After six years of data collection in our Minnesota Study Area, our research effort there has finally come of age. We now have measured the weight of enough chicks — 263, to be precise — to carry out a robust analysis of factors that affect the body condition of these youngsters.
You may recall that body condition of loon chicks matters for two reasons. First of all, heavier chicks more often survive to adulthood. Second, fatter chicks become healthier adults, which are more likely to survive long enough to return to the breeding ground and raise lots of chicks themselves. The weight of a chick, in other words, is a sensitive indicator of the how healthy and productive a loon will be throughout its life. And chick weight also provides a good indication of the quality of the habitat in which a loon is reared. Therefore, the weight of chicks in a loon population is our best single indicator of the health of that population.
What have we learned from this just-completed preliminary analysis of chick weight in Minnesota? A chick’s weight is affected by: 1) its age, 2) its parents’ weights, 3) the size of the family territory, 4) water clarity, and 5) the presence of zebra mussels. Chicks are heavier if they are old, have heavy parents, are reared on large territories, live on clear lakes, and — wait for it — share the lake with zebra mussels.
That is a lot to digest, but let’s try to pick it apart. Of course, chick weight depends upon age. That is obvious. And we have long known that body weight is heritable in loons — that is, heavy parents raise heavy chicks. So the meat of these findings lies in the remaining three results. Let’s look at those one by one.
Loon parents on large territories raise healthy, heavy chicks. This makes perfect sense. Large territories provide parents with more foraging options than small territories. If fishing is poor in one spot, parents with large territories can shift their families to another. I suppose loons are like anglers in this respect.
Let’s look at the water clarity pattern:
The finding shown in the graph just above — that chicks are heavier when water is clear during the rearing phase — echoes our findings from a few years back in Wisconsin. As visual predators, loons have higher foraging success in clear lakes than in cloudy ones. If parents cannot see fish, they cannot catch them for their young. This finding, of course, lends credence to our fears about excess nutrients like fertilizer, septic waste, and pet waste washing into loon lakes and reducing clarity. Furthermore, it shows that concerns about wake boats are legitimate, if the boats stir up nutrients from lake bottoms that spawn algal growth and reduce clarity.
What about the effect of mussels?:
The above association of zebra mussels with good chick condition was a complete surprise. Again, these are preliminary results, but it appears at this point that these exotic pests might help loons in some way. The fact that loons do better in mussel-infested waters is doubly puzzling because — remember — we have already accounted for the effect of better water clarity. So this finding — if it stands — indicates that the presence of mussels in a lake helps loons in some way other than their positive impact on water clarity. If you find this hard to fathom, join the club! We will have to explore the pattern further to see if we can discern its exact meaning.
Folks who are familiar with our publications from Wisconsin might greet these Minnesota findings — with the exception of the zebra mussel pattern — with a great big yawn. After all, we already knew from our closely-watched study population in Wisconsin that chick condition is highly dependent upon both water clarity and lake size. Why should it have been any different in the neighboring state?
There are at least two ways in which the brand new Minnesota findings shed new light on loon conservation. First, our Minnesota lakes are far larger, on average, than those in Wisconsin. The average lake size for a Minnesota loon pair nest is 1,133 compared to 453 acres in Wisconsin. Moreover, only 7% of all Minnesota lakes we cover are 50 acres or less in size, while 32% of our Wisconsin loon pairs nest on lakes in this tiniest size class. So the new Minnesota findings show that water clarity is not merely important on very small lakes.
Our Minnesota study lakes are also far clearer — 13.2 feet on average — than our Wisconsin lakes, whose mean clarity is only 9.4 feet. The dependency of chick condition on water clarity among this clearer set of lakes shows that clarity is not significant only among turbid lakes. In other words, we already knew that loon parents raising chicks on lakes with rather low visibility raise healthier chicks than parents in lakes where visibility is truly atrocious. But even loons on moderately clear lakes raise healthier young if water clarity happens to be high during the chick-rearing period. To put it another way, water clarity is important to loons across the spectrum of lakes, from very clear to very cloudy. This fact is important to understand if we seek to help loons regain the level of breeding success they enjoyed in past decades.
By the way, some of you might have seen an important recent publication showing that breeding success of common loons has been falling across most of Canada and the U.S. in the last few decades. Wisconsin was included in this study, but Minnesota was not. Yet we know, based on long-term data from the Minnesota DNR, that Minnesota loons are producing fewer chicks now than in past decades. Thus, Minnesota and Wisconsin echo the widespread pattern. With each passing year, it gets harder to maintain the falsehood that loon populations are healthy, yet many folks still profess that this is the case. Will this compelling study be the one that finally forces us all to acknowledge the problem and take steps to turn things around?
Photos are by Sheila Johnston (loon); P. Markham (clear water); and Adirondack Park Invasive Plant Program (mussels). Thanks to all!