Southern Ground Hornbill (Bucorvus leadbeateri) Population Decline
The Southern Ground Hornbill (Bucorvus leadbeateri) is disappearing from Southeastern Africa at an alarming rate. Human encroachment, habitat destruction, and the introduction of mercury into the environment play roles in this decline. The Southern Ground Hornbill behavior and ecology, paired with a lack of population decline research outside of South Africa, makes recovery difficult. Conservation groups are working to help the species rebound, not only for the environment, but also for the local people who feature the Southern Ground Hornbill prominently in folklore.
The number of Southern Ground Hornbills has dwindled in Southeastern Africa. The species was once found extensively in Africa south of the equator, covering an area of 4.5 million square kilometers (Marais, 1993). South Africa is the only country that has recorded the population decline, despite having just under two percent of this historic range (Marais, 1993). Between the 1940s and early 1990s, the South African population saw a 70 percent decrease in protected areas (Marais, 1993). Today, the South African population is only found in the eastern region of the country and are now extinct not only in Swaziland, but also in the Mpumalanga grasslands (Cilliers et al., 2013; Wilson and Hockey, 2013). Fewer than 1500 individuals, of which there are only 400 to 450 breeding pairs, remain in South Africa (Wilson and Hockey, 2013; Theron et al., 2013).
The number of breeding pairs that exist today in South Africa can be attributed to several factors, with the obvious reasons for the decline being human encroachment and habitat destruction. The Southern Ground Hornbill forages on the ground in open woodlands of the savanna and requires tall trees for nesting. Some of these areas have been forested with crop trees or cleared for grazing, reducing the amount of open foraging land and potential nest sites (Marais, 1993; Cilliers et al., 2013). Human encroachment has also lead to an increase in direct and indirect poisoning. Because of their territorial nature, the Southern Ground Hornbill will break windows by pecking at its reflection, thinking it is attacking an intruder. Owners of the buildings with broken windows shoot or poison these birds to stop future property damage (Marais, 1993; Cilliers et al., 2013). Farmers place poison to kill livestock predators, but the Southern Ground Hornbill often consumes this poison due to their feeding behavior (Marais, 1993). Mercury is also poisoning this species. Gold mining and phosphate plants contribute to an environment’s mercury levels, but coal plants are the leading cause in South Africa (Dase et al., 2015). Eight eggshells from wild-hatched Southern Ground Hornbills in the Mpumalanga and Kwazulu-Natal provinces in South Africa were tested for mercury (Dase et al., 2015). The eggshells averaged 4.63 micrograms of dry mercury weight, which exceeds the maximum of 1.5 micrograms of dry mercury weight before reproduction is comprised (Dase et al., 2015). Although mercury has only been linked to eggshell thinning in Southern Ground Hornbills, further investigation could reveal additional aberrations. In other avian species, this high level of mercury causes a decrease in clutch size, impaired embryonic growth, an increase in eggs laid outside of nests, and unusual juvenile behavior (Dase et al., 2015). Compromised reproduction from mercury could explain the rapid population decline in South Africa and should be studied further.
The Southern Ground Hornbill has had little success recovering on its own, primarily because of its reproductive behavior. They are the largest avian cooperative breeders and form groups consisting of a dominant breeding pair and up to nine helpers, oftentimes unrelated adult males or older juveniles from a previous breeding season (Marais, 1993; Bates, 2011; Cilliers et al., 2013; Wilson and Hockey, 2013; Theron et al., 2013). Southern Ground Hornbills are highly territorial and, in South Africa, each group requires between 100 to 200 square kilometers of openly wooded savanna (Cilliers et al., 2013). On average, the dominant breeding pair in a group will produce one offspring every 9.3 years (Marais, 1993). The Southern Ground Hornbill leaves their group between the ages of three and six, and reaches breeding maturity at about ten years of age (Marais, 1993; Coetzee et al., 2014). Eggs are laid in mid October during the wet season, but if the rainy season begins later than usual, the Southern Ground Hornbill will not breed (Kemp and Kemp, 1975; Theron et al., 2013). Females will lay two eggs, but will only raise the first that hatches (Marais, 1993). Southern Ground Hornbills are particular about their nesting site and environmental factors can determine if a particular breeding attempt is successful. The Southern Ground Hornbill begins to overheat at 26 degrees centigrade, and so must find a tree hole that is not in direct sunlight (Kemp and Kemp, 1975). Too much rainfall in a season can flood the nest and make fledging and foraging difficult due to a more dense brush, but too little rainfall will decrease food availability (Wilson and Hockey, 2013). The Southern Ground Hornbill is preyed upon by Leopards (Panthera paradus) and the Martial Eagle (Polemaetus bellicosus), so nesting spots must be out of direct sight (Cilliers et al., 2013). The slow reproduction rate, the large territory that each group defends, and the particular factors that must be met for successful nesting attempts keeps breeding rates low, slowing recovery.
Although cultural practices contribute to population decline, the Southern Ground Hornbill plays an integral and meaningful role in native folklore. In Zambia, Malawi, and South Africa, the bird’s feet and feathers are mixed with other items and spread around houses to protect families from drought, lightning, and evil spirits (Coetzee et al., 2014). Because the Southern Ground Hornbill call can be heard up to three kilometers away, the species is thought of as a timekeeper for the start and end of the workday, as well as a signal for the end and beginning of the rainy season in Kenya, Malawi, and Tanzania (Kemp and Kemp, 1975; Coetzee et al., 2014). Some will kill the Southern Ground Hornbill if they want the rainy season to come sooner or last longer to prevent the distinct booming call from sounding (Coetzee et al., 2014). Other cultures believe that they can gain leadership powers if they place the hornbill head in their bath, and some believe they will be able to see the world through an altered perception if they sniff or place the ashes of the bird under their tongue (Coetzee et al., 2014). These practices have been around much longer than the beginning of the population decline, and the species must be preserved to ensure these cultural practices can continue.
Due to the factors contributing to population decline paired with slow reproduction, the Southern Ground Hornbill is unlikely to recover without human interference. Several efforts have been made to increase population numbers. Most research and action has taken place in South Africa in the form of hand rearing eggs and the placement of artificial nest boxes. The Mabula Ground-Hornbill Project began research in the late 1960s and began hand rearing the second egg laid in nests in the mid 1980s (Marais, 1993). In 1990, the National Zoological Gardens in South Africa began a breeding program in an area encompassing 800 square meters where they raised the second eggs by hand (Marais, 1993). The first spatially explicit reintroduction plan took place in the early 2010s in South Africa (Cilliers et al., 2013). This plan included the creation of artificial nest boxes, which were readily accepted, but only 49 percent of all breeding attempts fledged one chick and only 31 percent of these birds survived to maturity (Cilliers et al., 2013). Another study involving nest boxes found that Southern Ground Hornbills that used the artificial nests were twice as successful as those that used natural nests, and were less dependent on open woodlands, a limiting factor in nest availability (Wilson and Hockey, 2013). Artificial nest boxes are successful because the proximity to good foraging areas can be controlled, decreasing the energy cost of traveling between the nest and foraging sites (Wilson and Hockey, 2013). Environmental factors, such as exposure to rain, sun, and predators, can also be controlled with artificial nest boxes. More of these efforts are needed in countries outside South Africa to ensure the recovery of this species.
Most research and conservation has been carried out in South Africa, but a single study in Zimbabwe suggests that not all populations of the Southern Ground Hornbill are the same. This study describes territories that were spaced only 40 kilometers apart compared to 100 kilometers apart in South Africa (Witteveen et al., 2013). It also states that just over 50 percent of juveniles were found on communal land and commercial farmland due to the abundance of available food, and were not found primarily in protected areas like they are in South Africa (Witteveen et al., 2013). Half of the South African population resides in the protected area of Kruger National Park (Broms et al., 2014). The Zimbabwe study reveals that not all populations of the Southern Ground Hornbill behave the same. More research must be conducted outside of South Africa and continued in Zimbabwe to determine population sizes and to create conservation plans specific to each region.
The Southern Ground Hornbill was once abundant across the savannas of Southeastern Africa and efforts must be made to restore this species. Despite the cost of hand rearing and reintroductions, captive breeding programs increase the survival rate of juveniles to adulthood and these programs should be enacted in more zoos. Artificial nests must be placed to counteract the limiting resource of available nesting sites to help increase populations in the wild. Conservationists ought to factor in the use of the Southern Ground Hornbill in folklore and cultural practices to engage the public in conservation efforts. Due to limited knowledge of the Southern Ground Hornbill outside of South Africa, it is critical that more research is conducted to find behavioral differences, to determine if eggshell thinning exists in populations outside of South Africa, and to establish baseline populations for each country that the Southern Ground Hornbill inhabits. More extensive research and collaboration between resources is necessary to increase populations throughout Southeastern Africa.
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Broms, Kristin M., Johnson, Devin S., Altwegg, Res., Conquest, Loveday L. (2014) Spatial occupancy models applied to atlas data show Southern Ground Hornbills strongly depend on protected areas. Ecological Adaptations. 24(2), 363-374.
Cilliers, Dirk., Evans, Steven., Coetzee, Hendri., van Rensburg, Leon. (2013). Developing a site selection tool to assist reintroduction efforts for the Southern Ground-Hornbill Bucorvus leadbeateri. Ostrich: Journal of African Ornithology, 84(2), 101-111.
Coetzee, Hendri., Nell, Werner., van Rensburg, Leon. (2014). An exploration of cultural beliefs and practices across the Southern Ground-Hornbill’s range in Africa. Journal of Ethnobiology and Ethnomedicine, 10(28).
Dase, Adegbenro P., Okonkwo, Jonathan O., Janson, Raymond., Bandae, Jose D.D.O., Kotze, Antoinette. (2015). Mercury concentrations in eggshells of the Southern Ground-Hornbill (Bucorvus leadbeateri) and Wattled Crane (Bugeranus carunculatus) in South Africa. Ecotoxicology and Environmental Safety. 114, 61-66
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Witteveen, Minke., Parry, Elspeth., Norris-Rogers, Mark., Brown, Mark. (2013). Breeding density of the Southern ground hornbill, Bucorvus leadbeateri, in the communal areas surrounding the Matobo National Park, Zimbabwe. African Zoology, 48(2), 274-278.