Tree Swallows are back and competing for both mates and nesting sites.
Tree Swallows don’t build their own nests but make use of cavities previously carved by other birds, often woodpeckers. The cavity in the piling, below, was made by a flicker a decade ago and has been taken over by swallows in the last few years. Males find and defend a potential cavity and then offer it to a female. A male without a cavity nest can father chicks, but a female without one cannot reproduce.
Owing to the competition among Tree Swallows, they are combative. Males attack other males; Females attack other females. Further, some apparent attacks may be acrobatic displays of fitness between potential partners. Only the birds know what they are doing.
It is probably a male that is looking out of this cavity in an attempt to attract a female to his find. But, is that a competitive male or an interested female on top of the piling?
The two birds then set about doing some acrobatic flying. The interpretation of this depends upon whether it is same-sex aggression or opposite-sex courting.
The acrobatics went on for a minute or two.