We always celebrate when someone identifies a new banded adult. “You got an exclusive!”, I say to one of my students, if they nail all bands on a loon that we banded as a juvenile in the study area and have not seen since. In fact, Melanie reported an exclusive ABJ (for Adult Banded as Juvenile) just yesterday; this one happened to be a four-year-old male hatched on Samway Lake that she ID’d on Elna. (Linda’s photo shows an exclusive that she nailed bands on, this one from Soo Lake.)
Sightings of young ABJs are valuable. These loons are all “floaters” — nonbreeding individuals that live on small lakes or parts of large lakes that are not used by breeding pairs — so they live a marginal existence. Still, they provide us with data on juvenile rates of survival and return to the study area, which contribute to mathematical models that tell us whether our population is stable, increasing, or decreasing. Young ABJs are also the future, because these green, reticent individuals — they are notably subordinate when interacting with territorial residents, have low fighting ability, and are well below optimum adult body weight — will ultimately replace our established and well-loved territorial breeders.
I spend most of my research time asking behavioral questions about our long-term territorial residents, which is regrettable, because it leaves young ABJs out. In fact, a vexing question concerning young ABJs has been lurking in the back of my brain for some years now: where do they all go? From the countless small celebrations the students and I have had over the years at new ABJ sightings, an expectation has formed that we would see a vast wave of new territory settlements by this youthful cohort. But it has not happened. Each year only a handful of young ABJs claim new territories by evicting a living territorial resident, replacing a resident that has died, or carving out a new territory where there was not one previously.
This morning I got fed up with waiting around for all of the young ABJs to settle — and looked at the numbers. They are pretty shocking. We have been able to celebrate sightings of 348 young ABJ floaters over the years. That is a lot of loons. Of these, though, only 124 settled on breeding territories and paired with a mate for at least 60 days. And an even smaller number — 94 — nested and hatched chicks. Since most of these individuals were last seen years ago, most died long ago. So only slightly over one-fourth of all young ABJs that returned the breeding grounds ever produced young. Even this rather low fraction is too high, because it presumes that we actually ID all young ABJs that come back to the study area. Clearly a typical young ABJ deals with many challenges — lengthy migrations between breeding and wintering grounds, bouncing around in the study area, probing here and there for territory openings, absorbing attacks and chases at the hands of territory owners — only to fall short in that last, most crucial test: territory settlement.
Why? Why would the rate of failure to settle be 75%, when vacant lakes with a successful record of chick production abound in the study area? If I were a loon (I know….but don’t say it!), I would look around for a prime breeding territory, pick a few fights to try and get one, and then settle for a less-than-perfect but adequate territory, if it came to that. Because failure to settle leads to evolutionary oblivion. That is, we expect that natural selection has acted on behavior in such a way as to maximize breeding success, and breeding success would appear to be maximized not by failing to settle but by settling wherever you can and cranking out as many chicks as possible there.
Of course, it is hubris to think that humans know how loons should behave. Loons have been molded by natural selection and other evolutionary processes for countless generations in a way that virtually guarantees that their behavior leads to high evolutionary fitness. However, as my students in Animal Behavior class know well, there are a few caveats to the expectation of sensible, adaptive behavior by animals. The main one is that rapid environmental change can sometimes outpace the capacity of animals to adapt, causing animals to behave in a way that does not maximize their reproductive success. In other words, if the environment that an animal faces — its predators, competitors, physical environment, etc. — changes so rapidly that the species cannot evolve suitable behavioral adaptations, then we might see animals behaving “foolishly”. So we might surmise that ABJs fail to settle on vacant territories because the availability of vacant territories has only recently increased, and loon settlement behavior evolved during a period when vacant territories were scarce. In effect, then, ABJs would be practicing behavior not suited to the territorial situation that now exists.
While we cannot reject the hypothesis that rapid environmental change has made loons look stupid, it is a bit hard to stomach. The hypothesis posits that young floaters are poor settlers because they are not used to territories being readily available. But whether territories are scarce or abundant, young floaters should have evolved to be able to occupy any available one readily. That is, the capacity to snap up a vacant territory is so fundamental that it is a trait that should be possessed by all young loons, regardless of the territorial environment in which they evolved. There must be a better explanation for the failure of so many young ABJs to settle on territories.
The reason for lackadaisical territory settlement by floaters is probably habitat preference. Some of you may recall that young loons show a peculiar but very strong preference for natal-like habitat. Specifically, young floaters from small, acidic lakes strongly prefer to breed on small, acidic lakes, and those from large lakes of neutral pH try to establish themselves as breeders on large, neutral-pH lakes. Strong habitat preference creates a situation where a young floater reared on one kind of lake does not see vacant territories on another kind of lake as a viable breeding option. If young ABJs are being finicky about the territories they choose to settle on, we should expect to see some “perfectly good” territories go unsettled, as we do. More to the point, we should not be surprised that many floaters fail to settle.
Natal habitat preference might help us understand the seemingly inefficient territory settlement of young floaters, but, if so, it merely shines a spotlight on another vexing question: why do loons strongly prefer to breed on lakes that resemble their natal one? I have speculated about this before, but no satisfactory answer has yet emerged. As we collect more and more survival data, we might find that loons have evolved to take into consideration more than just the potential of a breeding lake to produce chicks. Indeed, settling on a territory like your natal one might mean that you were prepared since day one for that kind of environment and might be able to survive well there. If so, natal habitat preference might allow you to offset with longevity any loss you suffer from settling on a territory that is less-than-stellar for producing chicks. The slow but steady approach of rearing a chick here and a chick there but surviving to a ripe old age might be the one that maximizes lifetime breeding success.