Influence of vegetation attributes on bird functional traits in a Pinus mixed forest at Yagirala forest reserve, Sri Lanka

Abstract

Although many studies have examined changes to overall species composition and diversity of birds following the conversion of natural forests into forest plantations, only a few have examined changes to functional trait compositions. The present study investigated the functional trait responses to the conversion of natural forests to forest plantations by comparing the richness and abundance of functional categories of three traits: i) feeding guild (categories: carnivores, insectivores, frugivores, nectarivores, omnivores and granivores), ii) nesting habit (categories: non-cavity tree dependence nesting, tree cavity nesting, ground nesting and parasitic nesting), and iii) foraging stratum (categories: ground foraging, mid-high foraging, and canopy foraging) of a bird community in pinus mixed forest and adjacent natural forest at Yagirala Forest Reserve located in the Southern part of Kalutara district, Sri Lanka. Further, the study examined the relationships between the functional categories and vegetation attributes to explore the factors that drive functional trait responses to conversions. The richness and abundance of functional categories were estimated using the data of point count surveys conducted at pinus mixed forest, which encompasses areas undergoing natural regeneration and enrichment planting, and in the natural forest. At each point count station within the pinus mixed forest, vegetation surveys were conducted to determine the number of standing dead trees, native tree species density, number of woody debris, percentage of canopy cover, percentage of herbaceous cover and the heterogeneity of tree height and tree dbh. The functional categories in pinus mixed forest areas and the natural forest were compared using One-way ANOVA followed by Tukey’s pairwise comparison. The relationships between bird functional categories and vegetation attributes were analyzed using linear mixed models (LMMs). According to the findings of this study, the abundance of six functional categories and the richness of three functional categories per point count station were significantly differed between the pinus mixed forest areas and natural forest (p < 0.05; one-way ANOVA, Tukey’s post hoc test). The findings further revealed significant relationships between the richness and abundance of functional categories including tree cavity dependents, frugivores, insectivores and canopy foragers with vegetation attributes such as number of standing dead trees, native tree species density, number of woody debris and percentage of canopy cover (p < 0.05; LMMs). This study emphasizes the potential of exotic forest plantations to support local bird communities by managing vegetation composition and structure effectively.