The Downside of Dual Incubation and a Ray of Hope

To be sure, 2014 is shaping up to be a dismal breeding season. Ice-out occurred weeks later than usual, owing to thick ice and cool weather in March and April, and delayed breeding for all loon pairs. The ensuing warmup in May caused the black fly population to explode to higher levels that we had ever seen. Pairs that had just laid eggs were blanketed by the relentless bloodsuckers and incubating males and females driven off their nests at rates never seen before. In short, almost all loons abandoned their first nests.

Although it is counterintuitive, loons are probably more vulnerable to nest abandonment than many other birds because males and females incubate the eggs equally. Consider the plight of loons trying to incubate their eggs to hatching for 27 days in the presence of black flies. Each pair member must incubate for stretches of several hours before being relieved by its mate. The rotation system must be efficient enough that eggs are incubated over 99% of the time. When not sitting on eggs, a breeder rests, preens and forages to replenish its energy reserves. If either pair member fails to incubate, the nest is doomed to failure, because its mate cannot compensate for missing incubation by remaining on the eggs at all times without long breaks for foraging. So we have a case where the weakest link breaks the chain. Even a tough, determined male incubator is destined to lose a clutch if his mate is less determined than he is and refuses to sit on the eggs and tolerate the torment of biting insects.

We have learned about the necessity of dual incubation from past observations. Nest abandonment commonly occurs following territorial eviction of a pair member during incubation. If its mate is evicted by an intruder, the remaining pair member usually continues to sit on the eggs for a time. With rare exceptions, though, its new mate (the usurper) does not incubate — why should it sit on eggs containing young to which it is unrelated? — the cycle of shared incubation duties breaks down, and the nest fails within a day. (If it is early enough in the season, such pairs will lay a new set of eggs that both will incubate.) A rarer cause of nest failure during incubation is death of a pair member. In 2005, the female on Alva Lake was killed by an eagle while sitting on the eggs. The male valiantly sat on the eggs, taking breaks to forage from time to time, even as female floaters competed in front of him to fill the breeding vacancy. Despite being within a week of hatching, this male could not complete incubation on his own, and the nest was lost.

Enough talk of failure! I will end on an up note by showing you the sweet photo that Linda Grenzer took of a breeding male sitting on a nest. This bird and his mate both tolerated the record outbreak of blackflies earlier in the season and were rewarded with a little fuzz-ball — a much-needed reminder that all is not lost this year. Let’s cross our fingers that other pairs, many of which are incubating a new clutch of eggs after abandoning the first, will be able to duplicate this effort.