Résumé
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 to 4.4 km width, displacing cohesively
over 1.2 to 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.