migration

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As many of you know, I am a worry-wart. Normally I get so stressed-out about my kids, my teaching, my research, my health — and many other matters that are going well — that I hardly have time to obsess about loons in the study area. But Linda Grenzer’s bleak photo of conditions on her lake today gave me a jolt. Could the late ice-out that we are facing in 2018 delay the season so much that it damages the breeding prospects of our loon population?

One might think that the later the ice comes off of the lakes, the later the loons nest, and the less time parents have to fatten up their chicks and prepare them for their first southward migration. Thus, a late ice-out might well lead to reduced breeding success for the population. Although there are many “if”s in this string of logic (and a preliminary analysis did not bear out the pattern), I felt concern  gnaw at me.

So I did what scientists often do to stave off despair: I looked at the data. First, I looked to see if loons nest later when the ice goes out later, which almost has to

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be true. It is true, but there is a lot of noise in the data. That is, loons are constrained to nest somewhat later in years when the ice goes out later, but the picture is not simple. The reason for the noise becomes clear when you look at the lag time between when loons settle on their territories and when they hatch their

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young. There is a very strong pattern here. When the ice goes out early (left side of graph), loons dawdle and wait weeks before nesting. But when the ice goes out late, as it will this year, pairs get down to business quickly, nesting within a week or so of territory return. So loon pairs are somehow able to catch up in years of late ice-out so that their breeding schedule does not differ greatly from other years. (Notice also that the orange line in the top graph is flatter than the blue line.)

What accounts for this pleasing pattern? We can make a pretty good guess based on findings in other migrant birds. Spring migration is an energetically costly process. In an early year, the ice is gone so quickly that loons settle on their lakes as soon as they return from the wintering grounds. In such cases, their fat levels are very low from migration when they first occupy their territories, and it takes a good deal of foraging before they return to good condition. In a late year, loons cannot settle on their territories right away but must wait on nearby rivers that have open water. There, they are able to forage and restore their bodies to good condition. As a result, loons hit their territories in prime body condition and fully recovered from the migratory flight in years of late ice-out. Thus, they can get down to breeding quickly.

Although I was heartened by the data I saw above, I had a look at the numbers that most directly addressed my concern about late ice-out and population breeding success. There is a no statistical tendency for the population to produce more loon chicks in years of early ice-out, despite the many years of data we have to look for such a pattern. Indeed, some of our best years for loon breeding (2013, for example) have occurred when the ice goes out late. So those many of you shivering in northern Wisconsin and other frigid regions can relax about one thing; the loons are no worse off in years when spring comes late than when it arrives early.

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I must confess that I had mixed feelings to learn this week from Nelson that the North Nokomis pair had hatched two chicks. Despite having seen scores of newly hatched chicks, I still enjoy watching the tiny fuzzballs bob up and down next to their huge parents while the adults, in turn, move gingerly around their tiny young to avoid injuring or drowning them.

So it was, in one respect, nice to learn that the North Nokomis pair had hatched the eggs from the conspicuous nest I had inspected on 25 July, after following the simple, clear instructions from my research team. But I recall thinking, “Oh geez!” on that date, because only two other breeding pairs of the 120 that we follow were still sitting on eggs. (Both of those, we had determined, were cases of infertile eggs that the pair had incubated for six weeks or more.) I have always presumed that chicks hatching in late July or August have too little time to mature physically, develop foraging skills, and learn to fly in time to make the fall migration.

Now we have the data to look at this question robustly. In other words, having captured and marked 983 chicks since 1991, we can determine whether hatching date is a predictor of survival to adulthood. Logically, there must come a date in late summer beyond which chicks run out of time. However, pairs might face a gradual decline in the likelihood of their chicks surviving migration, or there might be a rather sudden threshold date past which chicks that hatch cannot survive.

If we look at all chicks banded from 1991 on, and calculate how many have made it back to the study area as adults, we learn what the figure

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shows. In short, hatching date does not appear to influence survival to adulthood strongly. At the very least, we can say that chicks hatching in mid-July survive at a rate no lower than those that hatch a month earlier. There is a hint of a decrease in survival from early to late hatches, but it is only a hint.

As usual, our data are not perfect. In fact, we have too few cases of very late hatches to gauge the likelihood of the two North Nokomis fuzzballs (which hatched on about 28 July) making it off the lake this November. On the plus side, 470-acre North Nokomis Lake has one of the highest rates of survival to adulthood in the study area. (The territorial males on Gilmore and Cunard were hatched there.) I would like to think that the gutsy North Nokomis pair will be rewarded this fall with two healthy fledglings. So I am keeping fingers crossed for them.

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LMG2693 Loon Between the Ice

I always dread ice-out. While I am excited to know that I am scant weeks away from seeing the loons, ice-out now tends to occur during the high-stress period of the Chapman spring semester — and I am seldom ready for the event. As Linda’s photo from the Wisconsin River shows, loons are coming back, accumulating on the river, waiting for their breeding lake to open up enough to permit landing there. Meanwhile, my Animal Behavior students are sweating their behavioral experiments and write-ups.

Let’s focus on the positive of early ice-out for data collection. Rain and warming temperatures mean that ice-out is only days away. Soon we will begin to log the identities of returning veterans anxious to attempt another year of breeding. Since the lakes will be habitable two or more weeks before the historic mean date of ice-out, while many returning birds are still en route from the wintering grounds, we have an opportunity. In theory, adult loons that are in good physical condition generally should be those that can complete the breeding molt early and also migrate early. So the loons that show up first on breeding lakes should be those in good condition. Recent findings have shown us that this group comprises males and females about 8 to 15 years old — the prime of life for a loon. Thus, we predict that the early arrivals are in this age-class. The laggards should be breeders that are either very young — 5 to 7 years old — or very old — 20 years and older. We might expect the territories of such individuals to remain vacant for a week or more after the ice has come off of them.

If age does turnout to be a good predictor of date of territory occupation, then late return from migration could be another source of trouble for an old established breeder. That is, an old territorial bird whose body condition has begun to decline might not only need to worry about being evicted by a young, fit nonbreeder in the midst of breeding; danger might also come from the tardy return of the old bird to its territory in the spring, which could open the door for a youngster to seize the territory, pair with the old bird’s mate, and hold off the former owner when it returns.

I have painted a dire picture. We will have to use the increasingly early ice-outs like this year’s to measure date of return accurately and see if early ice-outs truly destabilize territory ownership. At the moment, I will tantalize you by reporting that breeding success across the population is higher when ice-out comes late. It is speculative at this point, but this pattern might indicate that early ice-outs lead to ousting of old, experienced breeders from their territories, which in turn suppresses chick production of the entire population. If so, I have one more reason to rue early ice-outs!