Two days ago, two of us — Mari and I — observed two instances of a curious and rare phenomenon in loons: adoption of an additional chick by pair with chicks already. In both cases, chicks on a very large lake shifted from their biological parents who resided in one territory on the lake to another breeding pair with two chicks on an adjacent territory within the same lake. Mari observed the Boom-Hodag Park pair adopt a chick from the Boom-Thunder territory; I saw the Pickerel-West pair accept one of two Pickerel-South chicks into their family. (All chicks had been banded, so we knew their origin.)
We are not the first to observe behavior of this kind. It has been reported occasionally on large lakes that chicks get “scrambled up” and end up with the wrong parents. But we had never observed it in our study area. To see it occur twice in the same day is quite extraordinary and makes us wonder why and how it occurs at all.
Certainly most chicks are best off if they remain with their biological parents, providing sufficient food is available for their parents to feed them. As I noted in a recent post, it is a desperate chick indeed who strikes out on its own in hopes of finding a new family. The odds are against finding a family willing to accept it — and one that has one or more chicks of the similar age as the wanderer, so that the wayward chick is not beaten up by its foster sibs. (In fact, it is a concern in both chick scrambles we saw yesterday that the host chicks are slightly larger than the foster chicks.)
So why does this behavior occur at all? We do not know at present, but I can offer one plausible hypothesis. This time of year is characterized by high rates of territorial intrusions. When intruders enter a territory with chicks, chicks hide out near shore and leave their parents to engage the intruders. Parents and chicks are reunited only after all intruders have left the territory, which might be an hour or more after intruders land. In most territories, which consist of single whole lakes, reuniting with parents is straightforward. Parents search where they left the chicks, and parents and chicks vocalize to find each other. But on large lakes, it is conceivable that chicks might lose track of their parents (and vice-versa) following intrusions. If chicks respond to the inviting calls of neighboring parents (which of course, are intended only for their own chicks), and neighboring parents accept non-biological chicks as their own, then chicks could wind up with the wrong parents. Scrambling of this kind would be most likely, of course, where two pairs with chicks use adjacent parts of a big lake.
It will take more than two instances of chick scrambling for us to be able to discern its precise causes. But the loss of a chick or chicks by a pair could well have a negative impact on their fitness (i.e. if the lost chicks perish). So this behavior pattern seems important and worthy of further thought and study.