Bluebird monitors each have their own area in which to monitor as many as 20 bluebird houses, at least every two weeks, and preferably on a weekly basis. To start out the nesting season (early March), the houses are cleared of any remnants of old nests or past occupancy. English sparrows are often described as the worst enemy of bluebirds. Their nests, eggs, and young can legally be destroyed. However, we do allow tree swallows and wrens to use the boxes. Monitors carefully keep track of all eggs and newly hatched young, and register the number fledged, after which they remove the nests and clean the boxes for the next nesting.