A classification scheme for mixed-species bird flocks

Abstract

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'.

Keywords: flocking behaviour; foraging groups; multi-species groups.

Figure 1.

(a) Map partitioning the reviewed flock studies according to major biogeographic realms and (b) the global accumulation of studies published each year, with the timeline denoting the first published study within each biogeographic realm.

Figure 2.

Percentage of mixed flock studies in each biogeographic realm that reported (a) territoriality and (b) seasonality.

Figure 3.

(a) Summary of the terminology used to report notable species roles in mixed flocks. (b) The accumulation of studies that reported these roles over the past 150+ years. The timeline in (b) is annotated with the first published manuscript to mention either sentinel, leader or nuclear behaviour. Note that ‘leader’ is labelled twice here. Even though Bates [13] was the first to mention this behaviour, the local native communities were the ones who communicated this to him, which Bates largely dismissed as a myth. Two additional important works are denoted on the far right, after which species roles have been reported more frequently.

Figure 4.

Summary of studies reporting species roles across each of the seven biogeographic realms. Across the top, capital ‘N’ represents the total number of published studies within each biogeographic realm, whereas the lowercase ‘n’ refers to the number of studies that reported participant roles, which were used to calculate the percentages shown.

Figure 5.

Standardized traits to report when conducting studies on mixed flocks. (a) Suggested traits related to habitat, such as the structure and configuration of the environment. Note that, as the physical complexity of the environment increases, clarification is needed about which forest stratum is used by the flock. Different bird species are exemplified with different silhouettes and/or colours. (b) Traits related to behaviour and the physical traits of participants, comprising four main sub-categories: (i) behavioural roles—whether the studied flock contains sentinel, nuclear or leader species. Simply put, sentinel species produce alarm calls, leaders (denoted in yellow) are the first to move in a particular direction, and nuclear species (denoted in violet) contribute to the formation and/or cohesion of flocks. (ii) Species evenness evaluates the numerical dominance of flock participants, with special attention to the numerical dominance of nuclear species (denoted in violet); (iii) social structure of the flock considers whether participants join and leave the flock over time, where the birds coloured in green represent the same individuals joining and leaving the same mixed flock at different times; and (iv) foraging guilds illustrate the need to supply descriptions about foraging techniques and the primary foraging guild.