Raking the ocean surface: new patterns of coordinated motion in seabirds

Abstract

Coordinated movements of seabirds exploiting a prey patch are known to increase prey encounter and capture rates of individuals. These behaviours, based on effective cooperation between seabirds, have only been reported at small scale, i.e. the scale of the prey patch. However, the efficient prey exploitation by seabirds in vast oceans require larger scale processes such as information transfers between individuals. Indeed, information transfers between foraging seabirds (e.g. changes in behaviour) reduce their search cost while increasing their prey encounter rate. Whether or not these information transfer processes imply active cooperation is unknown. Using images from fishing boat radars in the eastern tropical Atlantic, we show the existence of frequent medium‐scale patterns of coordinated flights of seabird groups, consisting in seabird fronts (‘rake’ patterns) of 0.3–4.4 km width, displacing cohesively over 1.2–10.6 km and lasting between 2 and 19 min. For these rakes to be maintained, seabird groups have to adjust their flight speeds and directions, while they are on average distant of 500 m from each other, what cannot occur by chance. These findings suggest the existence of collective and coordinated movements in seabirds during prey searching at several kilometres’ scale. This potential cooperation between foraging seabird groups brings new insight in the evolutionary trajectories of seabirds life‐style.

Assessing the effect of complex ground types on ground‐dwelling arthropod movements with video monitoring: Dealing with concealed movements under a layer of plant residues

“…This experimental device opens up the possibility for future analyses on the effects of a wide variety of ground types on the movement of ground‐dwelling arthropods. Additional video treatments or improved tracker algorithms (e.g., Assali et al, 2020) may be required in more complex environments, such as plant cover, where background noise management for trajectory extraction and individual recovery is challenging. In all cases, the spatial distribution of the cover units that could hide individuals should be set to allow the brief but regular appearance of the target species.…”

Mechanisms of group‐hunting in vertebrates

“…during searches and increase individual foraging efficiency for predators; indeed, this has been shown to be a vitalcomponent of the hunt when predators such as porpoises and seabirds search for fish schools (Norris et al, 1961;Hoffman, Heinemann & Wiens, 1981;Götmark, Winkler & Andersson, 1986;Barrett-Lennard, Ford & Heise, 1996;Benoit-Bird, Würsig & McFadden, 2004;Benoit-Bird & Au, 2009;Goodale et al, 2010;Assali, Bez & Tremblay, 2020). Flock leaders in colonies of black-billed gulls (Larus bulleri) call for flock mates to follow them during hunting outings (active information sharing; Evans, 1982) and during Cape gannet (Morus capensis) hunting trips aimed at fish schools, conspecifics act as directional cues and reduce the time to find a patch by half (passive information sharing; Thiebault et al, 2014a).…”

“…density waves that can propagate across many hundreds of metres) as well as normal locomotion through the environment (Swartzman, 1997; Mackinson et al ., 1999; Gerlotto & Paramo, 2003; Makris et al ., 2006, 2009). Because of the complexity involved in spatially and temporally tracking these sorts of groups, the use of social information by predators can reduce individual error during searches and increase individual foraging efficiency for predators; indeed, this has been shown to be a vital component of the hunt when predators such as porpoises and seabirds search for fish schools (Norris et al ., 1961; Hoffman, Heinemann & Wiens, 1981; Götmark, Winkler & Andersson, 1986; Barrett‐Lennard, Ford & Heise, 1996; Benoit‐Bird, Würsig & McFadden, 2004; Benoit‐Bird & Au, 2009; Goodale et al ., 2010; Assali, Bez & Tremblay, 2020). Flock leaders in colonies of black‐billed gulls ( Larus bulleri ) call for flock mates to follow them during hunting outings (active information sharing; Evans, 1982) and during Cape gannet ( Morus capensis ) hunting trips aimed at fish schools, conspecifics act as directional cues and reduce the time to find a patch by half (passive information sharing; Thiebault et al ., 2014 a ).…”

Visual trail following in colonial seabirds: theory, simulation, and remote observations

“…In this paper, “network foraging” will mean that birds match velocities and maintain a preferred spacing with each other while searching for food, i.e., flocking or schooling behavior (Suzuki and Sakai 1973, Okubo 1986, Parrish and Hamner 1997). The behavior I call network foraging has in fact been recently documented in the wild, in the kilometer‐scale “rake” patterns of seabirds observed on radar by Assali et al (2020). “Local enhancement,” in contrast, will refer only to the local phenomenon of birds joining compact flocks that are already feeding.…”