The male Bluebird spends a lot of energy and time defending the next site as it is challenged frequently by Tree Swallows looking for a nest box of their own. The Tree Swallows have an uneasy feeling when at this particular nest box, likely having been chased away countless times before, knowing the Bluebirds can't be far away. The Swallows linger at the box and peek inside but do not enter. Sure enough, the male Bluebird spots them and makes a beeline straight for the next box and the Tree Swallows. A fight sometimes ensues and the male Bluebird always prevails. But the Tree Swallows don't give up easily. I've seen the Tree Swallows come in groups of 3 but this one time 4-5 of them decided to make a go for the box hoping perhaps the larger number would suceed in getting a foothold on the next box. The Tree Swallows dive-bomb the lone male Bluebird who had to take cover inside the box (also to ensure none of the Swallows would get inside). Usually the male Bluebird does the nest box defending on his own but in some situations the female will come over to help as was the case this time. The Tree Swallows took off after a tussle but are quite a persistent bunch and will be back to test the Bluebirds again.