Birds as ecological indicators
Birds are often used as indicator for the state of the environment or the ecosystem’s health. An indicator species provides information on the condition of the ecosystem. Birds may reflect changes in the environment and ecosystems, as they are an integral part of the food chains and food webs. Changes in habitat quality is often expressed in a change in food availability for birds. This may lead to a shift to other areas, or when birds stay, to a lower condition and ultimately a lower survival or reproduction. When it is the other way round, survival and reproduction may be higher. For example, the bird populations in the Inner Niger Delta are depending on the flooding for their food, and population changes are a direct expression of the state of the Inner Niger Delta (Zwarts et al. 2009). As birds are often at the top of the food chain, they are sensitive to pollution, in particular chemical pollution. In the end, also pollution will be reflected by population changes.
Birds are also sensitive to habitat change as they are often bound to specific habitats. Even when small changes occur in habitat quality, this may already affect food availability or the exploitation of food resources. When human activity is increasing, this may lead to disturbance of feeding birds, and thus lead to a lower food availability. Also birds show direct response to poaching, like egg collection of the collection of young (from the nests).
The use of birds as ecological indicators is also promoted because the monitoring of visible birds is more easy than for instance mammals, amphibians, fish or invertebrates. Also, our knowledge about the impact of changes on bird populations is more extensive than for a other species groups.
Vulnerable birds in the Upper Niger Basin
This broad savanna zone is home to several hundreds of bird species, many of which are widespread throughout West Africa. When looking at threatened species, over 20 bird species are expected to occur in the UNB. This includes several birds of prey and a few species confined to wetlands or riverine habitats, like African skimmer and Kittlitz’s plover. Breeding of Black-Crowned crane is not recorded from the Upper Niger Basin. Most of the African vultures are now considered to be severely threatened, but these species are not linked to the wetlands or riverine areas of the Niger and Bani Rivers.
The source area of the Niger River in the Guinean Highlands may harbour species that are characteristic of the humid forest zone further south, and which marginally occur in the basin. These may include (near-)threatened species like Timneh parrot (EN) and Crowned eagle (NT). Rufous fishing-owl (VU) is tied to forested streams and has been recorded at the nearby Loma Mountains in Sierra Leone. A few range-restricted species occur in the wider area, including Turati’s boubou, Sierra Leone prinia (VU) and Emerald starling. These species are confined to forest edge or woodland savanna and are not linked to riverine areas.
In the Upper Niger Basin a few IBAs are identified by Birdlife International, in particular Kouakourou, Sirakoroni–Tyènfala (north of Bamako) and Mafou, which is part of the Parc Haute Niger in Guinea. Sirakoroni–Tyènfala and Mafou are savanna woodlands resp. forests. In these dry areas a high variety of bird species has been recorded. Kouakourou is a wetland area near the Niger. Generally the information on IBAs in Mali and Guinea is outdated due to a lack of recent data.
Vulnerable birds and bird concentrations in the Inner Niger Delta
The IND is one of the major floodplains in Africa and as such of paramount importance to both migratory birds and resident species (Zwarts et al. 2009). This refers primarily to waterbirds. However, the IND hosts also large populations of some other bird species, including birds of prey, which use the unflooded parts of the floodplains and the adjacent savanna landscape. The enormous international ornithological importance of the area has been documented since many decades.
The flood forests of Acacia kirkii host large mixed breeding colonies. Studies carried out in the 1980s and around the millennium show 17 nesting species, including Cattle egret and African cormorant, which were the most common breeding species. Some species have benefited from improved floods since the end of the Great Drought, or from flood conditions allowing a breeding cycle until March (Grey heron, Purple heron, Glossy ibis, African sacred Ibis) or by a higher breeding success (African darter). The high water with flooding in the central lake area until March became very rare, explaining the disappearance of nesting Grey and Purple herons and Glossy ibis. African openbill has also disappeared, but for this species the IND was rather a refuge during the Great Drought.
Breeding colonies and roosts of colonial breeding waterbirds in the Inner Niger Delta. Sources: Zwarts et al. 2009, Beintema et al. 2007.
other species are worth mentioning. Whiskered tern has been found breeding in the IND since the early 1990s, the only nesting site known in West Africa. During the great flood of 1999 a total of 200-250 breeding pairs was estimated. Despite annual breeding in the center of the IND, their prospect is not that good due to the disturbance of the floating nests by fishing activities.
The vulnerable Black crowned crane breeds again in the IND but the species is threatened by the fact that young birds are systematically collected and sold. A survey in 2001 revealed that in the cities of Mopti and Bamako the number of captive cranes was two to three times larger than the wild population of the IND, which is doomed to disappear.
Concentrations of waterbirds
The occurrence and abundance of (water)birds in the delta must be mirrored against the following processes related to bird movements and dynamics of the delta proper and the Sahel as wider environment:
- The migration window of migratory birds: large numbers of birds from the north arrive in the Sahel, and the IND, in the period September-October and either stay till March – April, or migrate further east and pass the are during their return migration;
- The breeding season and intra-African migration of resident birds: Resident birds breed mostly during the wet season (September-November) and (partly) migrate to other areas in Africa during the hot, dry season;
- Flooding cycle: the seasonal flooding cycle in the IND determines the temporal and spatial feeding conditions for birds. Flooding starts in September–November (crue), is at its maximum between early and end November (Akka), the flood recedes during December–April (décrue) and is at its lowest in May–July (étiage). During the décrue and étiage the resources for birds are scarce.
- Annual variations in flooding: The size of the large floodplains in the Sahel and the occurrence of temporal wetlands or lakes largely depends on annual rainfall. Depending on these conditions, migratory birds but also residents may move between these areas. This means that the abundance of birds in the IND is partly also driven by droughts and rainfall elsewhere in the Sahel.
In the framework of some large project funded by the Dutch government (PIN, BBI), since 1998 Wetlands International in Sevaré systematically counted the central lake area to map and analyse the dynamics of the waterbird populations in the central lakes, in response to flooding (Zwarts et al. 2009). Before 1998 a number of comparable surveys has been done, which means that a series of counts is available. The most recent count of the central lakes has been carried out in 2012. Since then no quantitative information on the status of the water birds in the IND is available.
The interactive graph shows the numbers counted in the central lake area (Lac Debo, Walado, Korientzé) between 1991 and 2012. Most censuses were performed in January - February, when the water level in Akka is below 200 cm. Birds then concentrate in the central lake areas, as most other areas in the Inner Niger Delta run dry. During 1998-2002 monthly counts were done, and in addition some counts in June. During high water levels birds are widely dispersed over the Inner Niger Delta and do not concentrate in the central lake areas. Breeding birds like the Long-tailed cormorant reach high numbers at high water levels; they breed and roost in the flood forests which are still present in the central lake area.
Sources and more information
- Beintema, A.J., J. van der Kamp & B. Kone (éds.). 2007. Les forêts inondées : trésors du Delta Intérieur du Niger au Mali. A&W-report 964. Altenburg & Wymenga conseillers écologiques, Veenwouden. Wetlands International, Sévaré. Pays-Bas / Mali.
- Zwarts, L, Bijlsma RJ, van der Kamp J, Wymenga, E 2009. Living on the Edge. Wetlands and birds in a changing Sahel. KNNV Publishing, Zeist. p. 1-564.