Loons Are Doing an Especially Poor Job of Mating for Life in 2020

I think I have made it clear already that loons’ allegiance is to their territory, not their mate. That is, when offered a choice between abandoning a territory and following their mate after their mate has lost a territorial battle, loons always choose to remain on the territory and form a new pair bond with the winner of the battle. This behavior seems heartless, but it makes perfect sense, because loss of a breeding territory is a crushing blow that makes it impossible to breed. Mates, in contrast, are easy to find.

While loons routinely say farewell to breeding partners, 2020 has seen more of our breeding pair members change mates than ever before. The graph below shows the proportion of adult loons on a breeding territory in one year that were not on that territory a year later. As the graph indicates, some years have very stable pairs (like 2008 and 2019), while other years see lots of turnover among pair members. In 2020, one quarter of all breeding pairs experienced a loss of at least one pair member, and a sixth of these — 6 pairs of the 145 we follow — saw both members of the pair

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replaced by a new adult.

Of course, eviction is not the only factor that causes a loon that is on a territory in May of one year to be gone from that territory a year later. In fact, there are two main causes: 1) eviction and 2) death. Both of these biological processes contribute greatly — and about equally — to the failure of breeders to return.

Looking closely at the graph, you can see that annual return rate jumps around a good deal. Last year, for example, was a year when a very high proportion of all adult breeders came back to their territories of the previous year. (We can only speculate here, but these jumps are probably caused by occasional “die-offs” during migration or winter.) If you were looking at the return rate as a proxy for adult mortality, you might breathe a sigh of relief at this point; the graph does not seem to show an increasing trend in failure to return to territory that might mean increasing adult mortality over the years.

One final point. While it is true that failure to return to the territory does not obviously shift upwards or downwards over the past quarter century, careful readers of the blog might remember that the eviction rate has fallen dramatically in our population in the past two decades, because there are now far fewer nonbreeders seeking territories than there were 20 years ago. If evictions have declined over the years, then deaths must have begun to account for a growing proportion of all cases of non-return shown in the graph. Thus, it might actually be the case that adult breeders die at a higher rate now than they did a few decades ago, but that the graph does not show us the data cleanly enough to tell. In short, whether adult survival is trending downwards or not will not be clear until we do a more detailed quantitative analysis.

Oblivious to the territorial chaos swirling around them in 2020 — I mean loon-related, not human-related chaos — loons have been going about their business. Whether members of a growing or declining population, and whether their mate is new or old, most of our study animals have picked up the pieces after a devastating black fly season and renested. We still hope that these late nests will allow the population to generate enough chicks so that 2020 will not be our worst year ever for chick production. There is some comfort in the fact that chicks are produced one brood at a time. Perhaps the stalwart seven-year-old Pickerel-South male in the video below and his mate will contribute a chick or two to the total.