Mating by birds — the passage of sperm from the male to fertilize eggs produced by the female — is quite unlike that of mammals.
It begins with the male Tree Swallow, in this case, finding an appropriate nesting cavity. Many Tree Swallows today use nest boxes. The male will defend the box. He will make a chattering call, perhaps fluttering wings. This can be indicative of fitness; female want fit mates. The birds will gape, touching bills with open mouth. The male might assume an upright posture on the nest box, pointing his bill at the sky. The female can signal acceptance by taking the male’s place. The birds might face each other and make bobbing head movements. There can be flight in unison.
The male can signal his readiness to mate by making a series of vocal clicking sounds. The responding female will take a position that allows the male successful approach.
To mate, birds press their cloacas together briefly. The cloaca is an all-purpose vent, used for digestive, excretory, and reproductive systems. The female is perched, the male in the air. Both birds move their tails to the side to give the male access to the female cloaca. Sperm is passed.
The birds may mate several times to increase chances of successful insemination. The female can store sperm until she begins to lay eggs. The event photographed lasted slightly more than minute, much of time the male working against a wind to accurately position himself.
These Tree Swallows were photographed in Wayzata on April 15.