Although some birds may have long and involved courtship displays, it does not take long to actually mate. Mating (the transfer of sperm from the male to the female) is very brief and takes place by the coming together of the two birds’ cloacas, which function as excretory and genital organs. During the breeding season, the area around the cloaca becomes swollen and reaches its peak size when the time is just right to reproduce. Sometimes it is called a “cloacal kiss.”
When bird banders catch a bird in their nets they “sex” it by blowing on the bird to part the feathers to look for a cloacal protuberance, which would only be visible in breeding male passerines.
Some birds have spectacular courtship displays. Scientists think that Bald Eagles find a mate and stay together for life unless one mate dies. When they are courting they do incredible acrobatic flight displays. One display is called the “cartwheel display”: the pair fly very high, lock talons, and tumble/cartwheel back toward earth. They let go of each other only at the last moment to avoid crashing into the ground. Watch an aerial courtship display by Bald Eagles: