Introduction
Globally, there are approximately 10928 species of birds distributed in 40 orders, 250 families and 2322 genera of the class Aves (Gill & Donsker 2020). Ornithologists have studied the geographic distribution, local and regional diversity and variation in birds for many years (Roth 1976; Hoyer & Canfield 1990; Clavel, Julliard & Devictor 2011; Dinesen et al. 2019). Many factors including geographic location, habitat condition and climatic factors affect bird species richness and abundance (MacArthur, MacArthur & Preer 1962; MacArthur 1964; González-Gajardo, Sepúlveda & Schlatter 2009; Moritz & Agudo 2013; Guevara et al. 2021). Studies have indicated that birds globally are at risk because of human growth, forest destruction, fragmentation and loss of suitable habitats (Brown & Dinsmore 1986; Jokimäki & Suhonen 1993; Chace & Walsh 2006). Ecologists have long been interested in how human induced ecological and environmental factors affect the abundance and distribution of communities in limnetic ecosystems. For over six decades a positive correlation between species richness and habitat area and cover have been established for terrestrial ecosystems (MacArthur, MacArthur & Preer 1962; MacArthur 1964; Venier & Fahrig 1996; González-Gajardo, Sepúlveda & Schlatter 2009; Cardinale et al. 2018). Furthermore, factors like wetland area, water surface, water level, habitat productivity and heterogeneity have been positively related to species richness, bird abundance and species guild (Mark & Daniel 1990; Edwards & Otis 1999; Babbitt 2000). However, which factors are most important remains unresolved. What is more, despite the growing recognition of wetlands as important environments for birds worldwide, wetland ecosystems are being altered and reduced at an increasing rate by human activities (Dugan 1990). Unfortunately, despite the value of wetland biodiversity, wetlands are still declining locally and regionally as a result of human pressure at global level (Ramsar Convention Bureau 2006; Sebastián-González, Sánchez-Zapata & Botella 2010).
Unlike this global situation, of decreasing and loss of wetlands, over the past two to three decades, wetlands have been created as a result of the construction of manmade reservoirs in Tigray (Asmelash 2009). The construction of such reservoirs has created a sizeable wetlands downstream and induced positive microclimate change in the surrounding areas (Haileselasie & Teferi 2012). However, majority of the research conducted on these limnetic ecosystems involves limnoilogical characteristics (Dejenie et al. 2008), Phytoplankton community structure (Asmelash 2009), fisheries (Teferi et al. 2013) and colonization of these reservoirs by the water flea-Daphnia(Haileselasie et al. 2018). There is little information on the avian fauna of these limnetic ecosystems since their construction for irrigation, livestock and household water consumption purposes. Globally there are a few studies that compared the diversity and composition of waterbird functional traits between natural, restored, and artificial wetlands (Almeida et al. 2020). But to the best of my knowledge, there is no study that examined the factors that affect bird population in limnetic ecosystems of Ethiopia in general and Tigray regional state in particular. Furthermore, it is not known whether modified habitats such as reservoirs, constructed wetlands could act as alternative habitats to attract and sustain bird communities and conserve regional species diversity. This study is focused on bird species richness as an indicator of biodiversity gain triggered by construction of reservoirs and has the following specific objectives: 1) to assess avian species richness in an age gradient of limnetic ecosystem constructed for irrigation, livestock and household water consumption in Tigray. 2) To investigate the effect of ecological and environmental variables of reservoirs on bird species richness and distribution in reservoirs of Tigray, Northern Ethiopia.