A Moment of Clarity on Woodcock Lake

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Two mornings ago, I saw my very first chick of the year on Woodcock Lake. My visit to Woodcock was memorable — not just for that reason, nor because of the rarity with which the loon pair there hatches a chick. Rather, I chanced to witness with great clarity one strategy that adult loons employ to defend their offspring. It is moments of  this kind, when loons’ behaviors and motivations become visible suddenly and starkly,  that fuel much of my thinking and writing about the species.

If you have read my recent posts, you know that we have learned a good deal about how loons defend their young from opportunistic intruders, which on occasion find and kill chicks less than two weeks old. We know that males possess a acoustic tool that females do not — the yodel — which conveys aggressive motivation and therefore can be used to discourage intruders within earshot of the territory from landing there and imperiling the chicks. What was less clear was whether males yodeled at a high rate as a generalized strategy to inform would-be intruders that visits to the yodeler’s territory would likely provoke an attack or whether, instead, yodels were targeted at specific intruders as they passed over a territory or began to land there.

On Saturday morning I arrived at Woodcock to find the male with a tiny chick riding on his back. The female was on the nest 200 meters away, incubating the second egg, which by now might have produced a second chick. As the sun rose above the horizon that day, intruders criss-crossed the airspace above the territory. Two such “flyovers” were especially enlightening. At 535, two intruders began to descend as they crossed the lake, preparing to land next to the male, near the lake’s center. The male crouched down, dumping the chick into the water, and uncorked a deafening yodel at just this moment. In response, the two flyers checked their glide down towards the lake, flapped rapidly to regain lost altitude, and flew off to try their luck elsewhere. Seventeen minutes later, this pattern was repeated. This time a lone flyer crossed just above my canoe, descended to within three meters of the lake surface — the whistling of wind across its wings easily audible in the morning stillness — before hearing the male’s acoustic objection and beating its wings desperately to abort the landing and ascend.

Though we had statistical evidence to suggest that male loons used yodels to repel specific intruders, I had never observed as clearly the effective targeting of flying intruders by a yodeling male. While songbirds sing incessantly during the spring in nonspecific fashion — that is, they sing repeatedly over hours, days and weeks to communicate their readiness to mate to any female in the vicinity and/or their willingness to defend their territory to any male that happens to be nearby — male loons seem to use yodels with surgical precision. In other words, loon males yodel rarely, and when they do, they aim their yodel at a specific target with a specific goal in mind.

Why are male loons so stingy with their yodels? Two possibilities come to mind. First, yodels appear to have a relatively high metabolic cost compared to songs of other animals. Perhaps, then, mere energy conservation places a limit on the frequency of this call. Second, Jay Mager’s work has shown that males reveal both their physical size and their physical condition when they yodel. By yodeling, therefore, a male might convey information about himself that he would prefer to keep private. Of course, a large male in good physical condition should be more apt to yodel, one might argue, whereas a small, ailing male should keep his bill shut so as to avoid an eviction attempt by a rival passing overhead.

I look forward to testing the prediction that male loons yodel rarely — and vary systematically from one to another in their tendency to emit the vocalization — as a means to avoid sharing information about themselves. That is a clear, well-grounded prediction that might produce an important finding. But my morning on Woodcock reminded me of a great benefit that I earn from spending time in the field observing loons. One can spend countless hours entering data and churning through statistical analyses to reach a rock-solid conclusion about animal behavior (and I do). But the occasional “Eureka” moment spent with animals in the field is invaluable.