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A few months ago, a loon naturalist and photographer from New England told me I was wrong when I said that males choose the nesting site for pairs. For 15 straight years, he said, he had watched a female look at potential nest sites that her mates had selected during the pre-nesting phase and then choose to lay the eggs in one of her own favorite sites, ignoring her mates’ suggestions. Thus, he claimed to have an exception to the rule that males choose the nest location.

Now let me say right off the bat that he might be correct. The paper we published showing that males choose the site where eggs are laid demonstrates an overwhelming statistical impact of male identity on nest location. That is, male identity is clearly far more important to selection of nest location than is female identity, but we cannot exclude the possibility that an occasional female might turn the tables on her mate and lay the eggs in a site that only she prefers.

Let’s look more broadly at avian breeding behavior to examine the possibility that a female might buck the usual trend in nest-site selection. Hundreds of studies have shown us that reproduction in birds requires tight behavioral synchrony in mated pairs. Coordination in behavior, in turn, leads to harmonious hormonal profiles of males and females. In other words, mates must be on the same page — both behaviorally and physiologically — throughout courtship, copulation, nest-searching, egg-laying, incubation, hatching, and rearing of young. (Linda Grenzer’s beautiful photo from a few years back illustrates this coordination nicely.) If one pair member is out of phase with its mate — say, not ready to incubate the eggs after they are laid, or unprepared to rear the young — breeding fails.

The dependency of reproductive success on behavioral and hormonal coordination between mates puts enormous evolutionary pressure on pair members to conform to the normal breeding roles and patterns of the species. In general, a male or female that behaves differently from others of the species will not find a mate, or if it finds a mate, will not nest. Weirdos generally do not leave offspring, so weird traits — to the extent that they are genetically based — do not persist in populations. For this reason, I am skeptical that the naturalist has found a female loon that flouts the “males choose nest sites” rule. Based on previous research findings across many species, I would expect a female that laid her eggs in a spot not selected by her mate would be faced with a mate unwilling to share incubation duties.

The naturalist’s claim of an exception to the rule has a more fundamental flaw. It is based solely on observations of a single female and her mates. As someone whose data consists mainly of behavioral observations, I am keenly aware of the limitations of  behavioral data that are not analyzed rigorously, especially observations from one or a handful of animals. Countless times I have thought that loons were behaving one way, only to find, on closer scrutiny and with a larger sample, that they were behaving another. If your sole evidence for a conclusion is “I looked closely at an individual and it looks as though she is doing this”, as in this case, you are on thin ice.

The story of how we learned that males choose the nest site illustrates well the pitfall of trusting limited observation to reveal true behavioral patterns, so it is worth saying a bit about how that analysis unfolded. On the face of it, we thought, how could males choose the site where a mated pair lays their eggs? After all, females, not males, lay the eggs. In a very basic sense, females must always control where the eggs are laid. Therefore, I expected my analysis to show that females controlled nest site selection. However, egg-laying in loons occurs only after many days of nest-searching within the territory. So it was conceivable that males might somehow influence their mates to lay the eggs in one spot or another. In fact, our statistical analysis showed that males take the lead in nest-searching and spend much more time than females in looking for a nest location. And an additional set of statistical tests showed unambiguously that males control where the nest goes. Here is the essence of our finding. Nesting pairs comprising a male that bred previously on the territory and a new female unfamiliar with the territory tend to reuse the successful nesting site from the previous year. Indeed, pairs composed of an experienced male and a new female select nest sites identically to pairs wherein both pair members are experienced on the territory. In contrast, pairs made up of a female with previous breeding experience on the territory and a male without experience there ignore the successful nesting location from the previous year and instead select entirely new, untested sites for nesting. Such pairs show no more knowledge of good nesting sites than do pairs in which both pair members are new and unfamiliar with the territory.

I was — and still am — puzzled by these findings. It seems absurd that a veteran female breeder permits her novice mate to choose an untested nest site, when she “knows” the best place to nest, based on her past experience. As the egg-layer, moreover, the seasoned female would seem to have absolute control over where the eggs are placed. But loon behavior defies common sense in this case. The data are very clear.

One more point about “knowing”. The naturalist who insists that he saw a female choose nesting sites is quite confident in his report. That is, he contacted me to inform me of his finding, not to try and reconcile his interpretation with mine. He “knows” that he saw a female select the nest site in the territory he observed. As humans, we often make observations, puzzle over their meaning, and then settle on an explanation of what we have observed. Then we get stuck. We become so invested in our explanation that we are unwilling or unable to give it up. In fact, reluctance to admit errors is a great problem in science, as we often make findings, build our reputations on those findings, and are unwilling to admit — even in the face of overwhelming evidence to the contrary — that we were wrong. I made a great error of this kind a few years ago and took many months to admit my mistake.

Human stubbornness of this kind makes sense….to a degree. In a world where we encounter many people who try to fool us or influence us to serve their own interests, we should show a strong tendency to “stick to our guns”. In the age of information, though, we also have access to useful knowledge from skilled practitioners — people who have rigorously and critically tested ideas and considered alternatives before settling on a conclusion. If we can see no reason why they would benefit from misleading us, we would do well to listen.