The literature on mixed-species flocks references a wide variety of bird associations. These studies, however, have used an array of unstructured characteristics to describe flocks, ranging from the temporal occurrence of flocking to the identity and behavioural features of constituent members, with little consensus on which key traits define and characterize a mixed-species flock. Moreover, although most studies report species-specific roles, there is no clear consensus about what these roles signify nor how to define them. This lack of consistency limits our ability to compare flocks from different habitats, regions and species pools. To unify this sizable body of literature, we reviewed and synthesized 538 studies on mixed-species flocks. We propose 13 categories to classify mixed-species flocks using behavioural and physical traits at the flock and participant level, as well as the habitat where the flock occurs. Lastly, we discuss the historical terminology for different species roles and propose definitions to clarify and distinguish among nuclear, leader, sentinel, and flock-following species. We envision that these guidelines will provide a universal language for mixed-species flock research, paving the way for future comparisons and new insight between different regions and systems. This article is part of the theme issue ‘Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes’.

Anthropogenic disturbance contributes to global change by reshaping the ecological niche space available to biological communities. Quantifying the range of functional response traits required for species persistence is central towards understanding the mechanisms underlying community disassembly in disturbed landscapes. We used intensive field surveys of cloud forest bird communities across seven replicate landscapes undergoing agricultural conversion in the Peruvian Andes to examine how a suite of 16 functional response traits related to morphology, diet, foraging behaviour and environmental niche breadth predict (1) species‐specific abundance changes in countryside habitats compared to forest and (2) differential changes to the ecological niche space occupied by communities. Our analyses relied on (1) hierarchical distance sampling models to examine the functional predictors of abundance change across the agricultural land use gradient while accounting for imperfect detection and (2) n‐dimensional hypervolumes to quantify the expansion and contraction of ecological niche space in countryside habitats. Key traits related to increased abundance in early successional and mixed‐intensity agricultural areas included (1) morphological adaptations to dense understorey habitats, (2) plant‐based diets (flowers, fruit and seeds) and (3) broad elevational range limits and habitat breadth. Species occupying mixed and high‐intensity agricultural land use regimes had mean elevational range limits 20%–60% wider than species found within forests. Collectively, ecological niche space expanded within agricultural habitats for traits related to diet and environmental niche breadth, while contracting for foraging and dispersal traits. Such changes were driven by species with unique functional trait combinations. Our results reveal the dynamic changes to ecological niche space that underly community structure in disturbed landscapes and highlight how increased niche breadth can ameliorate disturbance sensitivity for generalist species. We emphasize that functional traits can be used to predict changes in community structure across disturbance gradients, allowing insights into specific mechanisms underlying community disassembly beyond emergent patterns of functional diversity. By identifying key functional trait groups that align with different countryside habitats, we demonstrate how conservation practitioners can contribute to the retention of avian functional diversity in agricultural landscapes throughout the world.

Birds in mixed-species flocks benefit from greater foraging efficiency and reduced predation, but also face costs related to competition and activity matching. Because this cost–benefit trade-off is context-dependent (e.g. abiotic conditions and habitat quality), the structure of flocks is expected to vary along elevational, latitudinal and disturbance gradients. Specifically, we predicted that the connectivity and cohesion of flocking networks would (i) decline towards tropical latitudes and lower elevations, where competition and activity matching costs are higher, and (ii) increase with lower forest cover and greater human disturbance. We analysed the structure of 84 flock networks across the Andes and assessed the effect of elevation, latitude, forest cover and human disturbance on network characteristics. We found that Andean flocks are overall open-membership systems (unstructured), though the extent of network structure varied across gradients. Elevation was the main predictor of structure, with more connected and less modular flocks upslope. As expected, flocks in areas with higher forest cover were less cohesive, with better defined flock subtypes. Flocks also varied across latitude and disturbance gradients as predicted, but effect sizes were small. Our findings indicate that the unstructured nature of Andean flocks might arise as a strategy to cope with harsh environmental conditions. This article is part of the theme issue ‘Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes’.

The fragmentation of tropical forests remains a major driver of avian biodiversity loss, particularly for insectivores, yet the mechanisms underlying area sensitivity remain poorly understood. Studies in lowland systems suggest that loss of food resources, changes to light microenvironments, increased nest predation, and dispersal limitation are possible mechanisms, but these are untested for montane tropical bird communities. In this study, we related avian functional traits to area sensitivity (quantified using beta estimates from a multi-species occupancy model) to test the above four hypotheses for a cloud forest bird community (both resident species and just resident insectivores) in the Colombian Western Andes. We found that species with more specialized diets and those that use the canopy and subcanopy (loss of food hypothesis), larger relative eye sizes (light microhabitat hypothesis), and larger clutch sizes (nest predation hypothesis) were significantly more area sensitive. By contrast, there was no support for the dispersal limitation hypothesis; instead, we found that insectivores with more pointed wing shapes, and more aerial lifestyles, were significantly more fragmentation sensitive. These results suggest that reduced vegetation structure, loss of late-successional plant species, and loss of epiphytic plants may reduce food availability in fragments. Similarly, the ability to tolerate higher light intensity near fragment edges, or when traversing matrix habitat, may be important for persistence in fragments and suggests that habitat configuration may be of special importance in fragmented Andean landscapes. Overall, a lack of information on foraging, movement, and breeding ecology complicates avian conservation in the Andes.