Behind the paper: returning home to study how box-nesting birds fend off their parasites

Blog written by Sarah A. Knutie, Assistant Professor at the University of Connecticut, USA. Read the full paper here.

As a first generation college student, I had little knowledge of the college process and what it entailed. At the University of Minnesota, I started as a computer science major because my mother showed me a newspaper article promoting women in the field of Information Technology (IT) and I enjoyed using HTML to create unofficial fan websites for celebrities (you’re welcome Leonardo DiCaprio).

After several years of coursework, I decided that computer science was not my passion so I started exploring all other possible majors. I enrolled in a few ecological field courses at the UMN’s Itasca Biological Station in northern Minnesota and after a few days of the courses, I had found my career path. Over the next decade, I decided that I wanted to return to Minnesota someday to establish a long-term research program to inspire and train undergraduates just like myself. Therefore, while pursuing my graduate degrees on unrelated projects, I spent all my “free time” during the summer months building connections with local residents, including Christmas tree farmers, at and near Itasca Biological Station.

Banding birds in northern Minnesota in 2008. Photo by Erin Feichtinger.

After finishing my PhD, for which I studied the effect of introduced parasitic nest flies on birds in the Galapagos Islands, I wanted to find a local system in the US to understand the variation in host defenses against native parasites. The perfect system finally caught my eye: box-nesting birds and their parasitic nest flies Protocalliphora sp.

Nest box in a field in northern Minnesota. Photo by Sarah Knutie.

Not only are these flies similar to the parasites that I studied in the Galapagos (they live in the nests and the larval (maggot) stage feeds on the baby birds), but I also could experimentally manipulate the parasites and environmental conditions to establish links among host, parasites, and their environment in Minnesota. Protocalliphora flies parasitize a wide range of bird species, but some of the most commonly-studied bird hosts are the eastern bluebirds and tree swallows.

Adult eastern bluebird. Image at the top of the blog shows an adult tree swallow. Both pictures by Jeremy Cohen.

In 2014, my father and I started building wooden bird nest boxes for eastern bluebirds and tree swallows. Over the next 4 years, we built and established close to 150 new nest boxes near Itasca Biological Station, which added to the existing 50 or so boxes that had already been set up by local residents.

My father, Steve Knutie, with the first few nest boxes that he built in my grandmother’s garage. Photo by Sarah Knutie.

The first step of the project was to determine the effect of the parasite on hosts and whether hosts could defend themselves against the parasite. I had already established working relationships with local residents, including co-author John Hurlbert from Bemidji’s Christmas [Tree] Forest. I recruited University of Minnesota undergraduate students, including first author Kirstine Grab and co-authors Allie Parker and Dasha Pokutnaya, to help with the fieldwork. I also collaborated with Dr. Brian Hiller from Bemidji State University and his undergraduate student McKenzie Ingram. I was fortunate to receive two small grants from the Minnesota Ornithologists’ Union and North American Bluebird Society to get this work started.

Former Christmas tree farmer John Hurlbert shows his granddaughter the tree swallows in his nest box. Photo by Sarah Knutie.
Dr. Brian Hiller and McKenzie Ingram at a bluebird nest box. Photo by Sarah Knutie.

My team first reviewed published studies on the relationship between the parasitic nest fly and bluebirds and swallows. We found that across space and time, bluebirds had over twice as many parasites compared to swallows but both bird species did not suffer a negative consequence of the parasite on their survival. These results suggested that bluebirds and swallows are both well defended against the parasite, but might defend themselves in different ways.

Recently hatched tree swallow nestlings. Photo by Sarah Knutie.

Two field seasons went by in Minnesota before we had enough pairs of birds occupying the boxes for the study. In 2016-2017, we started our study by experimentally manipulating the nest parasite (in other words, hosts were either parasitized or not) to determine the effect of the parasite on the growth and survival of the nestling birds. We found that the parasite did not affect the survival of either species. Therefore, both hosts were tolerant to the effect of the parasite at their respective parasite numbers. However, swallows had half as many parasites as the bluebirds. These differences were likely because swallow nestlings produced an immune response to the parasite, which reduced parasite numbers; in contrast, bluebird nestlings did not produce a detectable immune response to the parasite. We could not identify the specific way that bluebirds defend themselves against the high number of parasites to which they are exposed, but the birds are likely investing in tolerance measures to compensate for energy lost to the parasite or effectively repair damage caused by the parasite.

Tree swallow parent visiting a nest box. Photo by Jeremy Cohen.

After gathering these two years of data, I decided that it was time to publish the work. Since undergraduate education continues to be a theme in this research, I invited Kirstine Grab to spearhead the paper, from analyzing the data to writing the first and final drafts. This was Kirstine’s first scientific paper, and writing the paper with her was an incredibly rewarding experience for both of us. I am so proud of and encouraged by these early-career scientists.

First author Kirstine Grab holding baby bluebirds. Photo by Sarah Knutie.

Since 2017, my team has continued monitoring the effect of the parasite on bluebirds and swallows to determine whether there is annual variation in host-parasite relationships over a long time scale. The team has also been working hard to determine what environmental factors, such as food availability or temperature, affects host defenses against parasites. We have also established a citizen science project to determine the spatial distribution of nest parasites across the entire range of bluebirds and swallows throughout the United States. Please get in touch if you are interested in participating (

University of Connecticut undergraduate Alyssa Addesso with a nest from a citizen scientist. Photo by Sarah Knutie

I am very fortunate that I have been able to come full circle in my career. The past 15 years – from taking classes at the Itasca Biological Station as an undergraduate to mentoring students of my own at that same station – has been a long, but extremely rewarding, journey. My advice to those who are interested in pursuing a career in field research is to think long term. Where do you see yourself in 10-15 years? Can you start setting up your system now? Often times, these side projects just need a few small steps at a time but once you’ve taken enough of these steps, you might just create the center piece of your research program!

Sunset over Lake Itasca at Itasca Biological Station. Photo by Laura Domine.

I would like to respectfully acknowledge that the fieldwork was conducted on the ancestral territory of the Chippewa Nationwhere Clearwater and Beltrami counties, Minnesota, are located.

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