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Ascension Frigatebird: The majestic seabird of Boatswain Bird Island

One Earth’s Species of the Week series spotlights an iconic species that defines the unique biogeography of each of the world. 185 bioregions from the earth.

The Ascension Frigatebird (Fregata aquila) is the only frigate species that calls Boatswain Bird Island home, a flat-topped crag near Ascension Island. Located in beautiful isolation between Brazil in South America and the Gulf of Guinea in the South Atlantic Ocean, Ascension Island provides a distinctive habitat for this remarkable seabird.

St. Helena and Ascension Bioregion (AT11), located in the southern Afrotropics subkingdom of the Afrotropics.

The Ascension Frigatebird (Fregata aquila) is the iconic species of the St. Helena and Ascension bioregion (), located in the southern Afrotropical subkingdom of the Afrotropics.

The graceful giant of the sky

The Ascension Frigatebird is majestic in shape and shares common characteristics with its frigate counterparts, showing subtle size differences between males and females. Adult sizes range from 89 to 96 centimeters (35 to 38 in) in length, with females exhibiting larger sizes and longer bills. A wingspan of approximately 2 meters (6.6 ft) is an example of its impressive aerial capabilities.

The adult male has brown-black plumage decorated with a sheen of purple and green. Its beak, pale gray with some flesh and black tones, contrasts with dark gray legs. Males also have a distinctive red angular sac, which they inflate to attract mates.

The female exhibits a similar color palette, but with less green and purple sheen. It is striking that it shows a striking dark brown to rust brown collar, supplemented with whitish to light blue-gray beaks and pink legs. Juveniles resemble adult females, but have an entirely white head and upper neck.

Diverse foraging at sea

Practicing surface dipping as a foraging strategy, Ascension frigatebirds primarily catch fish, with a notable preference for flying fish species such as Cypselurus, HirundichthyesAnd Exocoetus volitans. Additionally, they catch baby green turtles (Chelonia mydas) and sooty tern chicks (Onychoprion fuscatus). This dietary diversity underlines their adaptability to the rich marine resources surrounding Ascension Island.

Male Ascension Frigatebird with chick on Boatswain Island.  Image credit: Wiki Commons.

Male Ascension Frigatebird with chick on Boatswain Island. Image credit: Wiki Commons

A life anchored on Boatswain Bird Island

Characterized by a sedentary lifestyle, Ascension frigatebirds rarely venture further than a radius of 150 kilometers (93 mi) from their lonely breeding ground on Boatswain Bird Island. This unique ecological niche suits their specialized needs and emphasizes their dependence on the local environment for subsistence and breeding activities.

Ascension frigatebirds exhibit a flexible breeding cycle, with the ability to breed all year round, peaking between April and November or December. Nests are intricately woven among rocks and guano deposits, highlighting their ingenuity in using available materials. The loose colonies shared with Pelecaniformes underline the coexistence of different bird species on the breeding grounds.

A female juvenile Ascension frigatebird.  Image credit: Wiki Commons.

A female juvenile Ascension frigatebird. Image credit: Wiki Commons

Overcoming threats to invasive species

Boatswain Bird Island, which serves as a protected bird sanctuary, has undergone significant conservation efforts. Two centuries ago, the island faced problems with rat and feral cat plagues. However, a notable achievement occurred in 2006 when Ascension Island was declared cat-free. This milestone has contributed significantly to the breeding success of the Ascension frigatebird, illustrating the positive impact of eradicating invasive species on local bird populations.

Protection of this unique species

The conservation status of the Ascension frigatebird is precarious due to its limited global breeding range. BirdLife International estimates a global population of 17,000 to 21,000 adult individuals, which translates to a total population of 25,000 to 32,000 individuals.

Conservation efforts have focused on maintaining the bird sanctuary on Boatswain Bird Island and eliminating invasive species, demonstrating the continued commitment to conserving this unique bird species and its fragile island habitat.

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