Science, of course, is cumulative. Solving scientific riddles generally requires multiple studies by diverse authors using a broad range of scientific techniques. In fact, scientific conclusions are more compelling when they rest upon findings from many scientists using different techniques and with different backgrounds.
So I was quite anxious to learn what Kevin Kenow had found. Kevin, a scientist with the U.S. Geological Survey and also a member of the Scientific Loon Council at the National Loon Center, has studied common loons for about as long as I have. But while I have focused on the behavioral ecology of loons exclusively and established two fixed study populations for this purpose, Kevin has collected data on many species of migratory water birds, tackled questions related to human impacts and conservation, and worked across a variety of states and waterbodies in the Upper Midwest and beyond.
Kevin’s just-published article pulled together data from loons in Michigan, Wisconsin, and Minnesota. In contrast to my low-tech methods, Kevin’s team used satellite transmitters and geolocator tags to either: 1) track loons’ migratory movements in real time or 2) reconstruct their movements, following recovery of the geotags.
Kevin’s team confirmed several migratory pathways that we had known about or suspected through recoveries of dead birds. First, adult loons from the three Upper Midwest states typically “stage” on Lake Michigan in the late summer and early fall, before departing southwards to the Florida Gulf Coast. Second, juvenile loons (those only a few months of age) do not visit the Great Lakes prior to migrating south. Third, first- and second-year loons that are too young to return to the breeding grounds instead migrate northwards in the Atlantic, summering off of the Canadian Maritime provinces. The quirky patterns in loon migration and wintering behavior are important. They make it clear that a loon’s survival to the breeding stage requires that it survive and remain healthy across a period of many months, a variety of water bodies, and a number of geographic areas.
But one of the patterns that Kevin’s team identified loomed above the rest. They found a high rate of mortality among first- and second-year loons, especially in the spring. If you are following this blog and have an excellent memory, you recall that this finding appears to dove-tail with a recent one of ours. We found that “floaters” — the segment of adult loons that are two to five years old and are looking for breeding territories — have been disappearing at a high rate. Floaters alone, you might recall, account for most of the decline we have detected in the Wisconsin breeding population.
Do Kevin’s findings of high first- and second-year loon mortality solve the riddle of what is ailing the loon floater population in Wisconsin? Unfortunately not. In fact, we had long suspected that young adults would die at a higher rate than older adults, because they are less experienced. High mortality among young adults is a common feature of avian populations everywhere. But these new findings might help narrow down the “period of vulnerability” in the life history of loons. And our findings of high young adult mortality in the last decade combined with those of Kevin’s team might tell us where to look. Perhaps conditions along the Florida Gulf Coast and/or the Atlantic have deteriorated recently, causing higher than normal mortality during this trying time of life for young loons. This is only one hypothesis, and it will require rigorous testing. Thanks to Kevin’s team’s cool recent findings, though, the wheels of science are turning.
By the way, follow us on Instagram, if you like, at @loonproject. The whole LP team works together to post cool, informative “loonstagrams”!