Complex patterns of collective escape in starling flocks under predation

Abstract

Collective behaviour of animals has been a main focus of recent research, yet few empirical studies deal with this issue in the context of predation, a major driver of social complexity in many animal species. When starling ( Sturnus vulgaris ) flocks are under attack by a raptor, such as a peregrine falcon ( Falco peregrinus ), they show a great diversity of patterns of collective escape. The corresponding structural complexity concerns rapid variation in density and shape of the flock over time. Here, we present a first step towards unravelling this complexity. We apply a time series analysis to video footage of 182 sequences of hunting by falcons on flocks of thousands of starlings close to two urban roosts during winter. We distinguish several types of collective escape by determining the position and movement of individuals relative to each other (which determines darkness and shape of the flock over time) as well as relative to the predator, namely ‘flash expansion’, ‘blackening’, ‘wave event’, ‘vacuole’, ‘cordon’ and ‘split’. We show that the specific type of collective escape depends on the collective pattern that precedes it and on the level of threat posed by the raptor. A wave event was most likely to occur when the predator attacked at medium speed. Flash expansion occurred more frequently when the predator approached the flock at faster rather than slower speed and attacked from above rather than from the side or below. Flash expansion was often followed by split, but in many cases, the flock showed resilience by remaining intact. During a hunting sequence, the frequencies of different patterns of collective escape increased when the frequency of attack by the raptor was higher. Despite their complexity, we show that patterns of collective escape depend on the predatory threat, which resembles findings in fish. Significance statement Patterns of collective escape in flocks of starlings have always intrigued laymen and scientists. A detailed analysis of their complex dynamics has been lacking so far, and is the focus of our present study: we analysed video footage of hunting by falcons on flocks of thousands of starlings and show how patterns of collective escape (namely flash expansion, blackening, wave event, vacuole, cordon and split) depend on the preceding pattern and on details of attack. A higher frequency of attack during a hunting sequence resulted in a higher frequency of collective escape events. Flash expansion happened most often when the predator attacks at greater speed. A wave event was most likely when the raptor attacks at medium (rather than high or low) speed. These results provide a first quantitative approach to social complexity in collective avoidance of a predator. Electronic supplementary material The online version of this article (10.1007/s00265-018-2609-0) contains supplementary material, which is available to authorized users.

Damping of waves of agitation in starling flocks

“…One of the most common patterns of collective escape is the so-called "wave of agitation," where one or more dark bands (here referred to as pulses) of an approximately fixed width originate close to the attacking predator and travel within a second over the flock away from it (Procaccini et al 2011;Storms et al 2019); thus, the wave is transferring information rapidly across many individuals (Gerlotto et al 2006;Procaccini et al 2011). The function of this wave may be that it confuses the predator in whom to attack (Procaccini et al 2011).…”

“…When starlings swirl above their roosting site in huge flocks of thousands of individuals, they perform even without a predator attacking them complex maneuvers involving sudden collective turns that change the shape and density of flocks remarkably. When the flocks are attacked by a predator, they display patterns of collective escape that are even more complex (Feare 1984;Ballerini et al 2008a;Carere et al 2009;Goodenough et al 2017;Storms et al 2019).…”

Is social coordination during escape flights a general phenomenon in birds?

“…Such social coordination in escape is particularly likely in animals that live in open habitats in which the group itself is the only source of safety. Socially coordinated escape is known in many species of birds from open habitats (Brooke, 1998;Carere et al, 2009;Lima, 1993;Storms, Carere, Zoratto, & Hemelrijk, 2019). These birds escape together as a group and remain together in tightly coordinated flight while under attack from falcons and other raptors (Carere et al, 2009;Davis, 1980;Storms et al, 2019).…”

“…Socially coordinated escape is known in many species of birds from open habitats (Brooke, 1998;Carere et al, 2009;Lima, 1993;Storms, Carere, Zoratto, & Hemelrijk, 2019). These birds escape together as a group and remain together in tightly coordinated flight while under attack from falcons and other raptors (Carere et al, 2009;Davis, 1980;Storms et al, 2019). Such coordination gives the flock a superorganism-like behavior (e.g., Davis, 1980), with a variety of patterns of maneuvering and compacting during attacks (see Storms et al, 2019;Sumpter, 2010).…”

“…These birds escape together as a group and remain together in tightly coordinated flight while under attack from falcons and other raptors (Carere et al, 2009;Davis, 1980;Storms et al, 2019). Such coordination gives the flock a superorganism-like behavior (e.g., Davis, 1980), with a variety of patterns of maneuvering and compacting during attacks (see Storms et al, 2019;Sumpter, 2010). Modeling suggests that flocks of such birds achieve this coordination by interacting mainly with neighboring birds (Hildenbrandt, Carere, & Hemelrijk, 2010;Hemelrijk & Hildenbrandt, 2011; see also Bode, Faria, Franks, Krause, & Wood, 2010;Pita, Collignon, Halloy, & Fernández-Juricic, 2016).…”