It is late May again. With nesting underway, loons must confront the first of many hazards that stand between them and successful parenthood. Simulium annulus are blood-sucking black flies that attack common loon specifically. In a good year, they make life miserable for loons, forcing them to increase their diving frequency and decrease preening and resting — simply to avoid spending time on the surface, where they are at the mercy of the flies. In a big fly year, like 2014, black fly infestation can cause 70% of all first nests to be abandoned.
While we can understand how bites from flies would make life miserable for incubating loons, why should loons — like the male above photographed today on Tom Doyle Lake — leave their nests? Like many problems in biology and life, this involves a trade-off. A loon that incubates in spite of relentless attacks from black flies can hatch chicks from the nest. But tenacious incubators also face a high threat of blood-borne parasites from the flies, which might weaken them and shorten their lives. On the other hand, a loon that punts on its incubation duties in the face of the flies will lose that nesting attempt but be able to renest two weeks or so later, when black flies are all but gone. Although this delay lessens the chance of producing chicks for the year, it might make sense to a bird that must take a long-term view — favoring health and condition, rather than risking disease for a slightly greater chance of producing chicks.
As I was writing this last passage, I realized that this is yet one more case where we might expect the age of a breeder to have an impact on incubation behavior, based on senescence theory. An aging loon that stands to have only another year or two on its territory might well have greater fitness (i.e. lifetime chick production) by investing heavily in the current breeding year, rather than preserving its health for a future year that might never come. So we might expect male loons, which senesce mightily, to be tenacious incubators during their waning years. (In contrast, young males should readily abandon a nest, when black flies become thick.)
I will certainly look at the data to see if age has an impact on male incubation behavior; I am excited to do so. But there is a catch. As I have noted in a previous post, dual incubation by both males and females muddies the water. That is, we might expect that an old male would boost his eggs’ chances of hatching with heroic incubation during a heavy black fly season, but such a male is unlikely to be able to compensate fully for a mate that refuses to incubate. So at best we might expect that an old enthusiastic male incubator might decrease likelihood of nest abandonment by an amount great enough to justify his efforts.
I will let you know what I find out.