If you feel as though you haven’t heard from me in awhile, it is because I’ve been in the weeds. But my time in the weeds has been fruitful.
I have been analyzing loon reproductive patterns. Specifically, I am investigating what about adult loons themselves and their surroundings leads to success in hatching their eggs. The trove of detailed breeding data we have — 100+ territories followed closely for almost 30 years — gives us a unique window onto predictors of hatching success. I gave a hint of this analysis some months ago. But more data and a refinement of earlier statistics have clarified our understanding and led to new findings.
One of these findings might be familiar. Males, which choose the nest location, improve in hatching success as their familiarity with a territory grows. This is easy to understand. They use trial and error to find a good nest location. During the first few years on a new territory, they blunder about and lose many nests to predators. Over time, they improve their hatching success by avoiding sites where predators took the eggs. As the predictive curve shows, males make rapid progress in their first year or two on a new territory. Once three to four years have passed, their year-to-year improvement is small.
However, males continue to show incremental — but statistically significant — improvement in hatching success from year to year, even after 15 years on the same territory. Although the annual improvement is slight, it adds up. For example, a male with 20 years on a territory has a 27% better chance of hatching his eggs than a male in his second year. It’s a little hard to imagine the reason for this sustained improvement. Does a male only notice certain biological aspects of his breeding territory through prolonged exposure to them? For example, might a male observe over the course of many years that mammalian shoreline activity is higher at the south end of the lake than the north end and avoid placing a nest at the south end if — say, in his 17th year — circumstances should force him to relocate it? Maybe. In any event, the finding is exciting for the study of animals generally. It suggests that, for an intelligent animal inhabiting a spatially complex territory, adaptive learning — that is, learning that increases reproductive fitness — never really ends.
What about females? Like males, females improve in hatching success over time, but the pattern is completely different. Females have low hatching success in their first year on a territory but improve markedly in their second year. However, they do not continue to improve. Hatching success for females “tops out” after their second year on a territory. The female improvement pattern seems less difficult to explain than the male pattern. Since females do not control nest placement, they do not have any obvious means to get better at hatching eggs as males do. But they must learn something vital about the territory in that first year in order to enjoy greater hatching success in all other years. Perhaps they quickly learn where to forage in that first crucial year and retain that knowledge from the second year on. If so, that important knowledge might allow them to nest more often or better sustain the incubation effort.
Comparison of the two figures shows the contrast between males’ steady improvement in hatching success over their entire residency on a territory and females’ one-time jump up to higher hatching success after their initial year.
How can we be sure that the effects I have described are attributable to females’ and males’ experience on a territory instead of merely age? It seems reasonable to hypothesize that adult loons might get better at hatching eggs simply as they gain experience with nesting, regardless of the territory they are on. But this is not the case. I included age as a variable in the statistical analysis, and age turned out to be a poor predictor of hatching success. Thus, a 29-year-old male who loses his territory and settles with a new female on a new territory is right back to square one. He must learn again by trial and error on the new territory where to nest and where not to nest. He is no better off than a five-year-old male that has just settled on his first territory.
What about duration of the pair bond? Since males and females both incubate the eggs equally, they must coordinate their incubation schedules in order to hatch the eggs. So one might have expected that many years of breeding together would translate into greater hatching success for a male and female. But I included duration of the pair bond as a variable in the statistical analysis. Like age, pair bond duration is inconsequential. Yes, males and females that have been together for many years tend to hatch their eggs more successfully than new pairs, but they do so because: 1) the female is past her first difficult year of low hatching success, and 2) the male continues to improve his nest placement over many years on a familiar territory.
Why do I spend so much time making what must seem like fine distinctions? Does it really matter whether old males and females hatch a lot of chicks because they are old or because of experience on a territory? Who cares that old males and females hatch more young not because of more time spent together but because each of them has gained experience with a territory as individuals? We should all care. Since territory familiarity, not age or pair-bond duration, confers reproductive ability upon adult males and females, we should redouble our efforts to ensure not merely that loons themselves live long lives, but that we minimize disturbance to the familiar nesting areas on which their breeding success depends.
Beautiful photo of Clune sitting on his nest by Linda Grenzer.