Elevational distribution of birds in an Eastern African montane environment as governed by temperature, precipitation, and habitat availability

Abstract

Tropical species, many of which have limited thermal tolerances and narrow ecological niches, are likely to experience elevational shifts due to climate change. However, our ability to predict these shifts is limited by a scarcity of baseline data on current distributions and a lack of understanding of drivers of species ranges. To characterize the elevational distributions of a suite of Afromontane birds, we conducted annual point counts between 2013 and 2018 at 297 plots across habitats and elevations (2416–4303 m) in Volcanoes National Park, Rwanda. We assessed elevational and habitat preferences via indicator analyses, and modeled abundance distributions as a function of temperature, precipitation, habitat availability, and congeneric competition. Although most of the 35 focal species were distributed across habitats and elevational ranges, 20 species particularly associated with narrow elevational range (<300 m) and 24 species were strongly associated with one to three habitat types. Abiotic factors significantly correlated with abundance distributions of 33 species (temperature 33, precipitation 17), and biotic factors with distributions of 31 species (habitat 30, competition 7). Temperature and habitat availability were particularly associated with upper elevational limits (31 and 26 species, respectively, vs. 10 and 6 at lower limits), whereas precipitation affected both limits similarly (17 lower, 16 upper), and competition had a limited role at either limit (4 lower, 3 upper). That the elevational distribution of Afromontane birds results from a species‐specific combination of biotic and abiotic factors is crucial information in our effort to predict climate change effects in this region.

Density and characteristics of tree cavities inside and outside Volcanoes National Park, Rwanda

“…The particularly high prevalence of excavated cavities in branches may be related to excavators' preferences for softer wood for excavation of cavities (Schepps et al, 1999), though follow‐up studies would need to confirm that branches differ from trunks in wood tissue hardness. With regards to orientation, we find that most trees in our study area grow on slopes that face a southern or eastern exposure, and high decay‐inducing levels of precipitation and moisture may be reached on the northwestern slope‐facing sides of trees as precipitation increases with elevation (van der Hoek et al, 2021). Following this, we may also assume that excavated cavities are predominantly found on the northwestern side of trees where the relatively softer wood would be easier to excavate.…”

“…Indeed, our estimate of 134.7 trees per hectare is well below the interquartile range of tree densities for either tropical moist or dry forest (lower quartile cut‐off >250 trees per hectare for either forest type, Crowther et al, 2015). In addition, but related to low tree availability at high elevations near the treeline ecotone, there is a potentially low abundance and richness of excavators (avifauna) in VNP (van der Hoek et al, 2021; van der Hoek, Faida, et al, 2020; van der Hoek, Gaona, et al, 2020).…”

“…We opted for many relatively small (400 m 2 ) plots over few larger (e.g., the 0.5 ha plots used by Boyle et al, 2008) plots as (i) we aimed to sample across multiple vegetation types and elevations and were (ii) hindered by the steep terrain in our efforts to establish precisely measured large plots. Plots, the locations of which were governed by our efforts to link data collected for other research (e.g., van der Hoek et al, 2021) and logistic constraints, were laid out along transects that followed the elevational gradient. Given that each of these small plots will only include a few trees, we were wary of under-or overestimation of cavity densities and thus aimed to increase the surface area cov- In each plot, we measured the height and Diameter Breast Height (DBH) of all trees >1 m high and >10 cm DBH.…”

An environmental habitat gradient and within-habitat segregation enable co-existence of ecologically similar bird species

“…Together with the estimated reduction in residual pairwise correlations in the extended warbler model ( figure 4 ), these results provide strong evidence of niche partitioning along this Grinellian mechanism for ecologically similar bird species in the Virunga volcanoes, consistent with previous studies in the Cameroonian mountains [ 61 ], Himalayan highlands [ 26 ], tropical Andes [ 62 , 63 ] and in New Guinea highlands [ 64 , 65 ]. A recent survey of birds covering a third of the study area (Parc National des Volcans in Rwanda) identified 57% of species to be associated with narrow elevation bands (less than 300 m; [ 66 ]).…”

Effects of inbreeding and elevated rearing temperatures on strategic sperm production

“…Female availability naturally varies as a result of variation in the environment. Changes in abiotic factors, such as precipitation and temperature, often moderate habitat complexity, resource availability and movement between populations (Ferger et al 2014; van der Hoek et al 2022), altering mate encounter rates and the adult sex ratio. A classic example is the approach of spring, coinciding with longer days, higher temperatures and more food, which triggers increased conspecific interactions and reproductive activities (Kahn et al 2013; De Marchi et al 2015).…”