If aliens landed on Earth’s surface to investigate its life forms, they would be puzzled by the coexistence of common loons and humans. True, loons spend almost their entire lives on the water, where they are relatively free of dangers from humans and other terrestrial vertebrates. But loons require solid ground for nesting, safe “nurseries” for rearing their chicks, and abundant food to keep themselves and their chicks alive. How, then, can loon populations persist along the southern periphery of the species range, where humans and human recreation threaten all three basic requirements?
The ability of loons to thrive in regions of intense human building and recreation vexed me for several years in the 1990s, when I first began my long-term loon research. I could see that most people venerated loons and took pains to protect them. But the sheer abundance of humans surrounding, approaching, and fishing near adults and chicks during the spring and summer made the tasks of hatching young and rearing them to adulthood seem daunting. How did loons manage to raise any young in the northern U.S. and southern Canada?
I cannot answer this question completely. I am still amazed at the abilities of adults and chicks on busy lakes to dodge motorboats and jetskis as well as they do. And it continues to surprise me when adults fledge chicks from small lakes where food seems limited. However, detailed study of loons’ nesting patterns allowed me to solve one riddle: how loons enjoy high nesting success despite intensive shoreline development.
One would think that shorelines are essential to nesting loons. Loons have to nest along shorelines, right? And humans build summer homes along shorelines too. So loons and humans would seem to be direct competitors for shoreline habitat. But it is not so. Why not?
The answer is deceptively simple. Well-drained “upland” shorelines provide the best sites for building lakeside homes. Upland sites are free of boggy or marshy vegetation. At the same time, upland shorelines provide poor nesting habitat for loons. Most loon eggs placed on upland shorelines end up in bellies of raccoons that take advantage of the comfortable footing they provide to look for easy meals. Experienced male loons learn to avoid placing nests on dry, upland shorelines.* Instead, they usually locate nests on islands, marshes**, or bogs hard for terrestrial predators to reach.*** So one key to loons’ ability to coexist with humans is merely loons’ preference to nest where humans cannot build.
Hodstradt Lake in the Wisconsin Study Area illustrates the complementary use of shorelines by loons and humans. Hodstradt is a 119-acre lake that has beautiful clear water with a slightly greenish hue. The lake is full of fish but completely encircled by lake homes. There is no island, marsh or bog in Hodstradt — only a peninsula in the southeastern corner (see screen grab below from Google Earth). Almost all nesting attempts by loons on Hodstradt have been on the end of that peninsula. High water caused by heavy rainfall in the past decade submerged the narrow spit connecting the end of the peninsula to the mainland, making it a small island. Whether an island or a peninsula, though, the land is low lying and impossible to build on. Hence it provides permanent nesting habitat for loons that is off limits to humans.
A similar situation exists for many loon pairs in the Minnesota Study Area. The seven pairs that nested this year on massive Cross Lake provide a good example. Three of these pairs nested on small uninhabitable islands; three nested among dense cattail patches in marshy coves; and one used an artificial nesting platform. (Five of these pairs hatched chicks.) Thus, the “Jack Sprat” nature of loon and human shoreline use can be seen in both Wisconsin and Minnesota. With rare exceptions, sites safe for loon nests are sites where humans cannot or will not build.
So adult loons are able to put chicks in the water despite extensive shoreline development. This would seem to be cause for celebration. In a cruel twist, though, shorelines altered to support suburban-style homes, lawns, and driveways have increased runoff. Although we are still working out the details, it appears that higher runoff has, in turn, produced a decline in water clarity and decreased chick mass, probably because adults cannot see fish well enough to provision their chicks adequately. In short, shoreline development negatively impacts loons during the second critical breeding phase: chick-rearing. Indeed, the sharp increase in mortality of chicks and young adults in the past two decades has become our number one concern with respect to the Upper Midwest loon population.
If there is a silver lining, it is this. Loons are resilient. They have been able to find nesting sites and sustain a high hatching rate despite everything humans have thrown at them. Perhaps we can help loons reverse the decline in chick survival, if we can learn precisely what is driving the drop in water clarity. This will be a massive challenge. But I have to believe that loons can come back from this setback. It keeps me going.
* We learned 16 years ago through marking of loons and systematic tracking of nest placement that male loons choose the nest site. For those not interested in looking at the science, we know this from two facts. First, loon pairs learn where to nest by trial and error. That is, they tend to reuse a site where they hatched chicks the previous year but move to a new nest location after egg predation. This logical nesting strategy is called the “win-stay, lose-switch” rule. Second, loon pairs in which the female pair member returns from the previous year but the male pair member does not usually do not reuse a successful site from the past year. In contrast, pairs consisting of the male from the previous year and a new female tend to reuse successful sites. In short, pairs with new male members do not use the win-stay, lose switch rule. They act as if they have forgotten where the best nesting site is.
** The featured photo is of Clune, Linda Grenzer’s favorite male loon, who bred for many years in her lake in Wisconsin. He is incubating eggs in a marshy corner of the lake that, predictably, is devoid of homes.
*** Of course, humans often accomodate loons’ nesting preferences by placing artificial nesting platforms along lake shores. Platforms provide very attractive nest sites for loons.