Curious Behavior on Manson Lake

On Manson Lake, the long-time male breeder was displaced in May of this year by a younger male. After seizing control of the territory, the usurper proceeded to nest with the long-time resident female.

But the original territory owner did not give up on his territory easily. He remained in the vicinity and was a frequent intruder into his old domain. In fact, between the 13th and 19th of June, the old male drove off the usurper and regained ownership of his territory. At this point, incubation by the female and the usurper was well underway, and the eggs were within 10 days of hatching. The female continued to incubate the eggs, and the old male — though he had played no part in the reproductive effort and had not fertilized the eggs — defended the nest. (We did not observe him to incubate the eggs, however.) When the eggs hatched, he treated the chicks as his own.

Territorial usurpation creates some awkward reproductive decisions for loons. It is common for a breeder to lose ownership of the territory during a breeding attempt. In most cases, incubation ceases, and eggs are abandoned. If there are chicks present, they are often killed by the usurper, while the remaining pair member looks on helplessly. Manson is not the only lake where a usurper has seized control of a territory but continued to care for offspring that do not belong to him. In fact, we have observed at least 5 other cases of such behavior — all among males. That is, we now have 6 cases where a usurping male has wrested territorial ownership away from the male breeder but either permitted the young to survive and be reared by the female breeder (2 cases) or seamlessly stepped in to assume paternal responsibilities (4 cases). I am beginning to see a pattern!

Evolutionarily speaking, animals should behave so as to maximize the number of their own young they contribute to the next generation. It is generally unexpected to see animals care for young that are not their biological offspring.  I will continue to be on the lookout for more examples of such behavior in loons — and will puzzle over the apparent tendency of male usurpers, but not females, to be willing to adopt the offspring of their predecessors.