Location: The location of the bird houses may not be suitable for the blue birds. They prefer to build their nests in sheltered areas that are protected from the wind and rain. If your bird houses are in an exposed location, the blue birds may not feel safe building their nest there.
Size: The size of the bird houses may not be suitable for the blue birds. They typically build nests that are about 4-5 inches in diameter and 6-8 inches deep. If your bird houses are too small or too large, the blue birds may not be able to build a comfortable nest.
Material: The material of the bird houses may not be suitable for the blue birds. They prefer to build their nests in natural materials like wood or bark. If your bird houses are made of a different material, the blue birds may not be able to get a good grip on them to build their nest.
Competition: There may be too much competition for nesting sites in your area. Other birds or animals may be competing with the blue birds for the available birdhouses. If there are too many competitors, the blue birds may give up and find a different location to nest.
If you want to attract blue birds to your yard, you can try making some changes to your bird houses to make them more suitable for them. You can also try putting up more bird houses in different locations to increase the chances that they will find a suitable nesting site.