It is believed that they are very effective in ridding the yard of unwanted insects.
I found this Purple Martin family in northern Arkansas. It's a wonder I have any pictures at all. I was in the centre of a large lake when I spotted some small birds in a tree on the shoreline. I couldn't ask the tour guide to move us closer so I tried to steady myself as much as possible and started firing away with my Canon.
When I reviewed the pictures, I was quite surprised to find that I had caught a shot of the young being fed. That is the shot I would have tried for if I had been directly under the tree.