Purple Martins are quite unique in that most only live in bird houses.  They rely on man-made housing to nest in their breeding grounds.  A subspecies of Martins on the other hand, have developed an interesting alternative: using woodpecker cavities in Saguaros cacti. These cacti are enormous, sometimes growing over 20m!  Due to increased development in Arizona, (the state flower if which is the Saguaros blossom), this giant plant is endangered.  The plants are specially suited for the desert and provide food and shelter to more animals than just Martins.  

-Tehmeena Chaudhry