Tree Swallow
| Scientific Name | Tachycineta bicolor |
| Survival Strategy | Long-distance migrant |
| Arrival in Upper Midwest | Late March |
| Departure | July and August |
| Destination | Florida, Mexico, and Central America |
| Conservation status | Least Concern |
Spring
Tree swallows return to the Upper Midwest in late March, arriving a few weeks after the eastern bluebirds. The female lays 4 to 7 eggs shortly after she builds her nest. She will incubate the eggs for 14 to 15 days and brood the young for 18 to 20 days. During this time, the male provides food for the female and the hatchlings. Both parents continue to provide food for three days after fledging.
Summer
After the young have fledged, typically in late June, tree swallows begin forming flocks of increasing size. They leave the Upper Midwest in July and August. Flying by day in large flocks, sometimes numbering in the thousands, they follow the Mississippi River southward to the insect-rich wetlands of the southern Gulf Coast, Mexico, and Central America.
Fall
By mid-fall, most tree swallows have left their summer range.
Winter
In the winter range, tree swallows will feed on insects and berries. Cold weather can reduce the number of flying insects available and increase the risk of hypothermia.
Behavior
Battle for Nest Boxes
Tree swallows compete for the same limited number of natural tree cavities or bird boxes also favored by bluebirds. The early spring battles for prime nesting real estate begin in March and simmers to a truce when both species are incubating eggs.
Flocks
Shortly after the young have fledged, large groups of tree swallows will congregate, evenly spaced, on fences and powerlines. With any disturbance, they take to the air en masse and continue feeding. The flocks will increase in size as the time for migration draws nearer. These birds will roost at night in flocks that number in the thousands during migration.
Physiology
Molts
Tree swallows begin to molt in early summer. This replacement of flight and body feathers may continue even after the fall migration has started. The birds will complete their molt by late October.
Tree Swallows Have A Lot of Guts
Tree swallows can survive several days in cold weather, thanks to their long intestines, which allows these birds to digest plant material as well as insects, unlike their relatives. When low temperatures keep cold-blooded flying insects “grounded,” tree swallows can get the energy they need from berries and seeds.
Iridescence
The iridescence seen in tree swallow feathers results from light refracted by the proteins in the feather barbules. Acting like a million prisms (or raindrops), the color that is refracted changes as the viewing angle changes.
Diet
The summer diet of these beautifully acrobatic fliers consists almost entirely of flying insects. This high-protein food is essential for growing chicks, which must be strong enough to migrate in about three months. When available in their winter range, tree swallows will eat the fruit of the southern wax myrtle (Morella cerifera) along with insects. This waxy fruit’s high-fat content provides the needed energy to survive the cold weather of early spring.
Lifespan and Mortality
The tree swallow’s average life span is 2 to 3 years. Some individuals will survive eight or more years. About 80% of fledglings will not survive their first year. Causes of mortality include nest predation, starvation, and weather-related hypothermia.
Predators
Predators that feed on tree swallow eggs and nestlings include raccoons, American kestrels, American crows, northern flickers, deer mice, and feral cats. House wrens will take over nest cavities after destroying eggs and killing hatchlings.
Climate Vulnerability
One study showed that tree swallows are laying eggs nine days earlier, possibly due to increased spring temperatures.
Another study, using population data collected over 43 years, found a correlation between increasingly rainy spring weather conditions and a decline in the survival of nestling and post-fledging tree swallows. Early spring weather is typically unstable, with more significant temperature swings and precipitation. Insects do not fly in cold weather, making food scarce for aerial insectivore parents; at the same time, hypothermia threatens their young.
Never stop learning
Etymology: The tree swallow’s scientific name is Tachycineta bicolor. “Tachycineta” is from Ancient Greek “takhukinētos,” which means “moving quickly,” and “bicolor” is Latin for “two-colored.”
References
| Wikipedia: Tree swallow https://en.wikipedia.org/wiki/Tree_swallow |
| The Tree Swallow Farmer: David Winkler (Cornell Lab of Ornithology) https://www.allaboutbirds.org/news/the-tree-swallow-farmer-david-winkler/ |
| How to Create and Manage Tree Swallow Nest Box Projects http://www.treeswallowprojects.com/cafter.html |
| Quality of diet, not quantity, is key to health of baby birds https://news.cornell.edu/stories/2016/09/quality-diet-not-quantity-key-health-baby-birds |
| eBird: Tree Swallow Abundance Animation https://ebird.org/science/status-and-trends/treswa/abundance-map-weekly |
| Winkler, D. W., K. K. Hallinger, D. R. Ardia, R. J. Robertson, B. J. Stutchbury, and R. R. Cohen (2020). Tree Swallow (Tachycineta bicolor), version 1.0. In Birds of the World (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.treswa.01 |
| PO Dunn, and DW Winkler, Climate change has affected the breeding date of tree swallows throughout North America: Proceedings of the Royal Society of London, Series B: Biological Sciences [Proc. R. Soc. Lond., Ser. B: Biol. Sci.], vol. 266, no. 1437, pp. 2487-2490, Dec 1999. https://royalsocietypublishing.org/doi/10.1098/rspb.1999.0950 |
| Cox, A.R., Robertson, R.J., Rendell, W.B. et al. Population decline in tree swallows (Tachycineta bicolor) linked to climate change and inclement weather on the breeding ground. Oecologia 192, 713–722 (2020). https://doi.org/10.1007/s00442-020-04618-8 |
| Millions of Mirrors – Iridescent Coloration in Birds http://www.avianbiology.org/blog/millions-mirrors-iridescent-coloration-birds |
| How Birds Make Colorful Feathers https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/ |
| Creating and Managing Tree Swallow Nest Box Projects https://treeswallows.com/tres/ |