Nightjars

Poorwill on rock, Okanagan, BC

Mark Brigham

Nightjars are rather mysterious birds, heard at night far more commonly than they are seen. Their closest relatives are Owls. Many produce repetitive “nightjarring” calls, which have given the bird some of its names (e.g., poorwill and whippoorwill). In Ontario, an individual “whip” was counted to call over 16,000 times in a single night. Many calls are very familiar to those who live near them, such as the “good-lord-deliver-us” of the fiery-necked nightjar from southern Africa.

Nightjars’ nocturnal nature has in part contributed to superstitions about them which are captured in their scientific name (Caprimulgus, from Latin capra “goat,” and mulgere “to milk”). This results from the baseless belief that they suckle domestic animals at night. In fact, like Bats, most nightjars catch flying insects; but unlike bats, they detect prey visually, not using echolocation.

Currently there are eighty-nine species (family Caprimulgidae) known from around the world except Antarctica and some larger oceanic islands. Three species occur in Saskatchewan: common nighthawk, common poorwill, and whippoorwill. Most species occur in tropical woodlands, with temperate ones typically being migratory. Nighthawks nesting in Saskatchewan probably winter in northern Argentina.

Nightjars are large-eyed, nocturnal birds with tiny bills but a huge gape, and rather short necks and legs. They lack strong claws on the toes and do not use the feet to catch or hold prey. They are strong fliers. Some feed during sustained flight like swifts or swallows, but most sally in short flights from a perch or the ground. Virtually all are cryptically coloured, usually in shades of brown or grey. They seem to have few predators. Most are small, ranging in size from 30 to 100 g, but appear larger due to their long soft plumage. Most have well-developed facial bristles around the mouth, the function of which is not clear.

Typically, nightjars remain motionless at daytime roosts or on the “nest.” They do not build nests but lay two eggs in a shallow depression on the ground. Nesting is often synchronized with the lunar cycle, with chicks hatching so that when they need the greatest quantity of food the moon is full. Foraging occurs principally in low light: during the weak light around dawn and dusk, the dark parts of the night when the moon provides illumination, or around artificial lights. Nightjars’ eyes are adapted to low light levels thanks to the presence of a tapetum lucidum, which reflects light and enhances the light-catching power of visual cells thus accounting for “eyeshine.”

All three species found in Saskatchewan are capable of using torpor, an energy-saving strategy whereby metabolism and body temperature are reduced. The poorwill is perhaps most adept: its body temperature can fall to less than 5°C. In southern parts of the range (e.g., Arizona), it remains motionless for weeks at a time during the winter, in a manner akin to mammalian hibernation. This is the only bird in the world thus far known to do this.

Mark Brigham

Further Reading