The sheet metal should extend down at least 3 feet below the bottom of the box, rather than from the ground up. Some bluebirders recommend that the guard not be placed on the box until after the bluebirds have started their clutch, while others have had success leaving the guard on the box year round. This guard is available through NABS . This is an extra block of wood placed over the nest box entrance, with a corresponding hole in its center. Similar preventative methods apply to snakes and cats.
Snakes can climb smooth poles, even greased ones.
Snakes are more of a common problem in the southern states but some snake problems may occur all across North America. The stove pipe and cone guards are effective for snake control. Also effective is a 24 inch piece of hardware cloth placed directly under the box. Both domestic and feral cats pose a threat to bluebirds. The hanging guards mentioned may detour a cat but they may jump as high as 6 feet.