White-bellied boobies

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White-bellied boobies
White-bellied Booby (Sula leucogaster)

White-bellied Booby ( Sula leucogaster )

Systematics
Order : Suliformes
Family : Gannets (Sulidae)
Genre : Sula
Type : White-bellied boobies
Scientific name
Sula leucogaster
( Boddaert , 1783)
White-bellied booby in flight
White-bellied and masked boobies
On the left a red-footed booby , on the right a white-bellied booby
Brooding white-bellied booby
White-bellied gannet young bird in flight
Sula leucogaster

The brown booby ( Sula leucogaster ) is a goose large seabird from the family of gannets (Sulidae) in the order of the Pelecaniformes (Pelecaniformes).

The population of the white-bellied gannet was classified in the IUCN Red List of Threatened Species in 2016 as “ Least Concern (LC) ” = “not endangered”.

Appearance

When fully grown, the white-bellied booby reaches a body length of 65 to 75 centimeters and then weighs between 900 grams and 1.5 kilograms. The wingspan is 130 to 150 centimeters.

The body is streamlined, as is typical of boobies . It also has a long, pointed beak that makes it easier for it to grab fish . He hunts them by pushing himself down onto the surface of the water from a height of 30 meters or more and dipping between schools of octopuses and mullets . Air sacs on the head cushion the impact in the water.

The white-bellied booby is a skilled flier. It often glides low above the surface of the water and is reminiscent of albatrosses in its gliding phases. It is rarely seen at altitudes above 30 meters above the water surface. Like most booby species, it is a shock diver. On land, locomotion is sluggish and waddles like a duck.

distribution

White-bellied boobies live in the tropical Pacific , Atlantic and Indian Oceans . In some areas they are among the most common sea ​​birds . In the Pacific, they are mostly found in the vicinity of their breeding islands, but individual white-bellied boobies are also observed 300 to 1500 kilometers away from the nearest breeding site. Basically, they come closer to the mainland coast than other boobies native to tropical waters and are occasionally seen in harbors and estuaries. However, they rarely fly over land. The migratory movements of the white-bellied gannet have not yet been researched, but they often return to the same breeding colony once they have successfully incubated there.

Breeding colonies are located on tropical islands, which are often far from the coast of the mainland. Breeding sites are also found on atolls and cays . They occasionally use cays as breeding grounds, the surface of which protrudes only up to one meter above the sea surface. Here their nests are occasionally washed over by the waves during storms.

food

White-bellied boobies mainly eat flying fish and other fish species, as well as some cephalopods . White-bellied boobies, like most booby species, are shock divers. They plunge down from a height of 15 meters above the water surface, but this behavior occurs less frequently with them than with other booby species. They catch a lot of prey in shallow dives. When they dive, they usually do so from a height of three meters, 23 percent of the dives are vertical, 40 percent take place at an angle of more than 45 degrees. The other dives cannot be assigned because they take place in a spiral. While foraging, they look forward to find prey, which sets them apart from a number of other prey that watch the surface of the water directly below them. They usually catch flying fish just as they are back in the water. Catches with their entire body out of the water are rare. During the dive, the wings are spread apart until shortly before submerging. Dives can last up to 40 seconds.

White-bellied boobies also display kleptoparasitic behavior and follow other boobies when these fish carry fish in their beak. They also follow frigate birds and steal their prey. White-bellied gannets often follow ships and dive into their keel waves . White-bellied boobies forage for food either alone or in groups. White-bellied boobies have also been seen in tropical waters foraging for food near groups of dolphins.

Reproduction

Couple bond and nest

White-bellied boobies are monogamous birds, whose mating relationship exists over several reproductive periods. Couples may stay together until one of the two partner birds dies. Both parent birds are involved in the brood and care for the young bird for up to two months after it has fled.

White-bellied boobies are colony breeders who vigorously defend a territory immediately around their nest. Eggs are laid all year round. On Christmas Island , however, the main time for egg-laying falls between April and May. In breeding colonies on the east coast of Australia, however, it falls in the period from March to April and June to October. On Raine Island , a 32 hectare cay off the east coast of Australia, the peak of the egg-laying is in the period September to November.

The nests are built on the ground, the ground can consist of sand or gravel and can also be overgrown with plants. White-bellied gannet nests are also found in more densely wooded areas. They use a relatively broader spectrum of breeding areas than other booby species. The nesting density is greater than that of the masked booby. For example, on Raine Island there are 7.7 nests per 100 square meters. The amount of nesting material that is built up by the parent birds varies greatly. The white-bellied boobies lay their eggs on the bare ground on cays that are not overgrown. In other areas, plants, twigs, algae, bones, feathers, shells from turtle eggs and other materials are used. The material is usually carried by the male and either placed on the nest or given to the female. Both parent birds continue to build on the nest during the breeding season. White-bellied boobies often steal nesting material from other conspecifics. This behavior can be observed above all on cays, where only alluvial material can be found.

Fledglings

Clutches of the white-bellied gannet comprise between one and two eggs. The proportion of clutches with one or two eggs varies depending on the season and region. Clutches with two eggs predominate on Christmas Island. However, only one chick grows at a time. Usually the last hatched chick dies within four days because it is being pushed out of the nest by the older sibling. Egg laying is not very strongly synchronized within the breeding colony. Both parent birds breed. They do not show a brood spot, but cover the egg with the webbed feet of their feet. Occasionally the clutch is only shaded. The breeding season is 42 to 43 days. Newly hatched chicks are naked, their eyes are initially closed, but are usually opened on the first day of life. They show the dense downy plumage typical of gannets at three to four weeks. They reach their maximum weight in around 70 to 80 days when they weigh around 1.4 kilograms. They will then lose weight by the time they fled, but weigh more than the average adult bird. They usually fledge at 17 weeks.

Reproductive success

On Christmas Island off the Australian coast, young birds hatched in 68 percent of the nests, of which young birds also fled in 81 percent of the cases. A total of 58 young birds grew per 100 clutches. Breeding success was significantly lower on Ascension . Only ten young birds grew up here per 100 clutches. As a rule, no offspring grow up on Christmas Island in El Niño years because there is insufficient food available.

In Australia, humans have a considerable influence on the breeding success because eggs and young birds are removed for consumption. Where there are rats, they eat young birds and eggs. Seagulls, railings and possibly also the red-backed heron also eat eggs and occasionally chicks.

literature

  • PJ Higgins (Ed.): Handbook of Australian, New Zealand & Antarctic Birds. Volume 1: Ratites to Ducks. Oxford University Press, Oxford 1990, ISBN 0-19-553068-3 .

Web links

Commons : White-bellied Booby ( Sula leucogaster )  - Collection of images, videos and audio files

Single receipts

  1. Sula leucogaster in the endangered Red List species the IUCN 2017. Posted by: BirdLife International, 2016. Accessed August 15, 2018th
  2. ^ Higgins, p. 782.
  3. Nigel Redman, Terry Stevenson, John Fanshawe: Birds of the Horn of Africa: Ethiopia, Eritrea, Djibouti, Somalia, and Socotra - Revised and Expanded Edition . Princeton University Press , 2016, ISBN 978-0-691-17289-7 , pp. 44 (English, full text in the Google book search).
  4. ^ Higgins, p. 782.
  5. ^ Higgins, p. 782.
  6. ^ Higgins, p. 784.
  7. ^ Higgins, p. 784.
  8. ^ Higgins, p. 784.
  9. ^ Higgins, p. 785.
  10. ^ Higgins, p. 787.
  11. ^ Higgins, p. 787.
  12. ^ Higgins, p. 788.
  13. ^ Higgins, p. 788.