I did my graduate work at the University of North Carolina-Chapel Hill. My advisor and I spent tooth-chattering mornings from November to April sitting in blinds and watching 300+ color-marked white-throated sparrows compete for seeds at feeding stations. We learned a good deal from this work. We now understand how aggressive behavior affects the survival and local movements of this species.

Many lake residents and outdoorsy types in Wisconsin and Minnesota recognize white-throated sparrows from another time of year. These striking little birds sing distinctive, whistled songs in late May and June along forest edges in the Upper Midwest. Although they are more understated, the calls of white-throats typify northern boreal forests much as loons’ calls symbolize northern lakes.

White-throated sparrows live two very different lives in summer and winter. We witnessed part of the transition between these lives in North Carolina. During early May, the undisciplined hordes of sparrows we had grown accustomed to seeing broke up into smaller flocks. In mid May, they spurned the seeds we offered and fed instead on protein-rich buds they found in treetops. By late May, our sparrows had left for the north country.

The departure of our study animals left us with mixed feelings. On the one hand, it gave us a much-needed break from the daily grind of field work. On the other hand, it left a void and a puzzle. Where had our birds gone? Were they New England breeders that serenaded summer hikers in the White Mountains of New Hampshire with their plaintive calls? Or had they headed inland towards the Upper Great Lakes where they crooned to cottagers on lakes? Perhaps they had ventured to northern Ontario, Manitoba, or even the Northwest Territories. If so, they likely belted out their songs with no human around to enjoy them.

I spent many hours pondering summer destinations of our wintering sparrows. I felt — quite irrationally — that there must be some way to learn where they migrated to breed. Could some distinctive wisp of vegetation or a sticky residue from berry or insect from the breeding quarters become lodged in their feathers or stuck to the bill; survive the journey southwards; and still be detectable in North Carolina in October? Perhaps. But no researcher, to my knowledge, has ever determined the migratory origin of a small songbird by such a means. Eventually I gave up on my fanciful notion of learning where our sparrows spent the six months when they were not in our company.

Other scientists did not give up on the dream of linking breeding and wintering areas. Indeed, for a few decades now scientists have used stable isotope analysis to detect geographic movements of itinerant animals. Stable isotopes are different versions of a chemical element with different masses. For example, most hydrogen atoms (over 99.9%) occur as H-1, which has no neutron in the nucleus. But a few hydrogen atoms take the form of H-2, which possesses one neutron and weighs twice as much. Here is the key point: water droplets in rainfall in each geographic region contain a characteristic tiny percentage of H-2 (.02% in one place, .008% in another). And living things absorb water so that they too contain a ratio of H-2 to H-1 in their bodies that is distinctive to the region they inhabit.

Measurement of stable isotopes is especially useful to bird biologists because of feathers. Feathers are not living tissue. They are keratinized structures that grow from living tissue and, once formed, no longer receive a blood supply. So the stable isotope ratio within feathers does not track the current environment like that in skin, blood, or muscle. Instead, feathers are a time capsule that contains the stable isotope ratio at the time and place of their formation. When a bird grows new feathers in one place and migrates to another, its feathers retain the stable isotope signature of the molting location.

Loons grow fresh wing feathers on the wintering ground prior to migration. This means that wing feathers of breeding loons bear the isotopic signature of their winter quarters. Thus, a small wing feather we clip in July can tell us where a specific breeder spends its winters.

We have some understanding of loon migration and wintering patterns, thanks to the work of Kevin Kenow, Jim Paruk, and their co-workers. Furthermore, recoveries of banded loons have helped us sketch out the wintering range of our Wisconsin and Minnesota breeders (see map below). However, we cannot tell where any particular loon — like the territorial female on Roosevelt-Southwest — winters.

Sources and destinations of loons banded in Wisconsin and Minnesota. Light blue lines are paths of adults; dark blue lines indicate first-winter birds.

Why does it matter where a specific territorial loon spends the winter? Because we have burning questions about conservation of Wisconsin and Minnesota loons that require fine-scale understanding of migratory patterns. For example, does use of certain wintering areas lead to low survival for Upper Midwest breeders? Do poor years for adult returns in Wisconsin and Minnesota correspond to “die-offs” in specific wintering areas? Does the low annual return rate of adults in Minnesota (compared to Wisconsin) indicate unfavorable conditions in their more westerly winter range (see figure above)?

In fact, we have many more questions of this kind. To answer some, we will need ecological data from the Gulf of Mexico and Atlantic Coast. At the moment, however, subtle information locked within the feathers of our study animals and transported by them from wintering to breeding grounds is giving us hope that we will soon have a better understanding of common loon survival throughout the year.

Thanks to Linda Grenzer, who took the cool photo of the male from Halfmoon Lake coming in for a landing some years ago.