This post is the first of a two-part series on a day in the life of a behavioral ecologist, which features the stories and research of members of Dr. Kim Rosvall’s laboratory in the Department of Biology.
When most people imagine a scientist, they picture a person wearing a stark white lab coat and thick-rimmed goggles, hunching over a lab bench and watching a solution bubble and steam as a chemical reaction takes place. However, many scientists — including biologists, anthropologists, geologists, and sociologists — conduct part or all of their research in the field. While some scientists travel abroad to conduct fieldwork, members of Dr. Kim Rosvall’s laboratory in the Department of Biology have the advantage of conducting their research locally at field sites in and around Bloomington.
The Rosvall lab studies the genomic and physiological mechanisms of behavior in songbirds. One of the lab’s primary areas of interest is how physiological changes modulate same-sex aggression in females, a topic that has historically been understudied in birds and mammals, despite observations that females can be quite aggressive in nature. To explore these mechanisms, the Rosvall lab uses a charismatic songbird species, the tree swallow (Tachycineta bicolor). These birds, which are notable for the vibrant, blue-green iridescent feathers on their backs, migrate each spring from the Gulf coast to Indiana to breed and raise their young. Once they arrive in Indiana, tree swallows build their nests in cavities, including holes in trees and bird boxes. Importantly, tree swallows are obligate secondary cavity nesters, which means that their entire reproductive success depends on successfully acquiring and defending a nesting site; but, unlike a woodpecker, they are unable to excavate a cavity for themselves. Interestingly, this pressure to find a cavity means that females are actively involved in territorial defense against other females, making this species ideal for studying female-female aggression.
During their breeding season, tree swallows typically reach Indiana in late February and early March. As they arrive, the birds actively explore the 300 or so wooden nest boxes that Rosvall lab members have placed at their field sites around Bloomington and determine which cavities they would like for nesting and raising offspring for the breeding season, a process that often involves conflict between individuals. During this territory establishment phase, lab members give each bird a tiny set of unique bands on their legs, including a passive integrated transponder (PIT) tag, an electronic tag that is equipped with a unique code for each bird. As the birds travel into and out of nest boxes, an antenna, which is placed near the holes of the nest boxes, scans for PIT tags and transmits this information to a radio frequency identification (RFID) board located just below the nest boxes. These RFID boards log PIT tag information onto a data file, which can then be analyzed back in the lab. Thus, this RFID technology allows the Rosvall lab to track the birds’ movements to and from individual nest boxes, which can reveal which birds ‘own’ each nest box, how often intruders investigate boxes, and how the exploratory behavior of birds varies throughout the breeding season.
Once tree swallows have chosen their territories, the lab experimentally manipulates nest box availability to change the birds’ social environment using a design that is similar to the childhood game of musical chairs. For these manipulations, lab members venture to their field sites after sunset and take down nest boxes under the light of head lamps, during a time when birds are naturally roosting elsewhere in trees. The lab will then travel back to their sites before sunrise the following morning, which allows them to observe the birds as they return to their nesting sites, discover that their ‘homes’ have been misplaced, and aggressively fight to establish new territories. After the “winners” of each fight have been determined and the birds begin defending their new nesting sites, lab members will catch these female residents, as well as undisturbed control females, and use blood samples from these individuals to assess physiological changes that occur in response to social competition.
During the incubation period of the field season (e.g, after females have laid their eggs, but before their chicks hatch), the Rosvall lab uses simulated territorial intrusions (STIs) to measure aggression in female tree swallows. For these behavioral tests, which are typically 5 minutes in length, a custom-made 3D-printed bird decoy is placed in front of a female’s nest box hole. Tree swallow fighting noises are played directly under the nest box using a set of speakers, which gives the bird decoy a voice, and the female resident’s interactions with the decoy are observed and audio recorded. The aggressive behaviors displayed by individual females vary considerably and can range from passivity, such as circling or sitting on their nest box; to outward aggression towards the decoy, including diving, attacking, and pecking. These behaviors are ultimately used to calculate an aggression score, which is proportional to the amount of time the female spent aggressively responding to the decoy during behavioral testing.
A female tree swallow approaches a wooden nest box with a bird decoy during a simulated territorial intrusion (STI), a behavioral assay that the Rosvall lab uses to quantify aggressive behaviors during different stages of the breeding season. This female is particularly unaggressive, whereas other females attack the decoy repeatedly during behavioral testing. Video reused with permission by Dr. Kim Rosvall.
After their field season ends, the Rosvall lab returns to campus and begins the lengthy process of processing and analyzing the behavioral data and samples they collected. Two lab members, graduate student Elizabeth George and postdoctoral researcher Alex Bentz, are particularly interested in how female tree swallows physiologically adapt to increased competition. Elizabeth’s research has shown that female tree swallows are capable of modulating circulating levels of testosterone in response to their social environment, but only during early stages of the breeding season, when competition for territories is most intense and aggressive behavior is highest. This adaptation, she suggests, may prevent the disruption of maternal care, which takes place during the latter parts of the breeding season. Likewise, Alex’s recent work provided evidence that changes in ovarian gene regulation may contribute to seasonal variation in testosterone levels in females and that the brain changes dramatically from territorial establishment to later parenting stages. Currently, both Elizabeth and Alex are comparing hormone levels and the expression of hormone-related genes in female tree swallows across breeding stages and in response to short-term social competition, such as that produced by nest box manipulations. These techniques will allow them to determine whether plasticity in these particular mechanisms drives seasonal changes in aggression and how competition affects the female brain. Together, Elizabeth and Alex’s research will further our understanding of what biological processes enable animals to respond to changes in their social environment, an especially pressing issue in light of ongoing global change that is altering habitat availability for many wild animals, including the tree swallow.
Stay tuned for the second post of this two-part series, in which I will discuss how the Rosvall lab’s research is shedding light on the effects of early-life stress on aging!
Acknowledgments: I would like to thank Dr. Kim Rosvall, Elizabeth George, and Dr. Alex Bentz for sharing their research and field experiences with me and for their valuable suggestions for this post.
Edited by Taylor Nicholas and Katherine VanDenBurgh
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