Frigatebirds are striking seabirds that spend much of their lives soaring over tropical and subtropical oceans. They possess an extremely large wingspan relative to their body weight, making them masters of aerial life. The male is defined by a highly visible anatomical feature that becomes brilliantly red during the breeding season. This unique structure, often described as a balloon or pouch, is the answer to what the “red thing” is and why it exists.
Anatomy and Purpose of the Gular Sac
The prominent red structure is known scientifically as the gular sac or gular pouch, a featherless patch of skin located beneath the bird’s throat. This feature is a clear example of sexual dimorphism, as only the males possess the bright red, inflatable pouch, while females typically have a white throat and chest. The sac is essentially an air-filled membrane that remains deflated and less conspicuous outside of the breeding period.
The brilliant scarlet color is a temporary visual signal caused by increased blood flow to the skin’s surface, much like a human blush. The purpose of this feature is tied directly to sexual selection, functioning as an honest signal of the male’s health and genetic fitness. A larger, more deeply colored, and fully inflated pouch indicates that the male is well-fed, strong, and capable of attracting a mate.
Females judge the size and intensity of the color as they observe the males during the mating season. Males that maintain the largest and brightest display are often perceived as having superior genes or foraging abilities. This intense visual competition ensures that only the most vigorous males pass on their traits.
The Elaborate Courtship Display
The gular sac is central to one of the most elaborate courtship rituals seen among seabirds. Males congregate in large groups at breeding colonies, typically perched on trees or bushes, to begin their collective display. The male forces air into the pouch using his respiratory system; inflation can take 20 to 30 minutes to reach its full, heart-shaped size.
Once fully inflated, the male adopts specific behaviors to enhance the display for the females circling high overhead. He points his bill upward, spreads his long wings, and vigorously shakes his head back and forth. This head-shaking motion causes the pouch to wobble, drawing further attention to its size and color.
The visual display is often accompanied by an acoustic component: a rhythmic “drumming” sound. This sound is produced by vibrating his bill against the taut, inflated pouch, which acts as a resonator. Studies suggest that the frequency and consistency of this drumming significantly predict mating success, with lower frequencies potentially signaling a larger sac and greater vigor. The female inspects the display, including the sac’s size and color and the male’s drumming performance, before selecting a mate.
Frigate Birds’ Unique Life and Habits
Frigatebirds inhabit tropical and subtropical ocean regions worldwide. They are characterized by a massive wingspan, up to 7.5 feet, giving them the largest wing area to body mass ratio of any bird. This design allows them to glide and soar on wind currents for extended periods, remaining aloft for days or even weeks.
Their mastery of the air is necessary because, unlike most ocean-going birds, frigatebirds have non-waterproof feathers. They produce very little oil from their uropygial glands, meaning their plumage would become waterlogged if they landed on the ocean surface. If they were to get wet, their long wings and light body structure would make it extremely difficult to take off again.
This limitation means they must catch their primary prey, such as flying fish and squid, by snatching them from the water’s surface or mid-air without ever plunging into the water. Frigatebirds are also famous for a foraging behavior called kleptoparasitism, which involves aggressively harassing other seabirds, like boobies, until they drop or regurgitate their catch. This aerial thievery has earned them common names like “Pirate Bird” and the “Man-o’-War Bird.”