Setting up housekeeping

Checked all of the bluebird houses for activity. Of the 16 houses, all but three have nests started or completed. No eggs yet. Haven't installed the solar lights by each house. It is the eggs or young chicks that attract the bears but I don't understand why the solar lights work in repelling them. Thought I would wait until there is need for the lights and then remove them when nesting season is over. Now if I could just figure out something that would repel ticks. Give me time ... I'm working on it.