DOI: 10.5061/dryad.s7h44j1bn
title: Abundance variations within feeding guilds reveal ecological mechanisms behind avian species richness pattern along the elevational gradient of Mount Cameroon

description: Two distinct diversity patterns are observed along tropical elevations:
 (a) decreasing number of species towards high elevations and (b) a
 hump-shaped pattern with the peak at mid-elevations. As diversity is
 likely supported by ecological capacity of the environment, decomposition
 of the overall richness into ecological facets and considering number of
 individuals within them is crucial for the proper understanding of
 richness patterns. We examined abundances of different avian guilds along
 the forested part of the elevational gradient on Mt. Cameroon. We (a)
 compared richness and abundance elevational patterns, (b) assessed the
 effective contribution of multiple guilds to richness and abundance
 patterns, and (b) assessed to which extent observed abundances of guilds
 differ from those expected by chance. We sampled birds in 2011–2015 during
 the dry season at seven elevations (30 m, 350 m, 650 m, 1100 m, 1500 m,
 1850 m, 2200 m a.s.l.). For each assemblage, we estimated proportions of
 species and individuals that use particular diets, foraging modes, and
 feeding strata. We found that a rather decreasing pattern of species
 richness turns into a hump-shaped one if we look at the total abundances,
 implying different mechanisms behind these patterns. The number of species
 and individuals thus do not seem to be directly related, contrary to ‘the
 more-individuals hypothesis’. Abundances of foliage gleaners at
 mid-elevations, nectarivores at high elevations, and frugivores at low
 elevations deviated from random expectations. Our results suggest that
 parts of ecological space are filled partly separately by bird species and
 individuals along elevation of Mt. Cameroon.
description: <strong>Bird community
 sampling</strong> Bird communities were sampled in 2011–2015
 during the driest season (November - December) at seven forested
 elevations (30 m, 350 m, 650 m, 1100 m, 1500 m, 1850 m, 2200 m a.s.l.)
 using a standardized point count method (Bibby et al., 2000) and, additionally, random walks to capture the
 occurrence of rare species. Birds were recorded in the morning (6 am – 10
 am) at 16 points at each elevation. The sample sites had a radius of 50 m
 and were located at least 150 m apart to diminish the risk of multiple
 counting of individuals while remaining logistically tractable for the
 study. Each point was visited on three different days, the order of points
 was changed during each visit to avoid biases due to daytime, and birds
 were recorded in three consecutive 5 min intervals during each visit. The
 maximum number of individuals of a species recorded at any of these 5 min
 intervals was recorded as its abundance at the given point, and the sum
 across all 16 points as its abundance at the given elevation (hereafter
 “elevational abundance”). We gave 1 estimate to singing individuals (1
 pair) and a 0.5 individual estimate (half of the pair) to individuals that
 were just visually observed and were not singing. This is based on the
 assumption that singing individuals are usually accompanied by non-singing
 mates (Moller, 1991;
 Nowicki & Searcy, 2004). Duetting birds were counted as a pair, so 1
 was assigned to a duetting pair. Additional random walks by three
 observers have been conducted for three days to capture rare species not
 recorded during regular point counts. The abundance assigned uniformly to
 these rare species was 0.5 for the given elevation as only vagrant and
 non-breeding individuals may be present. We are aware that our approach
 has its limitations, but it is rather conservative and probably
 underestimates the actual number of individuals in the forest. However, it
 focuses on reproductively active individuals and since we did it uniformly
 across the gradient, and do not expect a higher abundance of singing
 females in a specific elevation, the comparisons between elevations are
 informative. The taxonomy used in this study followed the IOC World Bird
 List version 4.2 (Gill & Donsker, 2014).
 A distance sampling protocol
 was used, with birds recorded in five 10 m wide radial distance bands
 (Buckland et al.,
 2001). Here, we utilized
 the Distance v6.2 software (Thomas et al., 2010) to account for differences in species
 detectability and to estimate the number of unobserved individuals based
 on fitted detection functions (Buckland et al., 2001). Application of
 detection adjustments is, however, generally found problematic in tropical
 environments (Banks‐Leite
 et al., 2014) and we used
 the observed abundance estimates instead of the distance sampling-based
 estimates in further analyses. See Ferenc et al. (2016) and supplementary
 analyses therein for a detailed description of analyses on distance
 sampling-based data. Overall, estimates based on distance sampling
 provided worse results because (i) many rare species could not be included
 in the analyses, (ii) estimates derived from distance sampling were highly
 correlated with the observed abundance estimates, (iii) exceptions exist
 for which abundance estimates based on detection functions are
 unrealistically high. To standardize for number of individuals sampled we
 rarefied the observed diversities at particular elevations (Fig. S1) 
 <strong>Food and foraging niche
 estimates</strong> The data on food and foraging niches were
 obtained from literature (Brown et al., 1982; Urban et al., 1986, 1997;
 Fry et al., 1988; Keith et al., 1992; Fry & Keith, 2000,
 2004). The following niche
 dimensions (with categories) were recorded: food type – insectivores
 (insects and other invertebrates), carnivores (vertebrates), nectarivores
 (nectar and pollen), frugivores (fruits), and granivores (seeds); and
 foraging height classes - ground, low stratum, middle stratum, and canopy.
 Foraging behavior was categorized for insectivores as follows: (i) aerial
 feeding – catching prey actively in the air, (ii) foliage gleaning –
 gathering prey actively from leaves, mosses, twigs, and small branches,
 (iii) bark probing – gathering pray from branches and tree trunks, (iv)
 flycatching – waiting for prey and catching it in the air, (v),
 sit-and-wait – waiting for prey and catching it on the surface, (vi)
 ground search – gathering from the ground. We quantified the relative
 composition of the diet (% of diet or relative amount of time spent
 feeding on nectar) and relative use of foraging modes and foraging strata
 (% of time spent by each foraging mode and in particular strata while
 foraging).