During the mating period, the male Magnificent Frigatebirds (Fregata magnificens), as with all other frigatebirds, gather together up to 30 at a time, in an old nesting area, with some sitting in the old nests. They spread their wings and at the same time inflate their large, scarlet gular sacs and tipped upwards such that the bills are pointing skyward (above). This makes the males visible to females from far and near.
When a female flies overhead, the males all quiver their wings and heads, at the same time vibrating their bills against the inflated sacs creating a sort of drumming noise.
Once the female chooses a male, she flies down to join him and indulge in certain rituals that may involve the male taking her bill in his. Once the pair has bonded, nest building begins. The male is responsible for collecting nesting materials. The pair may go through a breathtaking aerial flight whereby the male throws nesting materials for the female to catch. The female builds the nest and at the same time ensures that others do not rob her of the nesting materials.
Copulation takes place intermittently during this period (above).
The inflatable gular sac becomes prominent only when the male frigatebird is about to breed. Other than display, the inflated sac is used as a sound box when the mandibles are being vibrated during the excitement when a female is around. The sac is also used to remove excess heat by fluttering.
(The images are by Howard Banwell and taken in the Galapagos Islands)