While sitting on the front porch I watched as a male and female bluebird landed on and entered the bluebird box I erected 4 years ago. This year will be the second year in a row that they did not nest in this box. I've seen them land on and enter it during those two years, only to find nothing more than a few strands of pine straw inside. Something had spooked them into abandoning this box as a nest site. Since I'm up here sporadically I'm unable to monitor the box like I do my purple martins at home. But the fact that they nested in 2007, 2008, and 2009 means those years must've been successful. Usually when something stresses them they will abandon the site and look for better digs. So what spooked them? There could be several causes:
Nest site competitors: such as Wrens, Chickadees, English Sparrows, etc. could've entered the nest and tossed the nestlings to the ground, causing abandonment, but then if that had happened I would've found the competitor's nest in the box.
Infanticide: happens with bluebirds (as well as with other bird species) so maybe another bluebird tossed the nestlings because of a territorial dispute, which inevitably led to the abandonment of the box. This usually occurs during breeding season when hormone levels are high.
Fire ant invasion: these ants are found scattered throughout our property, but if they were to blame I would’ve found the skeletal remains of nestlings inside. Plus I make sure to destroy any nests, especially those that are near the box.
Predators: (snakes, raccoons, squirrels, feral cats, etc.) raid the nestbox then usually the bluebirds will abandon it as a nesting site for obvious reasons. I have a predator guard on this box, and see no sign of a predator invasion, so I feel that it was not a predator that caused this behavior.
Parasites: When parasites (mites, blowflies, lice) become intense it will not only cause nest site abandonment, but will also force the adults to abandon young, or the young themselves to jump from the nest. So far I’ve never come across this when I’ve done nest checks.
Paper wasps: I did find paper wasps (Polistes) trying to build a nest several times inside. I did remove these nests and applied soap afterwards to the ceiling to hopefully prevent a repeat of this. Until I checked again recently and found a small paper nest that had been placed on one of the inside walls of the box. I guess since the wasp couldn’t get the pedicel (link) of the nest to stick they then went to the wall which I hadn’t soaped. So I removed it and then “soaped” the ceiling again and all of the inner walls.
After going over all of this through my mind, I have come to the conclusion that the wasps were the culprits. So I think I will try moving the box to a different location (moving it will make them think it’s a different box) and see if that helps. I will also have to make sure that I keep the inside of the box good and soaped to prevent this from happening again. Hopefully this will make a difference.