Tree swallows are small, migratory songbirds that breed across much of North America. They are aerial insectivores, meaning they catch insects while flying. Tree swallows build nests in tree cavities or nest boxes, and raise one brood per year. A common question among bird watchers and researchers is whether individual tree swallows return to the exact same nest site year after year.
Quick answer
Research shows that tree swallows exhibit high nest site fidelity, meaning they do tend to return to the same cavity or nest box year after year. However, it is not an absolute rule. Some swallows may switch nest sites from year to year, while others consistently reuse the same cavity for multiple years. Site fidelity appears to be stronger among older, more experienced breeders.
Do tree swallows use the same nest?
Multiple banding studies have found that the majority of tree swallows return to within 300 feet or less of where they previously nested. For example, one study in New York found that 78% of female swallows returned to within 300 feet of their prior nest box. Strong nest site fidelity has been documented across much of the tree swallow’s breeding range, from Ontario to Tennessee to Alaska.
Furthermore, even when provided plenty of unused nesting sites, tree swallows prefer to reuse old nests. In an experiment in British Columbia, researchers installed nest boxes in an open field. The boxes were left up year-round so old nests remained inside. They found that returning swallows selected boxes with old nests 90% of the time. Only 10% of pairs built new nests in unused boxes.
However, tree swallow fidelity varies between sites and regions. At more northern latitudes, cold weather may frequently destroy old nests over winter. One 20-year study in Alaska found that just 44% of female swallows reused the same box. Nest site fidelity also tends to be lower at sites with high competition for cavities.
Why do tree swallows reuse nests?
Researchers propose several potential benefits to reuse of old nesting sites by tree swallows and other birds:
- Revisiting a familiar site may improve breeding success compared to starting over in a new, unknown location.
- Old nests provide sanitary benefits like reduced parasite loads.
- Reuse saves time and energy compared to building a completely new nest.
- Familiarity with foraging sites near the old nest may increase provisioning efficiency.
Supporting some of these hypotheses, one study found that pairs nesting in reused cavities fledged more offspring than pairs nesting in new sites. Reuse may signal possession of a high-quality site worth returning to.
Do both males and females return to the same site?
Most research shows that male and female tree swallows exhibit equal fidelity to their prior breeding site. For instance, one 5-year study found no differences between males and females in nest box reuse rates or distances between old and new nests.
However, other factors may influence which sex is more likely to return. Males tend to arrive at the breeding grounds first in spring, and competition for cavities is fierce. If a male arrives to find his old site occupied, he may be forced to settle elsewhere. Females are more likely to return to an established mate if he secures the same nesting cavity.
Does age affect nest site fidelity?
Several studies indicate nest site fidelity increases with age and breeding experience in tree swallows. Young swallows nesting for the first time appear more likely to switch sites in future years compared to older birds.
For example, one analysis found that only 5% of yearling female swallows reused the same box the next year. But 45% of older females returned to the same box. Site familiarity and social bonds may strengthen fidelity in older, more experienced breeders.
How far do tree swallows migrate?
Tree swallows that breed in northern latitudes migrate long distances in the fall to warmer southern regions. Banding studies show winter ranges extending from the southern U.S. through Mexico, Central America, and into northern South America.
However, migration distance depends greatly on latitude. For instance:
- Swallows in Alaska may migrate over 4,500 miles to Central America.
- Swallows in Canada’s Northwest Territories migrate around 3,000 miles to the U.S. Gulf Coast region.
- Swallows in New York migrate around 1,000 miles to the southeastern U.S. coast.
Despite these long journeys, most swallows manage to return to within a few hundred feet of their prior nest site the following spring. Their navigational abilities are remarkable.
Do tree swallows remember their old nest sites?
Researchers believe tree swallows rely on spatial memory to recognize quality nesting habitat. Older swallows likely memorize visual cues around successful nest sites while breeding there. They then use these memorized landscape features to precisely return to the area the next year.
Several experiments support the importance of visual cues for nest site recognition. For example, one study rotated nest boxes between seasons, disrupting the visual cues. This reduced nest box re-occupancy rates compared to unchanged sites. Spatial memory apparently guides swallows back to familiar sites.
What factors influence nest site selection?
When initially selecting a nesting cavity, tree swallows likely assess several habitat factors:
- Cavity dimensions – Preferred diameter is around 4 inches wide x 6 inches deep.
- Height above ground – Optimal height is 5 to 15 feet high.
- Entrance size – Favor cavities with entrance holes around 1.5 inches wide.
- Protective cover – Shrubs and branches help shelter the cavity.
- Water proximity – Nest sites are often located near water bodies.
If a site successfully fledges young, swallows will remember and return to sites matching these criteria. However, suitable nesting cavities are limited, so even imperfect sites may be reused.
Do weather conditions affect nest fidelity?
Severe weather during the breeding season may reduce nest site fidelity in subsequent years. For example, one study found that after a year with unusually cold and wet weather, only 9% of female tree swallows reused the same nest box.
Poor weather likely reduced reproductive success, so swallows sought better sites the next season. In contrast, 54% of females returned to the same box after a more normal breeding season.
Cold snaps during spring migration may also lower return rates by increasing mortality. Tree swallows appear sensitive to ecological conditions in whether they reuse nests.
How does cavity competition affect nest fidelity?
Competition with other cavity nesters can override nest site preferences in tree swallows. Species like house wrens, chickadees, and house sparrows may fight swallows for control of nest sites.
At sites with intense competition, swallows are less likely to occupy their former cavity. One study found reuse rates declined from 60% with no competitors to just 29% with competitors present.
Dominant species forcing swallows to shift nests year to year is fairly common. Managing sites to reduce competition can improve nest reuse rates.
Do tree swallows ever switch mates between years?
Tree swallows typically form new pair bonds each breeding season, rather than maintaining long-term pairings. One study found that only 5-15% of pairs remained together in consecutive years.
However, even when they switch mates, swallows often return to the same nesting area. Strong site fidelity brings birds back to recently successful areas, where they may form new pairings.
For example, a female swallow may return to breed again with the male who nested in the next box over the previous year. Mate switching within tight nesting clusters is common.
What techniques confirm nest site fidelity?
Several methods help researchers study nest site fidelity in swallows and other birds:
- Banding – Attaching numbered metal bands allows tracking of individuals.
- Radio telemetry – Using radio tags to remotely track movements.
- Nest cameras – Recording footage to identify returning individuals.
- Feather isotopes – Comparing signatures helps match birds between years.
- Genetic analysis – DNA can confirm matches of the same individuals.
Combining banding with one or more other methods provides reliable confirmation that specific swallows have returned to the exact same cavity or nesting area.
Do any other bird species exhibit nest fidelity?
Many types of birds, from songbirds to seabirds to birds of prey, show nest site fidelity across years. Examples include:
- Barn swallows
- Ospreys
- Bald eagles
- Flamingos
- Penguins
- Wood ducks
- Burrowing owls
- Purple martins
- Common terns
Reuse of old nests provides familiarity and saves time and energy for pairs. It is an adaptive strategy observed across diverse avian taxa.
Conclusion
Research overwhelmingly demonstrates that tree swallows have a strong tendency to return to the precise cavity or nest box where they previously bred. High nest site fidelity, likely based on spatial memory, is well documented across the swallow’s range.
However, a variety of factors like weather, competition, and age may cause some individuals to switch nesting sites between years. Tree swallow fidelity is a tendency, not an absolute rule. Managers can support reuse behavior by providing clean nest boxes in optimal habitat locations.
