Effects of habitat degradation on mixed-species bird flocks in Indian rain forests

Abstract

Abstract:Habitat degradation affects mixed-species bird flocks (flock hereafter) through two mechanisms – changes in the bird community from which flocks are drawn and changes in the propensities of species to flock. We determined the relative influence of these two mechanisms by examining variation in flocks across nine rain-forest fragments (range 11–2600 ha) in a plantation landscape in the Western Ghats, India. We found differences between fragments in average number of species (range 10.8–15.2) and individuals (range 19.0–37.6) per flock, number of species that participated in flocks (range 34–59), encounter rates (range 0.5–2.4 flocks h−1) and flock composition. Multiple regression and randomization tests revealed that different mechanisms contributed to this variation. Three flock variables (open-forest individuals per flock, total open-forest species that participated in flocks in a fragment, flock composition) mainly reflected changes in the bird communities of fragments. Habitat structure strongly influenced three flock variables (open-forest species per flock, total and rain-forest individuals per flock) and flock composition to a lesser extent. Finally, flock encounter rate was strongly related to fragment area, but not to abundance of flock participants indicating habitat degradation-induced changes in propensities of species to flock.

Positive Relationships between Association Strength and Phenotypic Similarity Characterize the Assembly of Mixed-Species Bird Flocks Worldwide

“…b Higher average association strength observed across guilds compared with within guild. c Field methods used in this study were the same as those used in the study by Sridhar and Sankar (2008). d Field methods used to collect these unpublished data were the same as those used in the published study by Ewert and Askins (1991).…”

Decay of interspecific avian flock networks along a disturbance gradient in Amazonia

“…To date, research on how mixed-species flocks change along disturbance gradients have largely focused on species richness and encounter rates (e.g. number of detections per unit time [32,33]), yet no studies, to our knowledge, have examined how habitat modifications influence interspecific interactions and the stability of flock structure.…”

The structure of mixed-species bird flocks, and their response to anthropogenic disturbance, with special reference to East Asia

“…The first is fragmentation (Stouffer and Bierregaard 1995;Fernández-Juricic 2000, 2002Marini 2000, 2004;Tellería et al 2001;Van Houtan et al 2006;Sridhar and Sankar 2008;Cordeiro et al 2014;Mokross et al 2014). The general result of this literature is that flock qualities (size in individuals, species richness, encounter rate and even network characteristics; see Mokross et al 2014 about networks) decrease as fragmentation increases, with fragments below 10 ha being especially effected (Maldonado-Coelho and Marini 2004;Mokross et al 2014).…”

“…In studying how flocks respond to anthropogenic disturbance, several studies have tried to distinguish between how disturbance affects flocks and how it affects the pool of birds at an area available to flock, by taking into account data on the overall abundance of birds in the community (Fernández-Juricic 2002;Sridhar and Sankar 2008;Sidhu et al 2010;Goodale et al 2014). While Fernández-Juricic (2002) found that the total bird density in fragments was a good predictor of the probability of encountering flocks, the other studies found that flocking propensity decreased in highly modified environments (very small fragments in Sridhar and Sankar 2008; and areas of intense agriculture for the studies of Sidhu et al 2010;Goodale et al 2014).…”

“…If a species A is important to flocks, and a species B joins flocks to follow or derive benefits from A, then the habitat choices of A could alter the habitat preferences of B, a "within trophic cascade" analogous to trophic cascades in which top carnivores have large effects on communities (e.g., Terborgh et al 2001). Several studies have implicated the absence of nuclear species as a strong contributor to the breakdown of flock structure (Maldonado-Coelho and Marini 2004; Sridhar and Sankar 2008;Zhang et al 2013;Cordeiro et al 2014). New analysis of Goodale and colleagues' data of flocks in Sri Lanka (Goodale et al 2014) shows that at intermediate elevations, two different flock leaders, a babbler and a white-eye, vary in their sensitivity to land-use intensity, with babblers being more confined to forest (Mammides et al, unpublished manuscript).…”