Nightjars are a group of nocturnal birds in the Caprimulgiformes order that are known for their incredibly large mouths. Their scientific name Caprimulgiformes means ‘goatsucker’ which refers to an old myth that nightjars would drink the milk from goats. This myth likely started because of their very wide gape that allows them to swallow large prey items whole.
What is a nightjar?
Nightjars are medium-sized birds found worldwide. There are over 80 different species of nightjars that inhabit forests, grasslands, and deserts on every continent except Antarctica. Some common species include the common nighthawk, whip-poor-will, and European nightjar.
Nightjars have long wings, short legs, and very short bills. Their plumage is cryptic, meaning it camouflages well to their surroundings. Most nightjars are some variation of brown, grey, or black which allows them to blend into trees and vegetation during the day when they roost.
As their name suggests, nightjars are nocturnal. They have large eyes adapted to see in low light. They also have amazing hearing and their ears are not symmetrical. One ear is placed higher on their head than the other. This allows them to precisely locate insects and other prey in the dark.
Nightjar’s huge mouths
The most distinguishing characteristic of nightjars is their enormous mouths. Their gape extends far back past their eyes and their mouth cavities are very large compared to their overall head size.
For example, the common nighthawk has a gape of around 60 mm while its head is only around 45 mm long. This means it’s able to open its mouth wider than the entire length of its head!
Some key features of nightjars’ oversized mouths include:
- Hinged skull – The skull of nightjars has flexible joints that allow the upper and lower jaw to dislocate and open very wide.
- Small bill – With a tiny bill, the mouth and face open wide.
- Large mouth cavity – The mouth itself is very roomy allowing large prey to be swallowed.
- Backwards facing bristles – The corners of the mouth have backwards facing bristles that help contain and maneuver prey.
Why such big mouths?
So why have nightjars evolved these ridiculously large mouths? It mainly comes down to their specialized hunting strategy.
Nightjars hunt on the wing. At dusk they leave their roosts and fly out over open habitats scooping up insects and other prey at high speeds. Their huge mouths allow them to easily capture prey in mid-air.
Some key reasons for their oversized mouths include:
Swallowing prey whole
Nightjars are able to swallow their prey whole and on-the-go. Their large mouth cavities allow large intact prey to be stored before swallowing.
Most other insect-eating birds catch prey and transfer it to their bill to be broken down before swallowing. Nightjars eliminate this step which maximizes their aerial hunting efficiency.
Preventing injury
Swallowing sharp or spiky prey items like beetles, moths, and dragonflies would seem to pose the risk of injury. However, nightjars are able to instantly swallow them whole avoiding harm.
Their backwards facing bristles allow them to position prey so it slides straight down their throat feathers-first. This prevents any jabbing motions that could poke their mouth or throat.
Increasing prey capture
The immense gape of nightjars means they can simply open their mouth very wide and let prey fly or glide right in. This passive hunting strategy increases their chances of intercepting prey in mid-air even at high speeds.
Their very short bills do not obstruct the opening allowing for the largest possible gape to catch more insects.
Carrying more prey
Nightjars often hunt in bursts, flushing many insects from hiding. Their huge mouth allows them to scoop up and carry more prey during a single aerial feeding bout.
Scientists have found individual nightjars carrying over 50 prey items in their mouth at one time before swallowing.
Unique adaptations
Several other adaptations give nightjars the tools to take advantage of their expansive gape:
Sensitive bristles
The bristles around a nightjar’s mouth act like whiskers to help it feel and guide prey that enters their mouth. This assists with snap shut capture.
Wide-opening beak
The upper and lower parts of nightjar bills articulate widely apart which gets the bill out of the way for maximum gape.
Short legs
Nightjars have very short legs which may improve their aerodynamic profile for more efficient flight needed to catch prey on the wing.
Large wings
Long, broad wings allow nightjars to generate the lift and speed required for continuous aerial hawking.
Flexible neck vertebrae
Neck joints that bend far back let nightjars swing their head down and up to quickly grab prey anywhere around their body.
Other large-mouthed birds
While nightjars have the most extreme mouth gape, other birds have also evolved large mouths likely for facilitating faster feeding:
Swifts
Like nightjars, swifts are aerial insectivores that can open their mouths very wide to capture prey on the fly. However, their gape is not as exaggerated.
Kiwis
Kiwis are ground birds that probe soil and leaf litter with their long bills searching for worms and bugs. Their long bills and big mouth allow them to gobble up prey rapidly.
Shoebill
The prehistoric-looking shoebill has a massive hooked bill that opens wide to give it extra reach and swallowing ability to gulp down fish, amphibians, and reptiles.
Pelicans
Pelicans have expandable throat pouches that they use like nets to scoop up fish. Their large bill and jaws let them drain water and swallow fish from the pouch.
Why don’t all birds have big mouths?
While large mouths serve nightjars well, other types of birds are better adapted for smaller beaks and mouths. Some reasons most birds don’t have huge mouths include:
- Small prey – Many birds eat seeds, nectar, or tiny insects that don’t require large mouths.
- Perch feeding – Birds that feed from perches need dextrous smaller bills for handling food.
- Non-visual hunting – Birds like nightjars that catch prey on the wing in dark conditions benefit more than visual hunters.
- Slow flight – Fast continuous hawking like nightjars use favors big mouths more than slow flapping flight.
- Carrying food – Large mouths make carrying prey in bills to chicks more challenging.
Additionally, the nightjar lifestyle favors the evolution of large gapes that other birds may not require or find detrimental if it impaired vision or wind resistance.
Frequently asked questions
What family are nightjars in?
Nightjars are in the Caprimulgidae family of the Caprimulgiformes order.
What is the largest nightjar species?
The largest nightjar is the great eared-nightjar which can have a wingspan up to 55 cm (22 in) and weight around 130 g (4.6 oz).
Where do nightjars live?
Nightjars live on every continent except Antarctica. Different species occupy forests, open shrublands, deserts, and grasslands in various climates and elevations.
Why are nightjars called goatsuckers?
An old myth was that nightjars would drink milk from goats at night. This myth probably began because of their large gaping mouths.
Do nightjars migrate?
Many nightjar species migrate long distances between breeding and wintering habitats. Migration routes can be 2000 km (1240 miles) or more one-way.
Conclusion
In summary, nightjars have evolved huge mouths to maximize their aerial insect hunting strategy. By being able to open their mouths very wide they can quickly swallow flying insects whole without the need to carry prey. Their oversized mouths give nightjars a unique adaptation for efficiently catching insects at night.