The fact that all loons look alike is so widely known in the North that it scarcely bears mentioning. But this fact has impacts on our understanding of loons that range from annoying to devastating. From the public’s point of view, the difficulty of distinguishing one adult loon from another is simply confusing. No doubt it contributes to the enduring myth that breeding loon pairs mate for life. After all, the loons that showed up on your lake this year looked and acted pretty much like those from the year before — and ten years before — right?

A minor source of confusion for the casual observer is a massive obstacle to the scientist. Our inability to tell one individual from another means that we can catch only glimmers of knowledge about territory defense and settlement, aging, survival, nesting ecology, and mate fidelity without marking loons individually. To view the situation more positively, the individual marking of loons has produced huge breakthroughs in our understanding of their behavior and ecology.

Yet even the marking of loons for study has severe limitations. Consider our situation in Minnesota. Yes, we banded dozens of loons in 2021 and they seem, if anything, a bit easier to approach and study than the 120 marked Wisconsin pairs we have been observing for almost three decades. But we are, in a sense, starting from scratch in the North Star State. Why? Because, well, loons all look alike! The breeding pair we marked on Rush-Northeast consists of a male that we can only estimate as being 5+ years old and a female that we can estimate as being 7+ years old. (Females settle about two years later than males do, on average, leading to these estimated ages of newly-banded adults.) The same estimated ages apply to the Rush-Boyd pair, the Ossie-Island pair, the Roosevelt-Southwest pair, the pair on Big Pine, and the twenty-odd other pairs we marked in July. In fact, many of these Minnesota loons are in their teens and twenties; a few are likely to be in their early thirties, like the female in Linda Grenzer’s photo, above.

Knowing loon ages is not trivial. Some of the most valuable findings we have made in recent years have emerged from our knowledge of loon ages in our Wisconsin study population. Most crucially, information about age helps us refine our population models so that they yield more precise predictions about population trends. So we would very much like to know the ages of our new Minnesota study animals.

How might we learn the ages of freshly-banded loons in Minnesota? Alas, there is no obvious aspect of loon appearance (like wrinkles or grey hair) that can clue us in. However, one exciting possibility is telomeres.

Telomeres are simple segments of DNA that sit at the ends of chromosomes. They are not genes, nor do they contain genes. Instead, telomeres serve as protective “end caps” on chromosomes. Unlike genes, which reside on chromosomes and are always replicated in their entirety when a chromosome is copied, telomeres become shorter each time a chromosome is replicated. Why? Because the process of DNA replication is imperfect and can never quite replicate the entire ends of a chromosome. Since telomeres reside at the ends of chromosomes, a portion of each telomere is shaved off each time a chromosome is replicated. In effect, by allowing themselves to be shortened, telomeres sacrifice a portion of their length to prevent genes from suffering the same fate.

If telomeres get shorter each time a cell replicates, then they might serve as a clock within the bodies of animals. Young animals should have long telomeres, while old animals — whose cells have undergone many rounds of division — should have short ones. Studies in many vertebrates have confirmed this broad expectation. In fact a recent study showed that telomere length is quite closely correlated with age in a wild bird.

A new collaborator at Chapman is currently measuring telomeres of Wisconsin loons using small blood samples we collected. He is at a very early stage, but his findings so far are promising.

Ys/Gs,S/Gs Townline, 6-year old T/S = 1.02
Bs/Ar,O/S Two Sisters-East, 9 years old T/S = 1.06
Ts/S,W/W Mildred, 21+ years old T/S = 0.71
S/O,O/R Arrowhead, 22+ years old T/S = 0.91

Thus, two young males known to be six and nine years old had rather long telomeres, while two females in their 20s had short ones. This difference occurs despite a pattern that we had noted earlier about loon telomeres — that females generally have longer ones than males.

So we await further telomere measurements from our Chapman collaborator with great anticipation. If Wisconsin data show that telomeres are predictive of age in loons, we will begin to be able to separate the old-timers from the young whippersnappers in Minnesota.