I have just completed my paper on black flies. The paper presents evidence that black flies cause nest abandonment, which was lacking in the literature before. The evidence is pretty convincing, I believe. (We shall see what my scientific colleagues think when I submit the work for publication in the next week.)
In the course of looking at black fly impacts on nesting, I stumbled into two interesting findings. These findings were serendipitous, like much of what scientists report. That is, I was keenly focused on one topic — black flies and nest abandonment — when I made a finding related to another topic — other causes of abandonment. In fact, I analyzed statistically a whole set of factors, some seemingly unrelated to black flies, that might have predicted nest abandonment. Among these were age of the male, age of the female, duration of the pair bond between them, exposure to wind (which might have kept the flies at bay), size of breeding lake, and distance from the nest to the nearest flowing water (from which black flies emerge as adults).
I was excited, but also baffled, to discover two new predictors of nest abandonment. First, pairs on large lakes are less prone to nest abandonment than pairs on small lakes. Second, pairs containing an old female are far more likely to abandon a nest owing to black flies than are pairs containing young females.
Now, I like to think that I know everything about loons. When I am visiting a study lake and someone asks an easy one like, “Do loons mate for life?”, I puff myself up, lower my voice an octave, affect a mild British accent, and pontificate on the serially monogamous breeding system of Gavia immer. But I was wholly wrong-footed by these two new findings. I had been so laser-focused on black flies as the prime movers in nest abandonment that I had included age and lake size in the analysis almost as an afterthought. I had not even considered what it would mean to learn that age and lake size were significant predictors.
The statistical significance of lake size as a predictor of abandonment forced me to confront a complex variable. If numbers of black flies are correlated with nest abandonment (as they are), then it requires no great conceptual leap to infer that black fly harassment is causing loons to abandon their nests. But the fact that lake size predicts abandonment opens up a much broader range of explanations, because lake size is correlated with degree of human recreation, pH, wind exposure, wave action, available food, and numerous other factors. Having picked through the possibilities, an energetic explanation seems most likely to explain the lake size pattern. That is, large lakes provide more food than small lakes, so loon pairs on large lakes should be in better health and condition than those on small lakes. Well-fed, healthy adults with strong immune systems should be better able to cope with the blood loss and exposure to blood-borne pathogens (like Leucocytozoon protozoans, which cause a malaria-like disease in birds) than under-nourished individuals with weaker immune systems.
What about the higher abandonment rate of pairs that contain an old female? Here again, energetics might be the key. Old females senesce — they experience much lower survival and slightly higher vulnerability to eviction than young females. So it stands to reason that old females are in poorer body condition and are more likely to abandon nests when attacked viciously by black flies. Reproductive decline among old females is widespread in animals, and the tendency of old female loons to abandon nests more readily seems consistent with that pattern.
But what about males? As I have emphasized in recent blog posts, males senesce even more dramatically than females do. How is it possible that old males can continue to incubate eggs when being bitten mercilessly by black flies when old females cannot? Terminal investment appears to be the answer. Terminal investment — efforts to increase breeding output as death approaches — occurs only among male loons, even though both sexes senesce. As the months have passed, we have learned that male loons not only become hyper-aggressive when they reach old age (15 years) in an apparent attempt to hold their territory for another year or two of breeding, they also seem to show a more subtle willingness to try harder to hatch eggs and rear young to fledging. The new finding showing that old males do not abandon nests as readily as old females when beset by black flies is thus part of a growing pattern.
My tentative explanations for the impacts of lake size and sex on nest abandonment are not the end of the story, of course. Rather, they raise more vexing questions. Why on earth would a loon settle to breed on a small lake, when small lakes doom loons to poorer body condition, a higher rate of abandonment, and the likelihood of losing one or both chicks in the event they can hatch the eggs? And even if the higher rate abandonment of nests by old females fits a growing pattern, why do males and females differ so much in their life-history strategy? We do not know….and this is why I love my work!