Birds often check out several locations before deciding on which one they will use to raise a family. Sometimes a nest is started or even completed, but not used. This can be disappointing for the monitor who was looking for some real action in that box. Here are probable reasons:
Dummy Nests: A dummy nest is an eggless nest not used for breeding, usually built by the male. House Wren males build true "dummy" nests in a number of cavities within their territory. These nests are usually loosely constructed, and lack an egg cup lined with fur/hair/grass/feathers. These may be to offer the female some choices. She chooses the cavity she wants to use, and then reconstructs the nest in that location. Sticks in other boxes are left in place, possibly to prevent other competitors from nesting nearby. (As this does decrease the likelihood that those boxes will be used by another species, I remove dummy nests. See more on deterring House Wrens.) Carolina Wren and Prothonotary Warbler males may build dummy nests.
Indecision: Carolina Chickadee pairs sometimes start excavating and nest building at several sites before choosing one for egg laying.
Paired boxes: When two boxes are near each other, sometimes a Tree Swallow or Bluebird will build a partial or complete nest in the neighboring box. This may be because they can't decide which box to use (see indecision), or it may be to deter other birds from nesting in the second box. Sometimes the second nest is used for a second brood.
Problems and Predators : Sometimes a nest is started and then abandoned because of a problem. Maybe paperwasps have moved in, fire ants showed up, or a predator spooked them. (I had a Tufted Titmouse abandon nest building when I opened a box while she was inside adding material.) Maybe one of the pair is lost. House Sparrow or Tree Swallow harassment may drive them off. A squirrel might take the box over. Sometimes eggs are removed/destroyed or eaten (e.g., by House Wrens, Squirrels, etc.) before you even knew egg laying had begun. If unhatched existing eggs do not hatch after incubation, the female may lay a new clutch or move. Many birds will move to a different location after an unsuccessful nesting attempt.
If you are unsure whether the nest of a native bird is active or not, DO NOT REMOVE it. See more info on cleaning out nestboxes.
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