The bank swallow (Riparia riparia) is a small passerine bird in the swallow family. They are found in Europe, Asia and North America near areas of water where they nest in burrows dug into natural riverbanks and artificial structures such as quarries and highway cuts. Bank swallows exhibit a range of interesting behaviors related to their nesting, migration, feeding, and social dynamics.
What is the bank swallow?
The bank swallow is a small, sparrow-sized bird that is brownish-gray above and white below with a distinct brown band across the chest. They have a forked tail and pointed wings, allowing them to be agile, fast fliers. Bank swallows reach 5–6 inches in length and weigh approximately 0.4–0.5 ounces.
This species is found in open, cleared areas near water including rivers, lakes, and oceans coasts. They nest in colonies in burrows dug into vertical banks and cliffs made of soil, sand, or gravel. Originally a cliff-dwelling species, the bank swallow has adapted to nest in human-created sites like gravel pits and highway cuts as their natural habitat has declined.
When is the bank swallow breeding season?
The bank swallow breeding season generally runs from late April through August, varying slightly across their range:
- In Europe, breeding season is May to early August.
- In most of Canada and the northern U.S., it is early May through July.
- Further south in the U.S., it may start in late April and extend into August.
Bank swallows are strongly migratory. They winter in South America and return to their breeding range in spring to establish nesting colonies. Birds arrive at breeding sites and select mates by late April or May. Males establish nesting burrows before pair formation.
How does the bank swallow choose its nest site?
Bank swallows nest in colonies ranging from just a few to over 1,000 breeding pairs. Nesting sites have these key features:
- Vertical banks or cliffs near open water
- Fine-textured, crumbly soil or substrate like clay, sand, gravel, or silt that allows burrowing
- Low, sparse vegetation allowing clear access to burrows
- Overhangs or erosion to provide shelter from rain and sun
Breeding sites are often used year after year. As natural riverbanks have disappeared, bank swallows have adapted to nest in active and inactive sand and gravel pits, road cuts, railroad beds, and drainage ditches.
How does the male bank swallow attract a mate?
Male bank swallows arrive at the breeding site a few days before females to establish nesting burrows. The males use their burrows to attract potential mates by:
- Sitting at the burrow entrance singing and calling loudly
- Bobbing up and down and fanning their tail feathers
- Flying in circles over the colony
- Chasing intruders away
Unmated males may temporarily join in the courtship flight of a mated pair, trying to entice the female. Aggressive encounters between competing males over burrows are also common.
How does the bank swallow build its nest?
Bank swallow nest burrows are dug by both the male and female working together once paired. The burrows are dug horizontally into steep banks ranging from just a few inches to up over three feet long. Nest chambers are dug at the end and lined with a mix of grasses, stems, leaves, and feathers.
Bank swallows use their beaks and feet to loosen soil and shuffle it backwards as they dig out their burrows. It may take a pair 1-2 weeks to construct a completed nest. The female typically builds the nest cup while the male continues excavating the burrow.
How many broods do bank swallows have?
Bank swallows typically raise one brood per breeding season. However, if their first nest fails early in the season they may re-nest and try for a second brood. Some key points on bank swallow breeding include:
- Clutch size is 4-5 white eggs
- Only the female incubates eggs for 13-16 days
- Both parents feed the altricial young for 19-24 days till fledging
- Young return to the burrow at night for 2-3 weeks after fledging while still being fed by the parents
Most pairs only have time and resources for one successful brood each year. Second broods late in the season may not be successful if food becomes scarce as autumn approaches.
What does the bank swallow eat?
Bank swallows are aerial insectivores, meaning they hunt insects on the wing. Their diet consists mainly of flies, winged ants, bees, wasps, beetles, and mayflies. They capture insects in flight or pick them off surfaces in low vegetation over water.
Swallows forage mostly over open water like rivers and lakes where insect numbers are abundant. They may forage up to a mile and a half from their nesting colony, sometimes forming large feeding flocks numbering in the thousands.
How do bank swallows migrate?
Bank swallows migrate up to 5,000 miles between their breeding and wintering grounds each year. They migrate entirely during daylight hours, stopping to rest and feed at intervals. Migration takes place in large, loose flocks often containing other swallow species.
Their migration schedule is:
- Spring migration to breeding areas occurs from mid-March to May
- Fall migration south begins in August, peaks in September, and finishes in October
- They winter in southern South America in Brazil, Bolivia, Paraguay and Argentina
Bank swallows are one of the later swallow species to complete fall migration. Their winter habitat consists of open grasslands, agricultural areas, and savannahs with water for foraging.
How do bank swallows communicate and socialize?
Bank swallows have complex social interactions and vocalizations both within their colony and between flocks. Some key elements include:
- Colony members recognize one another by voice
- Mates use contact calls to communicate pair bond while nesting and foraging
- Special juvenile begging calls trigger parental feeding response
- Alarm calls alert colony to predators like hawks, owls, raccoons
- Chirping and twittering notes communicate in flocks during migration and feeding
Non-vocal displays like male courtship flights and displays of aggression through actions like bill-snapping also facilitate social interactions.
How does the bank swallow defend its nest?
Bank swallows are highly defensive of their nesting burrows. They have several tactics for deterring predators and dealing with threats:
- Mob intruders by diving at them while calling loudly
- Use fecal sacs dropped by young as ammunition to drive off threats
- May physically attack much larger intruders like humans
- Dig new dummy burrows to distract predators like raccoons
- Burrows provide safety for adults and young to retreat from danger
Colonial nesting offers protection through early warning from alarm calls and mobbing attacks from multiple birds at once. Adults continuing to feed fledglings for several weeks post-fledging likely also helps keep them safe.
How does colonial nesting benefit bank swallows?
Nesting in dense colonies provides bank swallows with multiple advantages:
- Safety from predators through group mobbing behaviors and shared vigilance
- Social opportunities like extra-pair mating to maximize reproductive success
- Available mates each season without having to attract a lone migrant
- Microclimate benefits like shared burrow warmth and reduced temperature fluctuations
- Ease of information sharing about threats, food sources, etc. through social communication
The major trade-off is increased competition between colony members for burrow sites, nest materials, and mates. However, the overall benefits appear to outweigh the costs in most cases.
How many years does the bank swallow live?
The typical bank swallow lifespan in the wild is approximately 2-4 years. However, individuals can live up to at least 7 years based on banding records. The main causes of mortality include:
- Predation, especially of young swallows and eggs in the nest by snakes, raccoons, raptors, etc.
- Severe weather like storms, prolonged cold/heat during migration, and breeding season cold snaps
- Accidents, particularly collisions during migration or with vehicles near colonies
- Parasites and diseases such as internal protozoan blood parasites
- Food shortages on wintering grounds or migration stopovers
With average annual survival estimated between 30-55%, relatively few bank swallows reach the maximum lifespan. However, their high reproductive rate allows the species to thrive despite short individual life spans.
How does the bank swallow benefit its ecosystem?
As aerial insectivores, bank swallows play an important ecological role in:
- Insect population control – A large colony can consume over a ton of insects daily
- Nutrient cycling from insects on land to soil and water via their excrement
- Providing a food source to predators when caught
- Possible seed dispersal from plant material in their mud nests
- Burrowing nests that provide habitat for other species after abandonment
- Indicating ecosystem health by their population trends as highly sensitive to environmental changes
Declines in bank swallow and other aerial insectivore numbers may disrupt food webs and nutrient cycling. Their protection is important for maintaining healthy ecosystems.
What are the main threats to the bank swallow?
Some major threats facing bank swallow populations today include:
- Habitat loss – Riverbank stabilization, quarrying, construction destroys nesting sites
- Pesticides – Reduces insect prey populations and contamination from runoff
- Climate change – Alters suitable range, timing of migration and breeding
- Extreme weather – Storms, drought, heatwaves can wipe out breeding colonies
- Natural bank erosion – Causes burrow collapse and failure
- Lack of management – Active and inactive banks not maintained for nesting suitability
Protecting and managing key breeding habitats is crucial for conserving bank swallow populations into the future as they face these mounting threats across their range.
Conclusion
In summary, bank swallows exhibit a variety of fascinating behaviors and adaptations related to their nesting, migration, sociality, communication, foraging, and defense tactics. Their colonial breeding and aerial insectivory make them an important component of many ecosystems. However, they face increasing threats from human impacts and will require ongoing conservation efforts to maintain populations into the future.