Abstract
Avian study in the southern Gulf of Lake Tana and head of the Blue Nile
River, Ethiopia was conducted from June 2016 to April 2017 during the
wet and dry seasons to examine species composition, relative abundance,
and habitat association of birds. The study area was classified based on
its vegetation cover and composition and point count method was used to
gather information about the occurrence of avian species and their
relative abundances. A total of 131 avian species belonging to 15 orders
and 53 families were identified. The wet and dry seasons data indicated
that 122 and 108 avian species, respectively, were recorded.Agapornis tranta and Oriolus monachal are endemic birds of
Ethiopia and Eritrea that are occurring in the study area. Moreover, two
Intra-African migrant, Threskiornis aethiopicus and Milves
migrans, and two Palearctic migrant, Delichon urbicum andMotacilla flava were identified in the area. The highest avian
diversity was recorded in the Debremariam wetland (H’=3.96), while the
lowest was at the adjacent areas of Lake Tana (H’=3.22). The relative
abundance score showed that 54.8% of the avian species were uncommon
and there was a significant difference in relative abundance of birds
between the wet and dry seasons (p<0.05). Different
anthropogenic activities and urbanization are the main threats to the
conservation of birds in the southern Gulf of Lake Tana. Therefore,
appropriate management actions should be designed and implemented to
ensure the conservation of birds in the Lake and its adjacent habitats.
Keywords : Blue Nile, endemic, Lake Tana, species diversity,
point count
1. INTRODUCTION
Ethiopia is known for its avifauna and the diverse avian species of the
country is mainly due to its different habitat types. Over 863 species
of birds are identified from Ethiopia representing approximately 9.5%
of birds in the world and 39% in Africa. It is known that nineteen
avian species are endemic to Ethiopia, three are rare or accidental to
the country, while fourteen species are shared with Eritrea and thirty
one are globally threatened (Lepage, 2021). According to Ethiopian
Wildlife and Natural History Society (EWNHS) (1996) forests, wetlands,
and riverine ecosystems are sites for wintering or migrant birds in
Ethiopia. As a result, there are 214 Pale-arctic migrant avian species,
and a large number of these birds have breeding populations in the
country (Pol, 2006).
In Ethiopia, most avian species are found in Important Bird Areas
(IBAs), but these areas cover only 5% of the country’s total area and
wetlands comprise 41% of the IBAs (Ayenalem & Bekele, 2008). Habitat
variables of wetlands differ in water depth, water level fluctuation,
salinity, topography, and vegetation (Colwell & Taft, 2000). Wetland
birds are categorized into wetland specialist and wetland generalist
(Airinatwe, 1999). Wetland specialists are completely dependent on
aquatic habitats, while wetland generalists are frequently found in
wetlands and they also use other habitats (Wondafrash, 2003). Wetland
birds are placed into four categories as resident, sporadic, summer
visitor, and vagrant (Clout & Hay, 1989).
The sizes of wetland habitats influence species richness and relative
abundance of water birds (Froneman et al., 2001). Larger wetlands, which
have high habitat heterogeneity can support a greater diversity of water
birds than smaller ones (Colwell &Taft, 2000). Water birds foraging
close to the shores persist in both large and small ponds and are
considered as area-independent species, while those species foraging in
open and deep-water habitats are considered as area-dependent species
and are restricted to relatively large ponds (Paracuellos, 2006). The
distribution of water bird species had a nested pattern among wetlands
of different sizes (Paracuellos & Telleria, 2004). The kinds and
amounts of resources available for breeding and foraging activities can
also affect avian communities (Lee & Rotenberry, 2005). Thus, larger
wetlands are of greater conservation value than smaller ones in
supporting diverse water bird species (Paracuellos, 2006).
A number of environmental factors including food availability,
temperature and the presence or absence of competitors and predators
have been found to influence bird species diversity and their relative
abundance. According to Robert & Kathleen (1992) habitat
characteristics at different spatial scales, habitat modification or
fragmentation, urbanization and the surrounding landscape composition
can influence avian species diversity and abundance. Habitat loss and
fragmentation are widely regarded as major factors contributing to the
decline of avian populations (Raman, 2006). One in eight of the world’s
avian species face extinction in the next millennium because of habitat
destruction (Delannoy, 2010). Therefore, the main objective of this
study is to explore species composition, relative abundance, and habitat
association of birds in the southern gulf of Lake Tana and head of the
Blue Nile River to design appropriate conservation strategies and ensure
conservation of birds in the Lake and its adjacent habitats.
2. MATERIALS AND METHODS
2.1. Description of the study area
Study on avian diversity and relative abundance was conducted in the
southern gulf of Lake Tana and head of the Blue Nile River, Ethiopia.
Lake Tana has a total drainage area of approximately
15,000km2 and the catchment of the lake is one of the
global top 250 lake regions most important for biological diversity
conservation (Barker, 2004). It has varied topography, and the altitude
ranges from 1784m a.s.l. in the south of the lakeshore to 3712m a.s.l.
in the northeast end. Lake Tana is the largest lake in Ethiopia with a
total area of 3600 km2 and is the source of the Blue
Nile River.
Lake Tana is located at 12o00’N latitude and
37o15’E longitude and it is 565km far from the capital
Addis Ababa. It has approximately 84km long and 66 km wide, with an
average depth of about 9m (Dejen, 2003) and has more than 30 islands
(EWNHS, 1996). It has more than 40 tributary rivers, but the major
rivers feeding the Lake are Gilgel Abay from the South, Ribb and Gumara
from the East, and Megetch from the North (Fig. 1).
Blue Nile River is the largest river in terms of volume of discharge and
the second largest in terms of area in Ethiopia, which comprises of
176,000km2 (17%) of the area of Ethiopia (Conway,
2000). There is a significant waterfall at Tis-Isat, roughly 25km from
Lake Tana, where the river drops 50m into the Blue Nile gorge. Most of
Lake Tana’s catchment area is characterized by croplands, while only few
limited areas of highlands are forest patches. Mean monthly maximum and
minimum temperature recorded at Bahir Dar are 30.3oC
and 7.82oC, respectively. The 10-year (2006 to 2016)
rainfall data of the area shows unimodal distribution. Small rains occur
sporadically during the months of April and May and heavy rainfall is
recorded from June to August.