Four newly hatched tree swallow nestlings and two unhatched eggs on a bed of feathers
BY JESSICA HERNANDEZ 

Four newly hatched tree swallow nestlings and two unhatched eggs on a bed of feathers

It’s a sticky, humid afternoon in southwestern Virginia. Trucks are spraying manure across a sea of rolling hay fields. I’m downwind and standing in front of a wooden nestbox. AirBnB #73. As I lift the opening of the box, feathery missiles begin dive-bombing me, sharply turning away at the last second and skimming the top of my head. Tree swallows! And by the look of things, their shrieks (‘alarm calls’) are attracting more swallows to dive bomb me. Hurry up, Jess! I peek into the box. Intricately woven nest, bed of feathers on top, four newly hatched birds, two white teardrop-shaped eggs. Noted! I close the box and move on to the next one. One hundred and forty-five AirBnBs to go.

Over the past four years, I have been studying a local breeding population of tree swallows (Tachycineta bicolor) at Kentland Farm. Female and male tree swallows arrive in Blacksburg around late March each year, pair up, and then spend the spring and summer breeding. Contrary to popular opinion, female and male tree swallows seek out mates in addition to their social partner – a pattern prevalent in many bird species. Paternity analyses conducted on tree swallow nests at Kentland Farm have confirmed that there is variation in the number of fathers per nest, with some nests having nestlings sired by one father and other nests having nestlings sired by multiple fathers. My research focuses on understanding the costs and benefits associated with having multiple mates, a question that has perplexed behavioral ecologists for decades.

A female tree swallow defends her nest box against another female tree swallow, while her male partner watches from above and another tree swallow flies towards the scene.

In addition to research, another goal in setting up these nest boxes was to promote the conservation of tree swallows and other cavity-nesting birds. Tree swallows, for example, naturally breed in tree cavities, which have been rapidly disappearing as woodland clearing practices increase. Such practices have played a prominent role in the approximately 50% decline of tree swallow populations in the last five decades. While artificial nest boxes are not the solution to helping populations sustainably rebound, they provide much needed breeding cavities. Plus, setting up a nest box is something that can be done by people in their own backyard (see link below). The nest boxes set up by the Moore Lab at Virginia Tech, of which I am a member, have provided breeding cavities for over 2,000 tree swallows, as well as several eastern bluebirds and Carolina wrens over the past four years (2016-2020).

In non-Covid times I would be joined in the field by a crew of high school, undergraduate, and sometimes even fellow graduate students. Instead, I am alone in the field today checking in on the feathery AirBnB tenants. Just me, the smell of manure, and several hundred tree swallows perched on wires or acrobatically flying around catching insects in midair. Field research in the time of Covid.

A tree swallow perched on a telephone wire, illuminated by the early morning sun.
Inaugural 2017 Tree Swallow Crew

Helpful links:

  • To document the birds you see and add to a collection of data provided by researchers, hardcore birders, and newbie birders alike, check out: https://ebird.org/


Jessica Hernandez is an Interfaces of Global Change fellow in the Biological Sciences Department under the advisement of Ignacio Moore. She studies a free-living population of box-nesting tree swallows (Tachcineta bicolor) that form social pair bonds throughout the breeding season yet also engage in extra-pair copulations.