Predators

Cats

Domestic and feral cats are a threat to bluebirds. As they are able to jump as high as 6 feets, you should take this into account when placing your bluebird house. 

Placing them on a high position on a pole, having an overhanging roof as Elijebirds bluebird houses or attaching your Noel guard will make it more difficult for cats to reach the bluebird house entrance.

Racoons

For racoons, as for cats, try to keep the racoon from reaching onto the box. Again, Noel guards are tremendously effective as racoons will not be able to reach the hole entrance and will not dare to step on it as they will feel it unstable.

Snakes

As for snakes, the best is to use a smooth pole with a cone guard. Snakes can climb even greased smooth poles but attaching a cone guard will impede snakes from climbing. A Noel guard will also help.