This blog post is provided by Walter Piper and Sarah Saunders and tells the #StoryBehindThePaper for the article “Interrelated impacts of climate and land-use change on a widespread waterbird“, which was recently published in Journal of Animal Ecology.

Loon capture is a peculiar annual ritual. Working from dusk until dawn, three members of the research team set out in a 12-foot motorboat to scour the surface of Wisconsin lakes. Pushing exhaustion aside with adrenaline, we sweep our massive spotlight back and forth across the murky water, trying not to let the beam fall upon cottagers sleeping along the shoreline. After three, thirty, or ninety minutes of intensive searching, we find a small, persistent spot on the lake’s surface that catches the light. If, after veering towards the spot, we find that it becomes two, three, or four spots, then we have located the loons, and we slow the boat to a virtual crawl as we draw near them. Since we hope to capture the entire family, we angle first towards the wary, agile parents. Often we are lucky and can safely net both adults, because they are reluctant to dive and abandon their young. The chicks themselves, more bemused than frightened by the spotlight, are easy to scoop from the lake.

Though it is without question the strangest of our research activities, loon capture is essential to our work. Marking of loons with leg bands is immensely rewarding; it permits us to convert unmarked, unknown individuals into study animals, in whose lives we suddenly take a keen interest. Over the long haul, marking and resighting of loons permits us to determine rates of survival, territory tenure, and reproductive fitness for individuals that can be used in population models. The field assistants take particular pleasure in helping to band the cute, fuzzy chicks that they had previously only watched from afar.

Despite the benefits of loon capture and the warmth it inspires in team members, it was this capture and banding that first exposed the troubles our Wisconsin loons are facing. It seemed to us, from routine weighings of captured chicks, that they had become progressively lighter over the years. Indeed, longitudinal analysis confirmed that chick mass has declined steadily during our 28‑year study – and with it, fledging success. Faced with a study population that was declining in both breeding success and survival rate, we first looked for obvious causes. However, neither bald eagles, which occasionally kill loon chicks and adults, nor small fishes, on which loons depend for sustenance during the breeding season, seemed likely to explain the worrisome patterns we had found.

We began to think more broadly. Could changes in climate and land use help us make sense of the drops in survival and fecundity that we were seeing? Sarah Saunders’ expertise in linking demographic parameters to environmental factors with the use of sophisticated population models offered a promising path forward. Indeed, our findings, which appear in the Journal of Animal Ecology, are startling and provide a new perspective on Wisconsin’s common loon population. First, we found that human development causes trouble for loons: the more humans nearby – humans who fish, boat, occupy shoreline habitat and release waste into the water unwittingly – the more adult loon survival declines. This result by itself is unsettling. We are unlikely to stem the tide of human development around northern lakes, even if we inform people that such development is hurting the population of a beloved avian denizen.

Our climatological findings are far more complex than the human development pattern but equally worrisome. The climatological factor that influences loon survival and breeding success is, alas, not something straightforward like temperature – it is the North Atlantic Oscillation (NAO). The NAO is important because it strongly influences patterns of winter temperature and rainfall across Europe and North America. But how does this climate pattern affect loons? A positive NAO during winter negatively impacts the survival rate of adult loons in the summer immediately following, yet positively impacts loon breeding success during the subsequent summer. Such “lagged” or delayed impacts of climate on populations are commonly found in seabirds, but they remain poorly understood. We speculate that populations of small fishes along the Atlantic coast, where our loons spend the winter, fluctuate with the NAO and might explain the correlation we found.

If your head is spinning from these complex effects of climate on loons, you are not alone! To understand these patterns more, we ran twelve scenarios where we varied land-use, NAO, and rainfall patterns and looked at how the loon population would be affected during the next decade. Alarmingly, the loon population declined under all twelve scenarios; in the most extreme case, the population was projected to decline by > 65% in the next ten years. Furthermore, the most severe declines are predicted under scenarios of a positive winter NAO, a trend that is expected to occur as climate change continues.

If knowledge is power, then our study has left us far more powerful than we were a few years ago. In 2018, we only had a vague sense that something was amiss with this loon population along the southern edge of the species’ breeding range. Armed with our new findings, we now have a better understanding of what it will take to conserve this charismatic resident of northern lakes, including adopting policies to mitigate the potential impacts of a changing climate on this beloved species.