Saddle-billed Stork Ephippiorhynchus senegalensis Scientific name definitions
Version: 2.0 — Published June 25, 2021
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Demography and Populations
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The population ecology of the Saddle-billed Stork is wholly unstudied. Indeed, basic, standardized population surveys are lacking for most of the range except for several historic aerial counts and more recent surveys in the Okavango Delta, Botswana. Surveys are probably the most glaring knowledge gap that needs to be addressed before any science-based status assessment can be developed.
Measures of Breeding Activity
Age at First Breeding
Intervals Between Breeding
Annual and Lifetime Reproductive Success
Nest success, chick survival, or other measures of reproduction have never been quantified, illustrating a significant knowledge gap. Ground and aerial surveys of the Kafue Flats, Zambia in the late dry season estimated no more than 5% immatures (100), but this certainly varies depending on the environmental conditions in a given year.
Life Span and Survivorship
The oldest captive stork was 48 years old when it died (M. Herry, unpublished data), but longevity in the wild is unknown. The status assessment for South Africa considered the mean generation length of Ciconia storks, 16.1 y, to be representative of the Saddle-billed Stork (69). However, its life history seems much slower than Ciconia species, so this may be an underestimate.
Disease and Body Parasites
Paronchocerca ciconiarum (heartworm) has been found in captive storks of wild origin (101, 102). The stork hosts, however, had been in captivity for years before the parasites were discovered, so it is unknown if wild birds carry them. A trematode species, Cathaemasia senegalensis, has also been described from a pair of wild-caught storks (possibly from Nigeria; 103). In this case, the female apparently died due to the parasites and 26 'worms' were found in her throat and esophagus. Keepers removed them from the male's throat before he succumbed. Beyond these brief cases, nothing is known about diseases and parasites that this species may be prone to contract/acquire.
Causes of Mortality
Causes of mortality are not known due to a lack of life history research and its long lifespan. Fledged young are doubtless prone to predation before they are capable of flying (12).
Population Spatial Metrics
The population estimate by BirdLife International is 1,000–25,000, equating to 670–17,000 mature individuals (104). However, this is not based on any population surveys, and is essentially useless for management given the huge range of the estimate. Aerial surveys from across the range are the best solution to remedying this shortcoming because ground counts are highly variable and do not provide any useful way to estimate population size. Additionally, ground counts like those commonly done under the African Waterbird Census have been found to be ineffective for surveying the related Black-necked Stork (Ephippiorhynchus asiaticus) because pairs occur at low densities (105). Counts –mostly aerial– from several areas across Africa provide the only reliable population estimates, yet most are decades old and not all had standardized survey designs. However, they provide some idea of local abundance.
The W-Arli-Pendajari National Parks complex of Benin, Burkina Faso, and Niger probably holds the largest population west of Chad. In February 2008, 102 were counted in Benin's Pendajri National Park (Wetlands International, unpublished data). Comprehensive aerial surveys of the Sudd in South Sudan in 1979–1981 during three seasons counted 3,640–4,158, with the maximum count in the late dry season (45). Aerial counts of a 368 km2 area of the Moyowosi-Kigosi Game Reserve complex in Tanzania during 1992 found a density of 1.2/km2. In 1993, surveys covered 4,188 km2 of the reserve complex and produced estimates of 1,398 storks at an average density of 0.33/km2 (Tanzania Wildlife Conservation Monitoring, unpublished data). Baker (106) suggested a possible population of 6,000 for Tanzania, but this was based on crude extrapolation.
The South African population is estimated at approximately 120–150 mature birds (69), but only the population in Kruger National Park has been empirically estimated at 40 (9). Surveys in Mozambique's Gorongosa National Park found 43 storks in 2014 and 2016 (M. Stalmans, unpublished data). In September 1999, 68 were counted in the Zambezi River Delta in Mozambique (Wetlands International, unpublished data). In October 1983, 275 storks (0.17/km2) were estimated from an aerial survey of Zambia's Bangweulu Swamp (107) but less than 40 were estimated in July 2002 (108). It is unclear if this major disparity represents an actual decline or if the differences in survey design or season are related. On Zambia's Kafue Flats, less than 20 were found during an aerial survey in the dry season of 2001 (109), which was similar to the 26–28 counted using ground and aerial surveys in the same season in 1964 (100). In the dry season of 1971, 43–58 were estimated from aerial surveys of the Busanga Plain in Zambia's Kafue National Park, but less than 20 were estimated there in 2003 (110). An aerial survey of Liuwa Plain in western Zambia estimated 198–235 from a count of 66 in the early wet season of 2001 (48). In July 2019, during a severe drought, an aerial survey counted 56, and 108 were counted in August 2020 (African Parks Zambia, unpublished data).
The most thorough and reliable population estimates have come from Botswana's Okavango Delta. The following are three population estimates from aerial surveys that covered most of the delta: 1,069 (CI: 879–1,259) in 2001, 812 (CI: 669–956) in 2002, and 812 (CI: 1,045–1,778) in 2003. These surveys also estimated 246 nests (CI: 165–327) in 2001, 62 nests (CI: 29–94) in 2002 (111), and 62 (CI: 23–105) in 2003 (112). More recent aerial counts in 2010 (113) and 2014 (114), albeit with slightly different methodology, estimated 1,040 (CI: 801–1,278) and 960 (CI: 775–1,145), respectively. The 2018 (115) estimate of 552 (CI: 429–676) showed a significant decrease since the previous survey. Density during 2001–2003 was 0.0.9–0.16/km2 and 0.01–0.02/km2 during 2010, 2014, and 2018.
Research on factors influencing population changes is lacking. However, environmental conditions, especially rainfall, are likely influential given the role in the species' ecology.