Mixed‐species bird flocks enhance the benefits of group aggregation by minimizing variation in body mass while maximizing variation in diet

Abstract

Many animal species participate in interspecific groups which can provide benefits such as better detection of predator presence, but may also lead to costs such as interspecific competition. In interspecific groups, species with particular functional traits should be aggregated in a way that maximizes the benefits and minimizes the costs of these interactions. Further, the balance between costs and benefits depends on the resource availability and the spatial scale in which species interact. We aim to determine how species traits are distributed within and among interspecific groups and how intrinsic (group size) and extrinsic (environmental conditions) factors relate to aggregation patterns. We surveyed 192 bird species distributed in 355 mixed-species bird flocks along an environmental gradient in southwest Brazil and analyzed data at both scales of individual flocks (n = 355) and sites (n = 29). We used mixed-effect and multiple regression models to test if the functional richness of coexisting species changes with increasing group size and in response to environmental covariates that may influence the selection of specific traits. Using a null model approach, we inferred the differences between observed and simulated associations (random expectations). We found that groups with more than 20 species have greater diet variation (i.e. more omnivores) and are more similar in body mass than expected randomly. Indeed, larger groups consume more resources, increasing competitive dynamics and decreasing facilitation among species. Thus such groups should take advantage of a more profitable trait variation among co-occurring species. Our results suggest that the distribution of species in close associations is determined mostly by ecological processes occurring at local scales (i.e. not dependent on regional spatial context). Additionally, larger groups maximize foraging trait variation (diet diversity) and minimize predator scape variation (similar body mass) potentially increasing the benefits of the aggregation.

Effects of land use change on the functional diversity and composition of mixed species avian flocks in the high tropical Andes of southern Ecuador

“…While numerous studies have examined impacts of anthropogenic activities on the taxonomic structure of mixed species flocks (e. g., Lee et al 2005, Knowlton and Graham 2011, Montaño-Centellas and Jones 2021, few have looked at responses of functional structure of flocks to the same activities (Jones and Robinson 2020) and to changing environmental conditions (Zhang et al 2020, Dri et al 2022. While species and functional diversities are related, they do not always respond similarly to disturbances and only looking at one or the other may obscure the magnitude of habitat disturbance impacts to mixed species flocks (Cadotte et al 2011).…”

“…Interestingly, functional richness of flocks was the only functional metric that did not differ across habitat types. Dri et al (2022) similarly found no effect of habitat type on functional richness of flocks after controlling for size differences. Meanwhile, Jones and Robinson (2020) found a significant difference in flocks' functional richness depending on the density of large diameter old-growth trees, but we found no increase in richness for native forest flocks.…”

“…Meanwhile, Jones and Robinson (2020) found a significant difference in flocks' functional richness depending on the density of large diameter old-growth trees, but we found no increase in richness for native forest flocks. Smaller flocks such as ours may show less variability in functional traits as compared to larger flocks, although habitat type still affects the clustering or overdispersion of certain traits (Dri et al 2022). As with species richness, reliable nuclear species and maintenance of cohesive flocks can also lead to homogenized functional richness due to specialized species persisting in disturbed habitats via facilitative interactions (Morse 1977, Mammides et al 2015.…”

“…Nearly 20% of the world's bird species participate in mixed flocks, including 38% of tropical species (Zou et al 2018). Members derive benefits from flocking, such as increased foraging efficiency and reduced predation risks (Morse 1977, Hutto 1994, Goodale and Kotagama 2005, Sridhar et al 2009, and interspecific competition associated with foraging in a group is often mediated in mixed flocks due to species with diverse functional traits, foraging guilds, and foraging strategies (Dri et al 2022). Flocking behavior not only increases the fitness of participants but can allow habitat-specialist species to utilize previously inaccessible or degraded areas via interspecific facilitative interactions (Morse 1977, Jullien and Clobert 2000, Mammides et al 2015.…”

“…Such changes may be reflected in altered niche occupancy and resource use by a mixed flock assemblage or avian community as a whole (Mason et al 2005, Rocha et al 2019. Several studies have analyzed the taxonomic and/or functional diversity of mixed species avian flocks, examining the effects of habitat fragmentation and disturbance within fragments (Jones and Robinson 2020), effects of flock size and environmental covariates (Dri et al 2022), and effects of an elevational gradient (Zhang et al 2020). Since functional diversity increases resilience to environmental change Cabido 2001, Díaz et al 2013), it can be inferred that mixed flocks with greater functional diversity will be less susceptible to impacts of habitat degradation and alteration.…”