How Loons Should Behave

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A week or so ago I gave a talk to the Northeast Loon Study Working Group. Inauspiciously-named and -initialled, NELSWG comprises loon conservationists from New England, the Upper Midwest, and a smattering of other regions within the loon’s breeding range. At present, NELSWG is the only group that attempts to pull together data on loon populations and brainstorm strategies for protecting the species. During my talk I shared our data showing that masses of adult loons and chicks decline as water clarity declines. I then updated the group on my analysis of male and female traits that lead to breeding success of pairs.

Impact of male (blue) or female (red) pair member on a pair’s hatching success. Both males and females are a drag on hatching success in their initial year on a territory. Females have a slight positive impact thereafter. Male experience on a territory continues to improve hatching success even after 10 years.

To remind you, a male’s knowledge of the territory makes a huge impact on the breeding success of a pair. Since males choose the nest location, males are a drag on nesting success in their first few years on a territory because they place the nest in lots of dangerous places. (Note the low blue bars for years 0 to 3 above.) On the other hand, males that have been on a territory for seven or more years are a boon to pair nesting success, because they have learned the safest spots to place nests. (Note the blue bars from 8 to 20 years on territory.) Females have an impact too. In their first year on a territory, females cause low hatching success for their pair. In later years, female territory experience boosts hatching success slightly.

It is almost more interesting to see the factors that do not affect breeding success. A male’s age does not affect his pair’s ability to fledge chicks at all. At first glance, this seems confusing. How can the male’s age have no positive impact on breeding success of a pair, when a male’s breeding experience on a territory is hugely important? The answer relates to cause. It is true that old males tend to have very high breeding success, but this is not because of their age but because, in most cases, they have been on a territory for many years. We know that age itself is not causing high breeding success because old males that nest on new territories have no greater breeding success than young males on new territories. It is familiarity with the territory and not age that is the salient factor.

Female age has only a weak negative impact on breeding success. In other words, older females lose chicks at a slightly higher rate than young females. This pattern is a bit difficult to make sense of, because the effect is so steady and gradual. Why would a 15-year-old female lose chicks at a higher rate than a 10-year-old female parent? Both females are in the prime of life, in the loon sense.

Effect of female age on a pair’s fledging success. Females cause a gradual decline in fledging success as they grow older.

To the listeners at NELSWG, though, the pattern that was most remarkable was the lack of a strong effect of mate familiarity. While pairs that know each other nest a few days earlier than pairs that are in their first year together, the pattern is weak (see below). Furthermore, the slightly earlier hatch date among pairs that know each other does not translate into a detectable advantage in overall breeding success. In short, pairs benefit only slightly from knowing their mate well.

Effect of pair-bond duration on hatching date. Pairs in their first year together nest later, on average, than pairs that have been together for at least one year.

How can this be? How can a male and female remain together year after year, raise young cooperatively — and still not benefit from this lengthy association? That was the question asked by Lee Attix at the NELSWG meeting. I don’t have a good answer for Lee. As a male in a 38-year relationship who has raised young cooperatively, I am well aware of the benefits that a long-term partnership can bring in the human species. But loons are different.