. Of its two subfamilies, the Caprimulginae occupies almost all of this wide area, whereas the Chordeilinae are mainly American, with representatives that are tentatively classified alongside them from tropical Africa (brown night-jar Veles binotatus) and ranging from southern Asia to Australia (genus Eurostopodus).

Habitat

The ancestral habitat of Caprimulgiformes was apparently in tropical lowland forests, which still form the main habitat of the oilbird, owlet-nightjars, potoos, and frogmouths. At least a few of the species of the latter three families have extended into more open woodlands, savannas with trees, or scrub. Nevertheless, species of all these groups still rely heavily on trees, for food in the case of the oilbird and for perches and nest sites in the case of the others. Only the Caprimulgidae has occupied more open habitats, with many species inhabiting savannas and open woodlands, although some occupy grasslands or even semi-deserts and a few are restricted to closed forests.

Behavior

Although details of behavior of many of these secretive nocturnal birds are little known, at least one or a few species of each family have been studied sufficiently to allow generalizations that should apply more widely. Thus it is clear that most species are monogamous. The pair bond may be sustained with the birds living together in a territory for several years in succession (in tawny frogmouth Podargus strigoides) or the pairbond may be maintained only for one breeding season (European nightjar C. europaeus). Two African nightjars, in which the breeding males develop elaborate display plumes, are exceptional in being polygynous (mating with more than one female) (standard-winged nightjar Macrodipteryx longipennis, pennant-winged nightjar M. vexillarius). With these two species, the displaying males are visited by females to mate, after which the female alone is responsible for nesting and care of the young. A similar system apparently prevails with three South American nightjars (genus Macropsalis), in which males have elaborately elongated tail feathers.

Just as darkness hinders human observers watching nocturnal birds, it also limits the scope for visual display and communication among the birds themselves. Many Caprimulgiformes have therefore developed white or pale patches that can be exposed as effective signals in low light; these signal markings are often surrounded or contrasted with darker areas similar to the bold coloration in the American striped skunk (Mephitis mephitis). Movement also emphasizes the signal markings, such as when the bold white patches in the outer primaries and on outer tail feathers of male European nightjars are shown off in territorial display flights and their bold white throat patches are exposed as the head is lifted to give territorial songs. However, all these conspicuous markings can be hidden from predators during the day.

Sounds are much more effective than vision for communication at night, therefore it is perhaps unsurprising that many Caprimulgiformes appear to lack elaborate visual displays and to rely instead on vocalizations. Indeed, so far as known, all caprimulgiform species give distinctive calls or songs. In many cases, these provide the best means of detecting the birds' presence, and many are among the most beautiful, unusual, and distinctive of all bird sounds (e.g., the call giving rise to the mnemonic "Good Lord deliver us" of the African fiery-necked nightjar C. pectoralis); many are also well-known to local people (e.g., the onomatopeic, echoing, calls of the whip-poor-will C. vociferus). However, a few are monotonous drumming or hammering sounds (e.g., the monotonously repeated t-chop, t-chop, t-chop of the large-tailed nightjar C. macrurus) while a few are harsh or raucous (e.g., the loud guttural snoring of the great potoo Nyctibius grandis).

The nocturnal calls of many Caprimulgiformes undoubtedly function in advertisement or defense of territories because playback of recordings commonly results in a "song-duel" that may be accompanied or followed by rapid approach and aggressive displays. Tawny frogmouths are known to spend the whole year as pairs in well-defended territories, whereas some nightjars only advertise and defend territories for the duration of one breeding attempt. The territories of these birds not only provide much or all of the food for their owners, they also provide nesting and roosting sites, and presumably freedom from interference by conspecifics that might include illicit mating attempts. A few species are known to have territories of much more restricted extent, reduced in the colonial-nesting oilbird to the immediate vicinity of the nest. Since up to 10 male standard-winged nightjars may assemble to sing and display at a lek, which is visited by females, the territory in this species also must presumably be greatly reduced or absent.

The frogmouths, owlet-nightjars, and potoos that live year-round in the same territory appear to be among the most sedentary of all birds, with few or no records of extra-limital

vagrancy. In contrast to these, some nightjars and nighthawks are long-distance migrants, and many others make shorter seasonal movements. Extreme examples of long-distance migration are provided by the European nightjar, which breeds widely across Europe and Siberia but vacates all of this area to spend the northern winter in Africa south of the Sahara, and the common nighthawk (Chordeiles minor), which breeds widely in North America north to southern Canada and winters in South America south to northern Argentina. Among many examples of shorter migrations, one of the best understood is with the pennant-winged nightjar, which breeds in savannas of Africa south of the equator but migrates to spend the nonbreeding season in the savannas north of the equator. Others, such as the standard-winged nightjar, move seasonally from one vegetation zone to another.

Activity patterns

Because so much of the activity of Caprimulgiformes occurs under cover of darkness, it was difficult to identify the birds' activity patterns until miniaturized radio-transmitters and other devices were used by researchers. It has, however, long been clear that all species of the order normally roost during the day, moving only when disturbed. Their activity commences at dusk and continues at least intermittently through the night until shortly before dawn. There often appear to be peaks in singing and other activity soon after dusk and before dawn, with less activity in the middle of the night. This crepuscular and nocturnal feeding undoubtedly gives these specialized birds access to much larger numbers of flying insect prey than are available during the day. Furthermore, potential predators such as hawks are inactive at night, although there is still a risk from owls. Radio-tracking studies tend to confirm that activity levels really are low in the middle of the night, when light levels may be too low for the birds to feed. However, at least some species show more activity throughout the night on moonlit nights, when feeding is presumably possible.

Several nightjar species have been shown to synchronize egg-laying with the phases of the moon, this having been demonstrated with the fiery-necked nightjar in Zimbabwe and the whip-poor-will in North America. The potential advantage of synchronizing egg-laying with a full moon is that better feeding conditions on moonlit nights will recur one month later when young are being fed by the parents and two months later when fledglings are beginning to feed themselves. It has been demonstrated that whip-poor-wills feed their young more often on moonlit nights than on dark nights.

The demonstration in 1948 that the common poorwill Phalaenoptilus nuttallii of the American Southwest hibernates in rock cavities astounded biologists. It has since been established that they may be dormant for months at a time during the winter, although they can become active briefly on warm days. When hibernating, the body temperature is reduced and the heart-rate is slowed, much as in hibernating mammals. The winter dormancy of the common poorwill allows it to survive through the months when there are few, if any, flying insects. Regular use of dormancy for much shorter periods of hours rather than weeks in order to survive cool conditions has been demonstrated in the common nighthawk, European nightjar, tawny frogmouth, and Australian owlet nightjar Aegotheles cristatus, therefore it appears widespread in the order.

Feeding ecology and diet

The oilbird feeds on fruits taken at night from forest trees. All other Caprimulgiformes feed largely or entirely on arthropods, mainly insects but also including spiders and sometimes millipedes. The larger species of frogmouths and potoos and the largest nightjar (Chuck-will's-widow C. carolinensis) also take a proportion of small vertebrates, such as frogs, small birds, bats, or mice.

The most widespread feeding technique in the order consists of sallying flights from a perch to capture prey on the ground, on vegetation, or in flight. Some species, such as the larger frogmouths, use this technique mainly to capture prey on the ground, while others, such as the potoos, rarely, if ever, visit the ground. However, many nightjars are more versatile. Many of the longer-winged species of nightjar and most nighthawks feed on insects captured on more continuous hawking flights, often high above the ground.

Reproductive biology

The extent to which nests are built varies widely among the Caprimulgiformes. Australasian frogmouths build a small platform like a pigeon's nest; Asian frogmouths build a neat but tiny nest cup; the oilbird makes a rough cup on a ledge; owlet-nightjars accumulate at least a pad of material in the bottom of the nest hole. In contrast, no nest at all is built by the potoos, which lay in a crevice of a branch or stump, or by any of the nightjars or nighthawks, which nest on the ground, rarely in a crevice of a branch. In North America, the common nighthawk commonly nests on graveled roof tops.

Eggs are an unmarked white in oilbirds, frogmouths, and owlet-nightjars but somewhat spotted in potoos. A few of the nightjars and nighthawks have unmarked white eggs, but many species have them richly patterned with spots, blotches, or lines. The eggs of some, such as the Chuck-will's-widow, have a layer of calcium deposited after pigment has been deposited on the egg, giving them a "creamy" mottled appearance. Clutches are comprised of a single egg in all potoos, some small frogmouths, and many tropical nightjars; of two eggs in other nightjars and a few small frogmouths; but regularly of up to four eggs in oilbirds, tawny frogmouths, and Australian owlet-nightjars. The degree to which the sexes care for eggs and young varies widely in the order, from being a concern only of the female in the pennant-winged nightjar, to a fairly equal sharing in oilbirds, to incubation predominantly by the male with young fed by both sexes in frog-mouths.

Conservation status

Eight species of Caprimulgiformes are listed by the IUCN as globally threatened, of which three are Critically Endangered (Jamaican poorwill Siphonorhis americana, Puerto Rican nightjar C. noctitherus, New Caledonian owlet-nightjar Aegotheles savesi), two are Endangered (white-winged nightjar C. candicans, Itombwe nightjar C. prigoginei), and three are Vulnerable (Heinrich's nightjar Eurostopodus diabolicus, Nechisar nightjar C. solala, Bonaparte's nightjar C. concretus). Several additional species are listed as Near Threatened or Data Deficient.

Significance to humans

Most species of the order are retiring nocturnal birds that are noticed only by their calls, if at all. A few, such as the whip-poor-will, large-tailed nightjar, and several potoos, have well-known voices, but calls of many others are regularly muddled with sounds from owls or nocturnal mammals. Voices of some are the subject of folklore and superstitions. How caprimulgiforms obtained their common names is also the stuff of legend and folklore. The ancient belief, dating to Aristotle's time, that the birds flew at dusk and sought out she-goats and cows to suck dry of milk, eventually killing them, gave rise to the name "goatsucker". The name "night-jar" undoubtedly has its origins in the loud, churring call made by the European nightjar that lasts several minutes and "jars one in the night." None of the species is of much economic significance, although nestling oilbirds were formerly collected for oil and several of their large colonies are nowadays significant as tourist attractions.

Resources

Books

del Hoyo, J., A. Elliott, and J. Sargatal, eds. Handbook of Birds of the World. Vol. 5, Barn Owls to Hummingbirds Barcelona: Lynx Edicions, 1999.

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

Other

"Caprimulgiformes." 0. Chaffee Zoological Gardens of Fresno, California. Jan. 2002 (24 Jan. 2002). <http://www.chaffeezoo.org/zoo/animals/caprimulgiformes.html>

"A Goat WHAT?!?" Mecklenburg Audubon, Charlotte, North Carolina. Jan. 2002 (24 Jan. 2002). <http://meckbirds.org/birdID/nighthawk.htm>

David T. Holyoak, PhD