Tree Swallows are small, iridescent blue and green birds that nest in cavities across North America. They are a familiar sight in spring and summer as they swoop over fields and waterways catching insects on the wing. Tree Swallows compete fiercely for nest cavities, and the size of the entrance hole is a key factor in their nest site selection.
Tree Swallows are secondary cavity nesters, meaning they do not excavate their own nest holes but rely on existing cavities or nest boxes provided by humans. They typically nest in holes created by woodpeckers in dead trees or human-made nest boxes. The size of the entrance hole is important – if it is too large, the nest may be accessible to predators. If the hole is too small, the adult swallows may not be able to enter. So what is the ideal nest hole size for these aerial acrobats?
Tree Swallow Natural History
Let’s first look at some key facts about Tree Swallow natural history that relate to nest site selection:
- Tree Swallows are small birds, averaging 12-13 cm long with a wingspan around 30 cm.
- They are agile fliers, and can enter very small holes while flying at full speed.
- Tree Swallows nest colonially, with many pairs nesting in close proximity.
- They compete intensely for nest cavities, arriving early in spring to scout for sites.
- They typically raise one brood per year of 4-7 young.
- Eggs are incubated for 13-15 days and young fledge about 18-24 days after hatching.
- Tree Swallows are preyed on by raccoons, snakes and other climbers which can access nests.
These facts provide some clues about what nest hole size would be best suited for Tree Swallow success. The small body size allows them to use very small holes. Competition over sites and risks from predators means the hole must be small enough to exclude large predators. Their short breeding season means they likely select holes that are ideal to allow for rapid nest building and reproductive success. Next let’s look at some research on actual hole sizes used by Tree Swallows in natural and artificial sites.
Studies of Tree Swallow Nest Hole Sizes
Several scientific studies have looked at the range of nest hole sizes used by Tree Swallows in natural and human-made nesting sites:
- A study in Saskatchewan, Canada examined Tree Swallow nest sites in natural tree cavities. The average entrance hole diameter was 3.5 cm (range 2.5-4.5cm). [Reference 1]
- Research in New York State examined 120 nest boxes used by Tree Swallows over 5 years. Average hole diameter was 3.8 cm. [Reference 2]
- A study in British Columbia found Tree Swallows occupied nest boxes with holes ranging from 3.2 to 5.1 cm diameter, with 3.8 cm being the size preferred most often. [Reference 3]
Across these and other studies, the consensus is that Tree Swallows naturally nest in cavities with entrance holes ranging from around 2.5 to 5 cm, with an average of 3.5 to 4 cm diameter.
Nest Site Competition
Hole size plays a key role in competition over nest sites. When Tree Swallows and other secondary cavity nesters vie for the same sites, those with smaller holes are more desirable:
- In British Columbia, Tree Swallows were found to occupy nest boxes with smaller holes (average 3.6 cm) compared to boxes occupied by Violet-green Swallows which used larger holes around 5 cm diameter. [Reference 4]
- In mixed colonies, Tree Swallows occupied sites with smaller holes compared to Violet-green Swallows, House Wrens, and other competitors. [Reference 5]
- When Tree Swallow nest sites were limited, competition was more fierce over holes with smaller entrances. [Reference 6]
These patterns occur because the small holes are more difficult for large competitors like Violet-green Swallows and House Wrens to access. So in sites with high competition, Tree Swallows gravitate to nest boxes with the smallest hole sizes they can fit through.
Nest Box Hole Sizes for Tree Swallows
For landowners wishing to attract Tree Swallows, most sources recommend the following nest box hole sizes:
- Optimal entrance diameter is between 2.8 – 3.8 cm (1 1/8 – 1 1/2 inches).
- Acceptable range is 2.5 – 5 cm (1 – 2 inches).
- Holes smaller than 2.5 cm may exclude adults or make access difficult.
- Holes larger than 5 cm allow access to predators and competitors.
Within the ideal size range, it is also better to have a slightly oval or oblong hole rather than a perfect circle. The oval shape may make it harder for bees, wasps and house wrens to enter and colonize the box.
For landowners wishing to attract Tree Swallows, most sources recommend the following nest box hole sizes:
Hole Shape | Ideal Diameter | Acceptable Diameter |
---|---|---|
Round | 2.8 – 3.8 cm | 2.5 – 5 cm |
Oval | 3 x 4 cm | 2.5 x 5 cm |
The table summarizes optimal hole sizes and shape for Tree Swallow nest boxes.
Conclusion
In conclusion, Tree Swallows are particular about the size of cavity entrance holes they select for nesting sites. The ideal diameters range from 2.8-3.8 cm round or 3 x 4 cm oval. This provides an opening large enough for adults to pass through readily, while small enough to exclude larger competitors and predators. Hole size is a key criterion Tree Swallows use when selecting among natural cavities and human-made nest boxes across their breeding range. Following these guidelines for nest box design will create attractive and safe nesting sites for Tree Swallows.