Data Zone
Justification of Red List category
Kilombero Cisticola is restricted to a single floodplain system that has been subject to rapid and ongoing conversion of natural wet grassland vegetation to agriculture such that it is suspected to be currently undergoing a rapid population reduction. In addition, the minimum estimate of the size of its declining population is small, and it has a restricted range with continuing declines in its habitat area and quality and population size. As such the species is assessed as Vulnerable.
Population justification
There is no direct estimate of the population size, however the density has been assessed at different times of year and in different habitats within the Kilombero riparian zone (Rannestad et al. 2015). In 2011 the species was evenly-distributed along a 60 km river transect and was among the 20 most common species encountered with 144 records (Rannestad et al. 2015). Highest densities were recorded in reeds in June (during the highest water levels), with 26.1 ± 6.9 individuals per km2, when the same period recorded densities of 17.7 ± 5.6 individuals per km2 in grass-bush habitat (Rannestad et al. 2015). These values are considered likely to relate to mature individuals. Densities in other periods were lower, suggesting greater dispersion of individuals, reduced detectability, or seasonal movements out of the surveyed area. All three factors may be operating, given the seasonal flooding of the habitat likely concentrates individuals and may be the trigger for breeding (with resulting increased detectability).
The extent of assumed suitable habitat is unclear such that it is problematic to assess habitat extent matched to the timing of the highest densities as reported above. Wilson et al. (2017) report 1,515 km2 of wet grassland and reeds in 2016, while Leemhuis et al. (2017) report 2,166 km2 in the 'wetland' land cover class in 2016. Msofe et al. (2019) report an area of 'wetland' (excluding waterbodies) of 261 km2, suggesting considerable inconsistency in the land cover classifications used. This range of values does however seem likely to contain the true extent of the area in which peak densities occur. Using a mean value for the June data of 21.9 mature individuals per km2 for the two wetland habitats results in a range of population size from 5,716 to 47,435, rounded to 5,700 to 47,400 mature individuals.
The population size is suspected to be declining rapidly due to the rate of habitat conversion within the Kilombero valley (Wilson et al. 2016, Proswitz et al. 2021). Projecting the most recent rate of wetland grassland loss forwards (estimated in 2004 and 2016: Wilson et al. 2017) indicates the loss of 31% of the remaining vegetation by 2032, and 32% for the ten years between 2023 and 2033. There is the potential for this loss to be greater, with values for the reduction in 'wetland' land cover given in Msofe et al. (2019) being higher.
Trend justification
Kilombero Cisticola is believed to be strongly tied to the presence of flooded wetland grasses and reeds (Fjeldså et al. 2021). Within the Kilombero Valley Ramsar Site the area of wet grassland (taken to approximate the available suitable habitat for the species and to be directly related to the potential abundance of the species) has been reducing at an accelerating rate (Wilson et al. 2017, Leemhuis et al. 2017). There are differing estimates of the rate of natural wetland loss to agricultural expansion (Wilson et al. 2017, Leemhuis et al. 2017, Msofe et al. 2019), which seem to be due to differences in classifying landcover but all indicate rates equivalent to a greater than 25% reduction in wetland vegetation over the past ten years. In Wilson et al. (2017) three estimates of extent of wetland grassland are presented; 2,273 km2 in 1991, 1,935 km2 in 2004, and 1,515 km2 in 2016 (Wilson et al. 2017). Assuming a linear reduction continuing at the rate of loss between 2004 and 2016 results in a 25.7% reduction in wetland grassland area over the past ten years. Projecting forwards based on this linear rate gives a reduction of 31% for the period 2022-2032 and 32% for 2023-2033. Separately Leemhuis et al. (2017) provide estimates of area classified as wetland of 5,436 km2 in 1990, 3,809 km2 in 2004 and 2,166 km2 in 2016, an even more rapid reduction over three generations though one which may incorporate areas not used by the species, equivalent to a 50.5% reduction over the past ten years. Noting that the rate of wetland grassland loss appeared to be accelerating, and projections from modelling also suggest a rapid or very rapid further future reduction in habitat at the site (Proswitz et al. 2021), it is suspected that the population is currently declining at a rate exceeding 30% over both the ten year period starting 2022, and the future ten years from 2023 to 2033. Monitoring of both habitat extent and especially of the species' occurrence and abundance is crucial to be able to further quantify this rate, which may increase given projections in Proswitz et al. (2021). Equally the strength of the connection of the population trajectory and the measured loss of wetland vegetation requires testing: it may be that the species is able to use modified habitat more extensively and is therefore less severely impacted than presently assumed.
Kilombero Cisticola is restricted to the Kilombero Valley floodplain in Tanzania, a very flat basin formed by the confluence of a number of rivers adjacent to the Udzungwa Mountains to the north and west and the Mahenge highlands to the east. Within this restricted area the species is apparently well-distributed and common within areas of flooded grassland, but it may be more restricted that the sympatric White-tailed Cisticola C. anderseni due to a stricter requirement for flooded vegetation, especially Phragmites mauritianus stands (Fjeldså et al. 2021).
The southern limit to the distribution in the valley is taken to exclude the southernmost extent of the floodplain, given the lack of records and confirmed absence from one site in this area (Fjeldså et al. 2021). To the north there are confirmed records to the Great Ruaha river (eBird 2022, GBIF.org 2022) and this is tentatively considered the edge of the range. One record further north in Mikumi National Park (Kaestner 1998) may not be precisely located and it is uncertain if it regularly occurs in this area, which is slightly lower than the elevation limits (240-305 m asl.) given in Fjeldså et al. (2021). Excluding areas north of the Great Ruaha river, the current extent of occurrence (EOO) is estimated at 6,900 km2.
There is no estimate of the area of occupancy given the uncertainty around strict habitat requirement and consequently the size and spatial configuration of the suitable area of the remaining wet grassland land cover in the Kilombero floodplain. The estimated extent of habitat is considered to lie between the value of 261 km2 for 'wetlands' given in Msofe et al. (2019) and 2,166 km2 given by Leemhuis et al. (2017), with a value of 1,515 km2 given by Wilson et al. (2016) for a more specific wet grassland habitat; all of these values are based on 2016 data. While the extent is uncertain due to inconsistencies in the definitions of the land cover classifications, all studies are consistent in demonstrating a moderately rapid to rapid reduction in the wetland area which is inferred to have continued due to ongoing conversion to agricultural use (Wilson et al. 2017, Proswitz et al. 2021).
The future threat to sustainable water flows from over-abstraction in the dry-season (Ministry for Natural Resources and Tourism 2018) would simultaneously impact a large proportion of the population, though the majority of birds are thought to occur along the main channels where impacts of this threat would be less severe. The remaining area is most significantly affected by conversion of wetland to agricultural uses, which is predominately carried out by individual farmers. There are a minimum of at least 9 further locations based upon this threat if the more restricted potential area of suitable habitat is considered, while larger extents would suggest a higher number of locations.
Occurs primarily in flooded Phragmites mauritianus dominated reedbeds at elevations of between 240-305 m (Fjeldså et al. 2021). It may forage in other mixed floodplain vegetation such as Polygonum spp., but appears to require dense stands of flooded grasses (Fjeldså et al. 2021). Juveniles have been observed in June and July (Fjeldså et al. 2021). The species has been recorded as present at the same sites throughout the year (Fjeldså et al. 2021) and no seasonal movements have been reported but densities vary in different habitats over the year (Rannestad et al. 2015).
Within the Kilombero valley there has been rapid conversion of wetland habitat, on which the species depends, to agricultural use over the past three decades (Wilson et al. 2017, Proswitz et al. 2021). The area of wetland/grassland is estimated to have decreased by 33% between 1991 and 2016, at a rate that appears to be accelerating (Wilson et al. 2017, Proswitz et al. 2021). The human population of the Kilombero valley has been increasing rapidly since the start of the 21st century, predicted to result in the measured rate of wetland loss at least continuing or more likely accelerating over the next decade unless complex interacting drivers are countered (Proswitz et al. 2021). Of particular concern is the lack of understanding of the hydrological system within the floodplain and therefore the inability to regulate irrigation and other water uses to within sustainable bounds (Wilson et al. 2017, Ministry of Natural Resources and Tourism 2018). Loss of wetland to agricultural use has come from large businesses purchasing land for irrigated crops, particularly rice, and the expansion of the area farmed by small-holders across the wetlands, including a rapid increase in pastoralists and cattle prior to a forced relocation by the government in 2012 (Nindi et al. 2014).
Additional threats come from the control of water upstream of the Kilombero basin. There were two hydropower generation schemes operating in 2017 with an additional hydropower projects planned for the upstream Kilombero basin (Ministry of Natural Resources and Tourism 2018). An assessment of the current hydrology of the Kilombero River mainstem and the tributaries showed largely unaltered flow regimes and ecological functioning, abstraction and regime modifications, but identified significant risks that the condition would decline and that an analysis of current irrigation plans threaten to reduce dry-season flows to below target sustainable levels (Ministry of Natural Resources and Tourism 2018). Additional irrigation is likely with the continuing increase in the human population within the floodplain, such that a considerable proportion of the flooded grassland area on which the species depends may dry out in future dry seasons. This is considered to be threat with the potential to affect the largest proportion of the population simultaneously, given the species' apparent strong preference for flooded vegetation (Fjeldså et al. 2021). On a precautionary basis this threat is suspected to have the potential to affect around a quarter of the range (given that main stem flows will continue and that the species is described as common along the main channels).
Conservation actions in place
The majority of the species' range lies within the Kilombero Valley Ramsar Site, designated in 2002 and covering 7,967 km2 (RIS 2002, Finlayson et al. 2016, Wilson et al. 2017). It is also an Important Bird Area (Fishpool and Evans 1998, Baker and Baker 2002) and with the recognition of this species and C. bakerorum now meets the criteria for an Endemic Bird Area (sensu Stattersfield et al. 1998). Conservation actions are a key element to the most recent Integrated Management Plan (Ministry for Natural Resources and Tourism 2018), within which this species is mentioned as a part of the qualifying criteria, albeit as one of two undescribed cisticola species. The plan contains recommendations to address concerns highlighted by an earlier Ramsar Advisory Mission report (Wilson et al. 2017), but the extent to which these have been subsequently implemented is unclear. There has been considerable research into the human pressure within the Kilombero Valley and the impact on the condition of the natural habitat and species (e.g. see Proswitz et al. [2021] for the complex drivers of environmental change in the valley).
Conservation actions needed
The key challenge is to retain sufficient suitable habitat (apparently tall flooded reed marsh) within an area subject to high pressure for agricultural conversion. It remains necessary to establish clear agreed conservation zones within the Ramsar Site where sufficient habitat can persist, noting that it is a dynamic landscape. Monitoring to assess the reduction in extent of apparently suitable habitat, flooded reedbeds/grassland should continue as part of the implementation of the Integrated Management Plan, and this should also include effort to monitor distribution and abundance of this and the other two bird species endemic to the area. The extent to which the species can persist in human-altered wetland areas is unknown: it is suggested that the species may be dependent on tall Phragmites mauritianus reed stands. To monitor rates of habitat loss, it would be helpful to determine more accurately the species' habitat requirements.
Text account compilers
Martin, R.
Recommended citation
BirdLife International (2024) Species factsheet: Kilombero Cisticola Cisticola bakerorum. Downloaded from
https://datazone.birdlife.org/species/factsheet/kilombero-cisticola-cisticola-bakerorum on 13/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 13/12/2024.