One of the quirks of the loon breeding system is that males choose the nest location. I have mentioned this fact many times in posts. I still scratch my head over the pattern, which makes no sense on many levels. Why would a breeding system evolve in which one sex or the other has sole discretion for deciding where the nest goes? In such a case, death or eviction of the choosing partner is devastating for its mate.

Consider this scenario. A male and female settle on a new territory on a protected cove in a large lake. At first they struggle to hatch young, but they begin to do so after a few years and enter a period of steady chick production. After 12 years, the male dies. Since he is the repository of information about safe and dangerous breeding sites in the territory, the veteran male’s death leaves his mate in the lurch. She must wait patiently while a new male takes over as breeder and stumbles badly for a few years by placing nests in the path of hungry raccoons. Ultimately, the new male learns by trial and error where eggs will be safe from predators, begins to reuse those favorable locations, and becomes an accomplished breeder. But, having endured several years of her mate’s reproductive ineptitude, the female has lost precious breeding time.

This scenario plays out incessantly in common loons. We field observers experience some of the frustration that veteran females must feel as we watch the poor nesting choices of their novice male partners. Until a few days ago, however, we did not appreciate the stark contrast between male and female breeding experience.

As the graphs above show, males do not simply improve in hatching success in their first year or two on a territory; they improve over a period of at least 20 years! And the cumulative impact of this steady improvement is massive. A male’s odds of a successful hatch are 35% better in year 20 than in year 1.

On the other hand, females do not experience better hatching success in year 20 on a territory than in year 1. In fact, there seems to be the faintest whiff of improvement for females over the first five years on a territory — and a tailing off around 20 years on — but these apparent patterns are not borne out by statistical tests.

By the way, these findings do not mean that a veteran female might not happen to enjoy higher hatching success as her tenure on a territory increases. What it means — and this can be confusing to think about — is that any boost in hatching success an individual female enjoys results from the growing nesting experience of her mate, not herself.

I am still scratching my head over this dramatic difference in breeding behavior between male and female loons. It still baffles me on many levels. But the dramatic, long-term gain in nesting ability of males does help me understand the viciousness of male territorial behavior. A male that has reaped enormous benefits while spending two long decades to learn the ins and outs of his breeding territory should fight desperately to keep it.