Tremolo Calls

Around 9 am this morning we could hear tremolo calls in the distance. Judging by the changes in volume and possibly some Doppler effect, we can be pretty sure most of the calls were from a single loon flying over the pond and making at least two passes. And some of the calls were from the incubating loon’s mate at the opposite end of the pond.
Unpaired loons looking for a territory to occupy use the tremolo as a means of scouting out a new lake. They fly over the lake while calling with a tremolo and wait for a response. If there’s no response, they know they can safely land. If they get a response, they can judge by the pitch and intensity whether or not they want to risk a landing.
With two eggs in the nest, we can be sure that the response was a warning to move on. At this time on this pond, visitors would not be welcome. Upon hearing the tremolos, the loon on the nest went into an alert posture and showed a lot of interest in the calls, but left it to the mate to answer.