Progress in Understanding Loon Migration and Movements

By Dr. Walter Piper, Chapman University

Lines connecting banding sites with recovery sites for wintering adult loons (light blue) and wintering first-year loons (dark blue), summering adults (orange) and summering first-years (dark pink). Data: United States Geological Survey. Map: Google Earth.

Much has changed in the past half-century. A half-century ago I was a young boy who fell asleep each summer night hearing loon calls echo across the surface of Lake Temagami, in central Ontario. Now I am planning for my 29th consecutive year of research on the territorial behavior, nesting ecology, and conservation of loons in Wisconsin.

Our knowledge of common loon biology has improved immensely during the past few decades. The most significant advances have come about through the efforts of scientists to mark loons with both 1) colored leg bands to help them tell their study animals apart and 2) numbered USGS leg bands that allow the Bird Banding Laboratory in Maryland to track banding and recovery of marked individuals across the continent. As the third decade of intensive marking of loons draws to a close, we have accumulated enough recoveries — mostly individuals that died along lake shores or beaches years after banding — so that clear patterns have emerged in seasonal movements of loons. These findings are most complete for the intensively studied breeding populations of Minnesota and Wisconsin, which together make up 75% of the U.S. loon population.

Here is what we have learned so far about loons that breed in the Upper Midwest:

1. Roughly 80% of all Upper Midwest loons winter along Florida’s Gulf Coast (light blue and dark blue lines). The remainder winter along the Atlantic coast, mostly between Vero Beach and the Carolinas.
2. Loons from the two best-studied state populations (Wisconsin — green endpoints; Minnesota, red endpoints) winter side-by-side — each state sending the bulk of its breeding loons to the Florida Gulf Coast and the remainder to the Atlantic Coast.
3. Most loons too young to return to the breeding grounds migrate a short distance northwards, along the Atlantic Coast (dark pink and orange lines), and spend their summers there. A few stragglers travel farther north, reaching New Jersey, Massachusetts, and even Labrador.

Loons that are only 5 to 6 months old and migrating south for the first time commonly “overshoot” the main wintering grounds along the Florida Gulf Coast, which extends from about Pensacola to Fort Myers. These youngsters often end up in the Florida Keys or off of Ft. Lauderdale. (Note dark blue lines extending to south Florida.) We do not know for certain, but it is likely that most of these first-winter loons adjust their later migrations so that they winter near the rest of the population in their second and later years.

Why do we care about the wintering range of Upper Midwest loons? Because, of course, our loons must survive all twelve months of the year, not just the summer. The Loon Project’s paper published two months ago showed that the population of adult loons in northern Wisconsin has been declining for 20 years! After narrowing down the possibilities, we think it is quite possible that increased loon mortality during the winter might explain this worrisome drop, at least in part. So now we must watch out for loons all year long.