This post was contributed by Caroline Wolfe-Merritt, co-author of a recent paper in The Wilson Journal of Ornithology on the types of feathers used in Tree Swallow nests.
On a balmy morning in the beginning of September, I sit on a rock promontory just southwest of Yaki Point, gazing north into the Grand Canyon and scanning the sky for migrating raptors. Noticing motion against the red rocks of the canyon below, I lower my binoculars, pointing out a swath of birds to my crew of fellow HawkWatch International migration counters. Before us, are perhaps 400 small iridescent blue and white birds—not raptors, but Tree Swallows. I feel a smile creeping onto my face. What wonderful little birds! Some birders talk about “spark birds,” defining them alternatively as the species that got them into birding or the first bird they “chased.” Tree Swallows don’t fit that definition for me. I can’t pinpoint a single moment when I became interested in birds, or nature, or science—all of those things have always been a part of my life. But what I do remember, with startling specificity, is the moment I realized I could turn this deep interest in birds and ecology into a career.
It began a little over five years ago, in June of 2017. During the previous semester, I had been invited to join a team of undergraduates studying Tree Swallow nesting behavior. As we sat in one of the labs at Earlham College discussing our study design, it didn’t quite seem real, but a few weeks later I saw my first Tree Swallow nest. Guided by professors Wendy Tori and Adriana Rodríguez-Ferraro, our research team (Lilly Hartman Brenna Hellman, Evelyn Barragan, Sam Pigott, and me) learned to handle, measure, and weigh nestling Tree Swallows, and even before we pulled them from their nest boxes, it was clear that their nests were different from other passerine nests I had seen before. Rather than sticks and spiderwebs flung together in organized chaos by a House Wren, or a tiny grass cup lined with moss and lichen by a Carolina Chickadee, or even the pine straw constructions of an Eastern Bluebird, the nest of the Tree Swallow was chock-full of feathers. Feathers galore!
Whose feathers were they? As I looked into each nest, it was clear from the many colors, patterns, and sizes present that the feathers came from many birds. Tree Swallows don’t pluck their own feathers; instead, they line their nests with the molted feathers of other birds. In our research, my team and I set out to identify the feathers and characterize which sorts of feathers were most frequently included in Tree Swallow nest linings in eastern Indiana and western Ohio. Though other researchers had touched on the topic in investigations of the function of feather linings (e.g. Winkler 1993, Lombardo et al. 1995), we hoped to add a new level of precision by identifying feathers molecularly (as opposed to using visual comparison and microscopic feather structures). Furthermore, ours is the first study of its type conducted in the midwestern United States, and we wanted to see how feather use there compared to results from previous studies in other geographic regions.
As the summer progressed, nestling Tree Swallows began to fledge, leaving their nests available for us to investigate more closely. As we carefully counted and characterized the feathers, we sorted them by appearance, setting aside both typical and unusual feathers from each nest. Extracting DNA from these feathers allowed us to match each one with a species on GenBank—a publicly accessible library of genetic sequences maintained by National Center for Biotechnology Information. We selected additional feathers for identification in 2018, sequencing a total of 121 feathers.
As identification results rolled in, we were excited by the breadth of species and orders included. Tree Swallows were using feathers from birds as diverse as Great Blue Herons, Turkey Vultures, Great-horned Owls, Wood Ducks, and Hairy Woodpeckers! Some nests even included feathers from domestic birds such as Indian Peafowl and Helmeted Guineafowl. Cumulatively we identified feathers from 26 species, 19 families, and 11 orders. We identified feathers from 20 species of birds not previously recorded in Tree Swallow nests, though there was also some overlap with species identified in previous studies, including Wood Duck, Great-horned Owl, and Canada Goose. The feathers most commonly included in nests were brown, medium-length contour feathers, and nests contained between 4 and 175 feathers.
Early in the process of counting, characterizing, and identifying feathers, we noticed that at a site located near Canada Goose and vulture roosts, nests contained a multitude of feathers, many of which seemed to correspond to those birds. This observation translated into another research question: How does the habitat where nests are constructed impact the number, type, and identity of feathers used? Analyses revealed that while the number of feathers used in each nest varied by habitat (with more feathers used in nests near lakes than in nests in agricultural habitats), so did the specific species present (with 18 species identified in early successional habitat, 14 in agricultural habitat, 8 in nests near lakes, and only 4 in nests in wetland habitat). The species identified from nests in each habitat corresponded to the avian communities in those habitats, suggesting that Tree Swallows collect whatever feathers are available in the vicinity of their nests, rather than seeking out specific feathers. Expanding on the idea that Tree Swallows use feathers available in their local environment for their nest linings, we expect that this explains the geographical variation displayed across previous studies.
Over the course of this project, we spent countless hours in the field, in the lab, and behind our respective computers puzzling through statistics and revising many renditions of manuscripts. We made observations and asked questions, then devised ways to test and quantify the patterns we noticed. We learned to handle nestling Tree Swallows, explored methods of data collection, applied molecular tools to ecological questions, and trial-and-errored our way through R analyses. For some of us, conducting this study served as a springboard for developing interests in ecology, birds, and the environment, for others it opened doors into graduate studies, and for others it was a continuation of a career in field ornithology. For me, sticking with this project from beginning to end opened my eyes to the realities of conducting research and allowed me to take my first steps into the world of avian ecology, a path I continue to follow as an early-career field biologist with a focus on birds and conservation.