Not only have dead trees and other sources of natural nest cavities been removed by rapid development, but the many agricultural barns which used to support a healthy Barn Owl population have been disappearing from our area as the farms and ranches have been replaced by subdivisions and shopping malls.
The numerous owls which used to help control rodents such as gophers, rats and mice have few places to nest, and their numbers have been falling.
Mt. Diablo Audubon Society has had Eagle Scout candidates building and installing Barn Owl nest boxes for natural rodent control in parks and open space to deal with gopher problems, but dangers from rat poisons placed nearby have curtailed the installations by scouts. However, scouts have made educational displays of skulls found in pellets inside barn owl boxes for places like Lindsay Wildlife Experience to show their visitors what these owls eat.
Most people don't realize what great assets barn owls are for rodent control - a textbook example of Integrated Pest Management, matching up rodent-feeding predators with rodent populations. Unfortunately, the City of Walnut Creek Parks & Open Space Commissioners voted to allow Public Works staff to continue to use rodent poison. In one park it was discovered that the reason why boxes we had installed were not being used by owls was that rodent poison had been used for so many years there were no owls left, despite the fact that poisoning raptors is a felony enforced by U.S. Fish & Wildlife Services.
It is crucial that the Barn Owl boxes are built following appropriate designs, as there are lots of unfortunate stories of what is found when they are not constructed correctly.
Important link: Build A Barn Owl Box (and much more!) 10MB file
Barn Owls are only one of many important species threatened by the use of anticoagulant rat poisons (an MDAS Conservation Concern), which can kill predators that eat rodents carrying these compounds.