Population Dynamics of the Endemic and Vulnerable Grauer's Swamp Warbler (Bradypterus graueri): Insights From Long-Term Monitoring in Volcanoes National Park, Rwanda

Abstract

Wetland-dwelling birds are indicators of wetland health. We investigated the population dynamics and conservation implications of the endemic and Vulnerable Grauer's swamp warbler (Bradypterus graueri; GSW) across three wetlands (Rugezi, Kabatwa, and Malalo) of varying sizes and elevations within Volcanoes National Park (VNP), Rwanda. We collected visual and auditory count data over 13 years from 19-point count stations systematically located across the wetlands and applied distance analysis to assess the abundance and distribution of GSW across years and wetlands. Then, we employed Generalised Additive Mixed Models (GAMM) to assess the relationship between GSW abundance and specific factors known to impact wetlands: trampling by large mammals, the presence of non-wetland plant species, or the absence of water. Additionally, we investigated how wetland sizes influenced this relationship. We found that the focal wetlands within VNP support an approximate mean of 3.143 (95% CI: 2.578–3.831) clusters of GSW per hectare, hosting a mean of 3.634 (95% CI: 2.960–4.462) individuals per cluster. Nonetheless, there were instances of GSW extirpation occurring in Malalo and Kabatwa VNP wetlands. Mammal trampling and non-wetland plants negatively impacted the GSW population (trampling: estimate = −2. 126, p < 0.043; non-wetland plants: estimate = −0.735, p = 0.05), while GSW abundance is positively associated with the presence of water (estimate = 0.862, p < 0.002). GSW density also increased with wetland size, though this trend was not significant (p > 0.05). Generally, despite fluctuations in GSW densities within the studied wetlands, VNP remains a crucial habitat for this globally threatened species: wetlands that might require actions highlight the need for efforts to avoid desiccation.