Widespread bird species show idiosyncratic responses in residual body mass to selective logging and edge effects in the Colombian Western Andes

Forest fragmentation is a major driver of tropical bird endangerment, yet the mechanisms underlying species losses in fragmented landscapes remain poorly known. Loss of foraging microhabitats and food diversity in fragmented landscapes are potential mechanisms explaining fragmentation sensitivity. We, therefore, measured the body condition (i.e., body mass adjusted for individual size) of 20 tropical understory birds as a proxy for food availability across gradients of fragment patch size and silvicultural histories in the Western Andes of Colombia. Specifically, we asked (1) if body condition correlates with patch size, edge effects, or selective logging disturbance; and (2) if species responses were driven by the same explanatory variables or varied idiosyncratically. We documented significant variation in body condition with covariates in 11 of 20 (55%) understory bird species, and 55% of significant correlations were with fragmentation and selective-logging associated variables. Species responses were idiosyncratic and contrasting, with at least 1 significant response to each fragmentation-related covariate. Most effects, however, were driven by logging-induced changes to vegetation structure including loss of large-diameter trees, reduction in foliage height diversity and canopy cover, and loss of understory vegetation. The body condition of frugivores and nectarivores increased with logging-induced shifts in vegetation structure, with canopy gaps potentially favoring early-successional fruiting and flowering plants. By contrast, some insectivores suffered lower body condition with loss of vegetation structure (reduced foliage height diversity). Overall, our data support the hypothesis that fragmentation and selective logging change food resources available to birds and that some widespread nectarivores, frugivores, and omnivores may benefit from increasing edge density and logging-driven vegetation disturbance. We also documented significant positive effects of breeding condition on body condition, however, highlighting the need to control for this factor. Body condition may therefore be a useful index of habitat suitability when paired with analyses of abundance and demographic changes. Lay Summary center dot Body condition, body weight adjusted for body size, is a measure of food availability for birds, but has never been used to evaluate the hypothesis that loss of food leads to the local extirpation of birds in tropical forest fragments. center dot We used mist-net captures to measure the effects of forest patch size, amount of forest edge, and selective logging on the body condition of Andean birds across a range of patch sizes in Colombia. center dot Half of significant correlations of environmental variables with body condition were with fragmentation- and logging-related covariates, but responses were variable across species (both positive and negative). center dot Changes to vegetation structure from selective logging explained two-thirds of significant responses, likely due to differences in the availability of food plants and foraging habitats. center dot Contrasting responses to logging disturbance were related to the foraging behavior of each species, suggesting that gain or loss of food resources is a mechanism explaining positive or negative effects, respectively, of logging on tropical bird communities.

Automated summary

The study found that fragmentation and selective logging have different effects on the body condition of different species of tropical understory birds, mainly through changing vegetation structure and food resources. Some bird species have better body condition under logging-induced vegetation disturbance, while some insectivores have poorer body condition with loss of vegetation structure.