The social structure of southern ground-hornbills
Ground-hornbills have an unusually complex social structure. Generally there is only one female per flock of four to five males. “Groups can be as large as 12 birds and as few as two, depending on the habitat,” explains Lucy Kemp, the project manager of Mabula Ground Hornbill Project (MGHP). Large flocks are more reproductively successful as there are more birds to tend to the young.
As cooperative breeders, it is only the alpha male and female that breeds, producing two eggs every couple of years. The second egg is an insurance policy as ground-hornbills exhibit obligate brood reduction. This means the second chick does not survive to fledge unless something goes wrong with the eldest chick. The group is then responsible for raising the single chick.
A female chick is forcibly removed from the group after 11 months. She will either die, as she is more vulnerable to predation on her own, or if she survives to adulthood, she will pair up with another male forming a new group.
Ground-hornbills reach sexual maturity at about six years of age. Males are able to breed when, and if, they become the alpha male in the group. “They can be up to 25 years old before they have the opportunity to breed,” says Kemp.
From observations in the field, there is an equal chance of an egg being male or female. However, later on, the sex ratio of ground-hornbills becomes skewed towards more males. An adolescent female displaced from her group is vulnerable and don’t always survive. Because males remain in their natal group, they have a better chance of survival.
If the project is not careful there will be surplus females in need of a group and a viable territory, as each reintroduction group needs a small male army to each female. For this reason, it is preferable for the MGHP to manipulate the sex-ratio of the metapopulation to be skewed towards males. This is achieved by sexing the eggs.
In-ovo sexing of ground-hornbill eggs
In order to try boost southern ground-hornbill numbers in South Africa, the MGHP has been successfully harvesting the second egg from groups in the metapopulation for twenty years.
Recently, the MGHP has started in-ovo sexing eggs before they harvest. “Staff members have been trained to ‘candle’ an egg in the first two weeks of incubation, using a strong LED light that enables us to see the embryo through the shell,” says Kemp.
A microscopic hole is drilled through the shell accessing the blood vessels near the surface. A tiny drop of blood is removed and sent to the laboratory for DNA sexing.
A strategic decision can then be made to only hand-rear the chick if it is a male until more females are needed. There is currently an excess of females in captivity that are not breeding because there are not enough males to form a group.
Creating a new ground-hornbill group
Should a new viable territory become available in a nature reserve or national park, Kemp and her team will bond a female to a beta male, forming a new group to occupy the reserve. One or two young males are also added to the flock.
The oldest beta male, ranking one below the alpha, is bonded with an available female. The birds can be picky and sometimes do not bond with their chosen mate, in which case a new male is selected. Once released the new group is monitored on a daily basis.
“There is a growing population gap between southern Kruger National Park and southern Mpumalanga,” says Kemp. “It is highly likely that the northern Zululand region will become a new location for releases.”
Managing the southern ground-hornbill metapopulation
The Mabula Ground Hornbill Project has three experimental groups of roughly five birds under their management. These flocks are monitored daily and reside in reserves across South Africa. This year, two newly established groups bred for the first time.
In an unsuccessful breeding year, a hand raised male chick is introduced into each of the nonbreeding groups to make sure there is a succession of males to improve their reproductive success in the future. The male chicks are then able to learn from the experienced wild birds.
Written by Georgina Lockwood
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