A quick Google search turned up this from mouse planet-
During the annual Epcot Flower and Garden Festival each year, purple martins, the largest of the North American swallows, return to comfy white homes behind Mouse Gear to start new families. As crowds gather to enjoy the festival, the martins go about their daily routine of raising their next generation, paying no attention to the hustle and bustle of nearby guests.
Some guests imagine that those white PVC gourds with an alphabet letter on them are cameras or special lighting fixtures on a white pole, but they are actually safe havens for birds who sometimes have to battle other sparrows and starlings for the treasured space. The letters help identify each gourd for documentation by the Animal Programs team.
As part of the Disney Company's commitment to conservation efforts, there are more than 200 man-made white gourds on property. At Epcot, the temporary residences are located on stage between Mouse Gear and Test Track, while an area over twice that size exists backstage at Disney's Animal Kingdom Park.
There are approximately 60 houses at Epcot and between 160 to 180 houses backstage at Disney's Animal Kingdom. In 2005, Disney's Animal Kingdom set up their first six houses for martins and used speakers to play a recording of a special song that the male purple martins sing to attract other martins. It worked, and two pairs set up house, one pair in each of two martin houses.
https://www.mouseplanet.com/10213/Purple_Martins_of_Epcot
