Bank swallows are small, migratory songbirds that nest in colonies in natural and man-made settings with vertical surfaces. They build nests out of mud in cavities or on vertical surfaces, often on riverbanks, ocean bluffs, and under bridges and highway overpasses. Bank swallows are found throughout North America and are drawn to open areas near water where they can gather mud and insects.
When do bank swallows nest?
Bank swallows arrive at their breeding grounds between early April and early May. The exact timing depends on the location and weather conditions. They are one of the earlier swallow species to arrive in the spring. Once bank swallows return from migration, they immediately begin searching for nest sites and excavating burrows. Nest building and egg laying occurs from mid-May through June. The birds raise one brood per season. Bank swallow nesting colonies are very active places in late spring and early summer as adults fly in and out provisioning their young.
Where do bank swallows build nests?
Bank swallows nest in colonies ranging from a few to a few thousand nests. Their preferred nesting sites are vertical banks and bluffs with fine-textured or sandy soils near open aquatic habitats like rivers, lakes, ocean coasts, ponds, and quarries. Some common nesting areas include:
- Riverbanks and bluffs
- Ocean cliffs
- Gravel pits and quarries
- Reservoir banks
- Roadcuts through hilly areas
- Under bridges and overpasses
- Stock pond dams
The vertical nesting sites provide bare soil that makes excavating burrows easier. Proximity to water provides a source of mud for nest construction and abundant insects for feeding nestlings. Nest height above water or ground ranges from just a few feet up to around 80 feet.
What are bank swallow nests made of?
Bank swallow nests are intricately constructed burrows dug into vertical banks or cliffs. The nest cavity is made entirely of mud. To build the nest, the birds collect mud in their beaks from nearby wetlands or stream edges. They carry globs of mud back to the colony and sculpt them into a gourd or flask shaped structure attached to the vertical surface. The nest entrance is a rounded opening leading into a tunnel ending in a nest chamber. Typical dimensions of bank swallow nest burrows are:
- Entrance diameter: 2.2 – 3.9 in (5.5 – 10 cm)
- Tunnel depth: 16 – 39 in (40 – 100 cm)
- Nest chamber depth: 4 – 16 in (10 – 40 cm) below tunnel end
- Chamber width: 4.7 – 6.3 in (12 – 16 cm)
- Total depth: 16 – 59 in (40 – 150 cm)
Bank swallows excavate their burrows using their bills and feet to chip away at the soil. It takes the birds 1-2 weeks to construct a complete nest burrow. Both males and females participate in nest building. The burrows are clustered close together, often sharing walls between adjacent nests. Nest density varies by site but may reach up to 2-3 nest entrances per square foot. The tight quarters provide shelter and help stabilize the colony in the soft soils.
What does the inside of a bank swallow nest look like?
The interior of a bank swallow nest consists of simply the bare earthen chamber and tunnel. Unlike many bird species, bank swallows do not build an inner cup or add any nesting material inside their burrows. The nestlings rest directly on the dirt floor:
- No nest lining or nest cup
- Bare earthen chamber and tunnel
- Smooth interior molded from mud
- Nestlings rest on dirt floor
The lack of nest lining may be an adaptation to reduce parasites. The smooth mud interior likely helps repel crawling insects that could infest nests. Bank swallow chicks have sparse feather tufts that help insulate them against the dirt.
How do bank swallow eggs and chicks stay in the nest?
Baby bank swallows have several adaptations that help them survive in their unique nest environment:
- Eggs have blunt, symmetrical shape – less likely to roll
- Chicks have flank feather tufts – insulate against dirt
- Feet located far back – grip nest wall as they grow
- Short flattened wings – avoid hitting burrow walls
The semicircular chamber provides just enough room for the adults to access the nestlings. The nestlings are unable to move around much within the nest. As they grow, their feet and flattened wings help wedge them in place.
How do bank swallow parents care for the young?
Bank swallow parents share duties feeding and caring for their clutch of 4-5 eggs and nestlings. Typical behaviors include:
- Incubate eggs for 13-16 days
- Brood hatchlings for first few days
- Make frequent trips to collect flying insects
- Return to nest up to hundreds of times per day
- Deliver whole insects to chicks
- Remove fecal sacs from nest
- Nestlings fledge at age 22-27 days
The nestlings remain in the burrow relying completely on their parents to supply food. The adults catch insects during continuous aerial foraging then quickly fly back to deliver the intact insects directly into the gaping mouths of the chicks.
What features do bank swallows need in nesting sites?
When selecting nesting colonies, bank swallows require sites with stable vertical banks or cliffs that provide:
- Vertical banks with fine-textured, stabilized soil
- Open habitats with clearance for aerial foraging
- Bare soil without thick vegetation for burrowing
- Overhangs for shelter from rain and sun
- Proximity to water for collecting mud
- Height above land for protection from predators
If the soil is too hard packed, too crumbly, or covered in vegetation, the birds may abandon the site. Erosion and slumping banks can ruin nests. Colonies are more successful when conditions allow burrows to persist over multiple years.
How does weather affect bank swallow nesting?
Bank swallow nesting success is heavily influenced by seasonal weather patterns. Some effects of weather include:
- Rain – Delays nest building; erodes burrows
- Drought – Reduces insect prey abundance
- Cold snaps – Slows insect development; increases chick mortality
- Heat waves – May overheat vulnerable chicks in burrows
- Strong winds – Makes aerial foraging difficult
- Severe storms – Can destroy nest colonies
Heavy rainfall early in the breeding season can be especially detrimental if it collapses burrows and washes away nesting substrate. Bank swallows exhibit lower reproductive success during wet, cold springs compared to warm, dry years.
How do bank swallow populations change over time?
Bank swallow populations naturally fluctuate from year to year depending on nesting conditions and mortality factors. However, long-term trends over past decades show steep declines in many regions:
- Declined 33% from 1966-2015 across North America
- Classified as Threatened in Canada
- Endangered listing in IL, IN, OH, PA
- Steep declines in eastern and midwestern states
Habitat loss, climate change, pesticides, invasive species, and extreme weather events may contribute to diminishing populations. Protecting and managing key nesting areas will be important for recovery efforts.
Conclusion
Bank swallows exhibit remarkable nesting behaviors tailored to their specialized habitat. By burrowing into steep banks near water, they create highly efficient mud nests that become lively bird apartments each spring. Paying close attention to suitable substrates and protection from the elements allows colonies to successfully fledge young. However, bank swallow populations face increasing threats from human impacts and climate change. Maintaining suitable nesting habitat will provide critical refuge for these aerial acrobats to continue their elaborate construction projects each breeding season.