Cliff swallows (Petrochelidon pyrrhonota) are small passerine birds found throughout North America. They build gourd-shaped mud nests on vertical surfaces like cliffs, under bridges, and on buildings. Their nests have a fascinating structure that allows them to be securely attached to vertical surfaces.
What is a cliff swallow?
The cliff swallow is a member of the swallow family Hirundinidae. They are sparrow-sized birds about 5-6 inches long with a wingspan of 12 inches. Cliff swallows have blue-black upperparts, a buffy forehead, throat, and rump, and a rusty orange face and collar. The tail is slightly forked.
Cliff swallows breed across North America and winter in South America. They migrate long distances, traveling up to 5,000 miles between their breeding and wintering grounds. Cliff swallows often forage in large flocks during migration, catching flying insects on the wing.
Where do cliff swallows nest?
As their name suggests, cliff swallows nest on vertical surfaces, especially cliffs. They also build their mud nests under the eaves of buildings, bridges, and highway overpasses. Cliff swallows nest in colonies ranging from just a few to over 3,000 nests. Large nesting colonies provide safety from predators.
Cliff swallows need an overhang or vertical wall to build their nests. They attach their nests to surfaces using mud pellets as an adhesive. Colonies are often built just above a source of mud near a stream, pond, or puddle. The birds use this mud to construct their nests.
Structure of Cliff Swallow Nests
The nest of a cliff swallow is an intricate structure made primarily out of mud. The nest is gourd or vase-shaped with a long entrance tube protruding from the bottom. The mud nest provides insulation and protection for the eggs and chicks. Let’s look at the key structural elements of a cliff swallow nest:
Entrance tube
The most distinctive feature of a cliff swallow’s nest is the long entrance tube, which can be over a foot long. This tube hangs down from the bottom of the main nest chamber. It has a narrow opening just big enough for a cliff swallow to pass through.
The entrance tube helps ensure the safety of the eggs and chicks. It makes it difficult for predators like snakes to enter the nest. The smooth sides of the tube also deter other climbing predators. Only adult cliff swallows can easily enter their nests through the narrow opening.
Main nest chamber
The entrance tube leads into a gourd or vase-shaped nest chamber. This chamber is lined with grasses, feathers, and fur to provide insulation. The nest chamber is typically 6-8 inches across and 4-6 inches high.
Here the female cliff swallow lays her eggs and rears the young. The curved shape and small opening of the chamber help protect the contents from weather and predators.
Construction material
Cliff swallow nests are constructed primarily using mud. The birds gather mouthfuls of mud from sources near their colony. They mix the mud with grasses and form it into pellets that are carried back to the nest site.
At the nest, the mud pellets are attached using more mud as an adhesive. The nest gradually takes shape as more rows of pellets are added. The mud dries into a hard, durable substance.
The cliff swallows also weave grasses, feathers, algae, and other materials into their nests for insulation. The combination of stiff mud and soft lining makes a secure home for raising young.
Attachment to surfaces
Cliff swallow nests have the remarkable ability to adhere to vertical surfaces. The mud pellets act like cement, bonding the nest to the underlying cliff, bridge, or building. The nests are attached so securely that they can often remain in place for many years.
The weight of the nest also helps press it against the surface. Nests are often built in a corner or under an overhang, using the sides for additional structural support. The nests are able to withstand rain, wind, and the movements of the adult birds.
Nest Construction
Building a cliff swallow nest is a demanding process accomplished by both male and female birds working cooperatively. Nest construction proceeds in stages over several weeks. Let’s look at the key steps involved:
Site selection
In the spring, existing pairs of cliff swallows return to their former colony sites. New pairs also scout for appropriate nesting locations. Sites are chosen based on overhangs to build on and access to mud and water. The colonies need vertical walls or surfaces like cliffs, buildings, bridges, or highway overpasses. Proximity to feeding sites is also important.
Foundation
The nest construction starts with the foundation. One of the cliff swallows clings to the vertical surface and begins laying down globs of mud. These form a half cup shape against the surface. More mud is added to build up the structure and form the basic shape.
Entrance tunnel
With the foundation in place, the birds begin constructing the long entrance tube. More mud pellets are added in rings. As the tube elongates it starts to curve upwards. The length and narrow opening help prevent access by predators.
Nest lining
Once the mud structure is completed, both birds gather soft materials for lining the interior. They collect grasses, feathers, algae, and plant down. This lining provides insulation and a soft bed for the eggs.
Nest maintenance
Existing nests are often repaired and reused from season to season. The birds may rebuild portions damaged by weather or expand the nests. The colonies are busy hubs of activity as new nests are built and old ones maintained.
Cliff Swallow Nesting Behavior
The unique nests of the cliff swallow support their colonial breeding habits and mating behavior:
Colony formation
Cliff swallows like to nest in dense colonies with their nests stacked closely together. Their gourd-shaped mud nests can be attached underneath an overhang in groups of tens to thousands. These dense colonies provide some safety from predators.
Nests are often reused from year to year, so colonies may gradually grow in size over time as more pairs return to the site. The bustling social nesting sites are hubs of activity during the breeding season.
Monogamous breeding
Cliff swallows form monogamous breeding pairs. The male and female cooperate to build a nest together. A mated pair will usually return to the same site and nest over multiple years. Though cliff swallows mate for life, DNA evidence shows some instances of extra-pair mating.
Shared nest duties
Both the male and female cliff swallow participate in nest building, incubating eggs, and rearing young. The female typically spends more time incubating while the male brings food. But the male may also incubate or brood, allowing his mate to occasionally forage. Shared duties ensure good care for their offspring.
Multiple broods
Cliff swallows often raise 2-3 broods per breeding season. After a successful first brood, the pair will mate again and lay more eggs. Using the same nest, they can reuse it with some new lining. The long breeding season allows time for multiple clutches. More broods improve the odds of successfully fledging young.
Adaptations for Nest Building
Cliff swallows have several key adaptations that allow them to build their remarkable nests:
Mud gathering
Cliff swallows have short, wide beaks that are ideal for picking up balls of mud. Their beaks help transport many loads of mud back to construct the nests efficiently.
Strong toes
Their feet also adapt them for nest building. Cliff swallows have short legs but very strong toes. Their toes allow them to tightly grip vertical surfaces while constructing nests. The toes help counteract the weight of mud as they work.
Saliva
Cliff swallows have sticky saliva that helps bind mud pellets into place. Their saliva acts as an adhesive to hold the nest to surfaces before the mud fully dries and hardens. The saliva also helps shape and smooth the nest walls.
Flight skills
Expert flying skills allow cliff swallows to deftly maneuver while carrying mud and nest materials. They use their agile flight to attach materials high up on vertical surfaces. Strong wings provide the stamina for frequent flights to and from the nest.
Social cooperation
Cliff swallows cooperate in large colonies to build nests. Their complex nests require both the male and female working together. Social nesting also provides networks for information sharing and learning nest building skills.
Cliff Swallow Nest FAQs
How long does it take cliff swallows to build their nests?
The nest building process takes 1-2 weeks for most cliff swallows. The birds make hundreds of trips carrying mud to construct their gourd-shaped nests. Working cooperatively as a pair allows them to complete the intensive project of nest construction in a reasonable timeframe.
Do cliff swallows use their nests more than once?
Yes, cliff swallows will often reuse the same nest across breeding seasons. The sturdy mud structures can persist on surfaces for many years. Each spring the birds will refurbish old nests by re-lining the insides with fresh soft materials. Repairing old nests is faster than building an entirely new one.
How do cliff swallow nests stay on vertical surfaces?
The mud pellets act like cement to adhere the nests to walls and cliffs. The sticky saliva of cliff swallows also helps bind the nest to surfaces as the mud hardens. The weight of the nest pressed against the wall adds extra gripping power. Overhangs provide additional structural support.
What happens if a cliff swallow nest gets knocked down?
Cliff swallows have been observed rebuilding nests even after severe damage. But if a nest falls, the eggs or chicks likely won’t survive. The birds may start over and construct a new nest from scratch, but the breeding attempt will be lost for that season.
How many eggs do cliff swallows lay in their nests?
A typical cliff swallow clutch contains 3-5 white eggs. The female incubates the eggs for about 15 days before they hatch. Both parents feed and care for the chicks until they fledge at around 4 weeks old.
Threats to Cliff Swallow Nests
The unique mud nests of cliff swallows face a variety of threats from both natural and human sources:
Predators
Snakes, rats, raccoons, and house sparrows may invade cliff swallow colonies looking for eggs and chicks to eat. Large nesting colonies help provide safety in numbers against such nest predators. Very long entrance tubes also make access more difficult.
Weather
Heavy rains can damage cliff swallow nests, especially early in nest building before the mud fully hardens. Storms may destroy sites, forcing birds to rebuild. Extended cold snaps can also kill swallows and their eggs since they rely on flying insects for food.
Declining mud supplies
Humans have diminished sources of mud through activities like damming and filling in wetlands. Without adequate mud, cliff swallows struggle to construct quality nests. They may be forced to nest in suboptimal, less secure sites.
Human disturbance
People intentionally removing or disturbing nests remains a problem for cliff swallows. Their habit of nesting on man-made structures like bridges puts them in conflict with human activities. Vehicle traffic and construction can disrupt breeding as well.
Habitat loss
Cliff swallows rely on vertical structures for nesting sites, but suitable habitats are shrinking. Destruction of cliffs and changes to buildings are reducing nesting surfaces. Conversion of land for agriculture also eliminates the mud supply.
Cliff Swallow Nest Interdependence
Cliff swallow nests highlight the interdependence between wildlife and their environments. The swallows rely on specific habitat resources to construct their unique nests. In turn, many other species benefit from the empty nests after the breeding season:
Mud supply
Cliff swallows need adequate wetland areas as sources of mud for their nest construction. They transport this mud to form the sturdy walls of their nests. But human activities that reduce wetlands can jeopardize the mud supply.
Insects for food
Insects like flies, wasps, beetles, and moths are vital food for cliff swallows. Adults catch insects on the wing and bring them back to feed nestlings. But pesticide use can reduce insect populations and food availability.
New nest materials
The birds rely on plant fibers, feathers, and fur to line their nests each season. But habitat loss and changing land use may limit access to these soft materials needed for nest insulation.
Abandoned nest reuse
Once cliff swallows fledge their young and migrate, the empty nests provide shelter for other species. Birds, bats, insects, and rodents may reuse the old nests. But human disturbance can prevent this hand-me-down nest use.
The complex nests of cliff swallows highlight the connections between nature. Their unique mud structures rely on ecosystem services. In turn, the nests give back by providing habitat after the breeding season. Understanding these relationships can help promote nest building success and biodiversity.
Conclusion
Cliff swallows construct intricate nests perfectly adapted to their habitats and lifestyles. Their gourd-shaped mud nests have a long entrance tube and attach securely to vertical surfaces. Nest building requires extensive cooperation between mating pairs. The nests support cliff swallow breeding behaviors and protect vulnerable eggs and chicks. While facing some threats from humans, these aerial architects continue to create impressive structures that provide far-reaching benefits. Their mud-crafted nests are wonderful examples of avian engineering.
