We have known it for some time. Young loons looking for territories observe chicks on a lake and return the following year, hoping to evict one of the resident pair members and take over the breeding position. The effect is dramatic; the intrusion rate increases by 70% following a year with chicks.
From the viewpoint of a young floater (a young bird that has not yet settled), chicks are a boon. The mere sight of young encapsulates all of the information necessary to size up a potential breeding location. Instead of having to learn about breeding success by trial and error, the floater need only seize an existing territory that has proven to be a chick producer. (Of course, seizing a territory has costs.)
From the breeders’ standpoint, raising a chick is like painting a great big target on their backs. Those little brown fuzzballs look cute in the moment, but their presence portends many battles with floaters the following year. No wonder parents take steps to hide their chicks, when they can.
A week ago, during the writing of my proposal to NSF, I made another finding related to chick production and its impact on a territorial pair. In this case, I was examining our data related to aggression between breeders and floaters, which can take the form of grappling battles between two loons, lunges by one loon at the other, chases across the water, or severe feather damage to the head of the breeding bird — evidence of a recent violent encounter. In a few cases, I could infer aggression because a pair member was injured on its territory after having been healthy a few days beforehand. When I threw all of observed aggression or evidence of past aggression together and asked whether it was related to chick production, I found that more aggression occurs per visitor both in a year in which chicks are present and also in years following chick production.
Now you might wonder whether I have moved the needle here. We already knew that chick production causes a spike in territorial intrusions the following year; what extra information do we get from knowing that aggression also spikes? The answer relates to costs. Just visiting a chick-producing territory more frequently expresses interest in the territory. The fact that floaters elicit a greater rate of aggression per visitor tells us that floaters are willing to incur costs (i.e. the cost of injury) while visiting such territories. In short, floaters flock to successful loon territories — and they mean business when they do so.