Eat Prey Loon: lessons from juvenile loons in Wisconsin

by Brian A. Hoover, Andrew L. Rypel and Walter H. Piper

Do you remember when you first moved from home, and were completely on your own in new surroundings? How did you decide where to live, or which restaurants to try for the first time? Did you try places randomly, or did you seek familiar food chains and businesses where you knew what to expect? 

Familiarity comes from experience, and familiarity can be important when an animal finds itself in a new habitat. Familiarity means you have previous experience with a specific situation, and so have a good idea of what to do (and what not to do!) if you find yourself in similar conditions again. For example, a young animal that learns how to find food and avoid predators in one habitat can apply those same lessons in a new similar habitat. But what if this same individual visits new habitat, which offers a different type of food that needs collecting in a different way? Learning new skills requires practice, and practice takes time. For young animals just starting out on their own, there might not be much time, or margin for error.

Identifying familiar habitats can be beneficial, but which habitat traits actually matter? A new study (https://onlinelibrary.wiley.com/doi/pdf/10.1002/ece3.7134) examines this question for juvenile common loons (Gavia immer) in lakes in northern Wisconsin. In central California we generally see loons in the winter, mostly in coastal ocean waters and also at some large reservoirs in Solano and Yolo County. But in summer, these large birds are icons of northern Minnesota, Wisconsin, New England, and Canada (e.g., the Canadian one-dollar coin is engraved with a loon and affectionately known as the “loonie”). There, loons thrive within landscapes carved by glaciers and dotted with natural lakes. Loons are highly territorial, with a single pair often defending an entire small to medium-sized lake. This territoriality leads to the loons’  famous vocal wails and yodels, which carry across the lakes and warn of dangers or intruders. Below is a short clip of one of the study bird adults wailing on Silverbass Lake in northern Wisconsin in 2020.

Loons raise 1-3 chicks in the summer, feeding them a variety of small fish, crayfish, leeches, snails, and other small food items. In the fall, chicks increasingly forage by themselves, and by the time they fledge, are on their own. After fledging, juvenile loons start visiting nearby lakes. Working with loons, the difficult part is identifying which birds are which and where they go! Fortunately, the Loon Project, started by Dr. Piper in 1993, has been banding adult and juvenile loons in Oneida and Vilas County in Wisconsin for 28 years, and the unique color band IDs given to each loon enable observers to quickly ID birds from shore or canoes. Consequently, observers can quickly track which juvenile loons are present when they start to visit different lakes in the study area.

Juvenile loons showing color bands on Muskellunge Lake. Photo credit: Linda Grenzer

This banding system allows us to understand where these juveniles are going, and what kinds of lakes are they visiting. And how the lakes of north Wisconsin differ from each other, in terms of their habitats.  One of the authors, Andrew Rypel, has been examining this last question in terms of fish communities and physical lake features (Rypel et al. 2019), and identified “classes” of lakes, based on distinct fisheries and their relationship to lake size and trophic complexity (complex – simple), temperature (warm – cold) and water clarity (dark – clear). Using these lake classes and physical lake variables, the study found that juvenile loons in the region follow a specific pattern:

1) Regardless of the lake that  juveniles grow up on, ALL juveniles prefer visiting large and complex lakes.

2) Juveniles prefer visiting lakes with pH similar to their home lake.

Randomization test showing the average pH difference (red vertical line) between the origin lake of juvenile loons and the destination lakes they visited. The histogram simulates what the pH differences should look like randomly if you calculate the pH difference between loon lakes and all other lakes in the study area (not just destination lakes)…. and then repeat this 10,000 times! The red line shows that juvenile loons are choosing to visit lakes much closer in pH to their home lake than would be randomly expected.

Point 1 makes sense – loons are large birds (large males can weigh up to 11 lbs) that require ample food supplies. Large complex lakes are likely to still have abundant fishes to feed on in late fall, especially energy-rich pelagic fishes, like Cisco (Coregonus artedi). In contrast, smaller lakes may be depleted by fall and generally lack larger, calorie-rich fish that would be especially attractive to a young bird trying to add mass for its first migration south.

Point 2 is just…interesting! Why would a juvenile prefer a lake with a pH similar to where it was raised? Interestingly, adults show a very similar behavior, as they prefer to breed on lakes that are similar in both size and pH to the lakes on which they were born (Piper et al. 2013). This behavior is believed to provide strong advantages to individuals that practice it. The idea is that if a young animal survives, the environment it grew up in likely indicates a reasonably high quality habitat. Thus, seeking out similar environments as an older individual could help improve survival and fitness. For example, loons might not really care about pH in particular, but pH may indicate or reflect something that loons DO care about, such as the smell of a particular habitat type where they know how to find and catch fishes. Perhaps a wandering juvenile loon looking for the right lake to visit is just like a human faced with an unfamiliar new menu, searching for a familiar and trusted childhood food? These patterns raise another question: if loons prefer habitats similar to home, what do other birds do? These types of questions might be especially relevant to the ecology of birds in California. For example, are some reservoirs/lakes in California more heavily used by migratory water birds than others? Do similar “rules” apply here? And ultimately, the behavior is intriguing, but the mechanisms and reasons for this behavior still need to be disentangled. Coupling loon movements with critical limnological dynamics promise more insights to come.

A parent looks on as a juvenile loon eats a bluegill (Lepomis macrochirus). Photo credit: Linda Grenzer

Brian Hoover is a GCI Postdoctorate Fellow in the Schmid College of Science and Technology at Chapman University in Orange CA. Andrew Rypel is an Associate Professor and the Peter B. Moyle and California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and Co-Director of the Center for Watershed Sciences. Walter Piper is a Professor of Biology in the Schmid College of Science and Technology at Chapman University in Orange CA. 

Further Reading

North Woods loons adopt duckling. 

BA Hoover, KM Brunk, G Jukkala, N Banfield, AL Rypel, WH Piper. (2021). Early evidence of natal‐habitat preference: Juvenile loons feed on natal‐like lakes after fledging. Ecology and Evolution Published Online

WH Piper, MW Palmer, N Banfield, MW Meyer. (2013). Can settlement in natal-like habitat explain maladaptive habitat selection? Proceedings of the Royal Society B: Biological Sciences 280 (1765), 20130979

AL Rypel, TD Simonson, DL Oele, JDT Griffin, TP Parks, D Seibel, CM Robers, S Toshner, LS Tate, J Lyons. (2019). Flexible classification of Wisconsin lakes for improved fisheries conservation and management. Fisheries 44 (5), 225-238

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