2003), and may correlate with the ability to perform certain cognitively demanding skills, such as inhibition ( Amici et al.
On the one hand, a high degree of fission–fusion dynamics may increase the socio-cognitive challenges faced by a species ( Aureli et al. 1999), spider monkeys Ateles geoffroyi ( Symington 1990) and chimpanzees Pan troglodytes ( Smuts et al. 1997), bottlenose dolphins Tursiops truncatus ( Connor et al. 2001), hyenas Crocuta crocuta ( Holekamp et al. Such fluidity is resplendent of some mammalian systems, such as those found in elephants Loxodonta africana ( McComb et al. Alternatively, Eurasian Nutcrackers Nucifraga nucifraga form large congregations at fruiting trees, but small groups at roosts in the core areas of their home ranges ( Haffer 1993 Vander Wall and Balda 1977). Flocks of Common Ravens Corvus corax, Western Jackdaws Corvus monedula and Rooks Corvus frugilegus vary from large groups of several hundred birds at the night roost to small groups of a handful of individuals during the day. In contrast, free-living corvid flocks are characterised by a strong fluidity, changing their flock size often within a single day. However, all these results were obtained from captive birds, where group size and interaction patterns are limited by space.
2006) and when pilfering the caches of others ( Bugnyar 2011 Bugnyar and Heinrich 2005, 2006). 2004) and seemingly attribute knowledge about others’ perception and knowledge state when protecting their own food caches ( Emery and Clayton 2001 Dally et al. 2004), employ tactical deception in competition for hidden food ( Bugnyar and Kotrschal 2002 Emery et al. 2007) moreover, they infer social dominance from observing interactions among others ( Paz-y-Mino et al. 2007 Fraser and Bugnyar 2010a, 2012), reconcile conflicts ( Fraser and Bugnyar 2011) and console distressed partners ( Fraser and Bugnyar 2010b Seed et al. As such, corvids may use their bigger brains to tactically manoeuvre the social domain: they form alliances and differentiated relationships ( Emery et al. This hypothesis states that life in complex societies provides special kinds of intellectual problems leading to increased brain size. Indeed, the most supported hypothesis for cognitive evolution in mammals, the social brain hypothesis ( Byrne and Whiten 1988 Dunbar 1998), has also been successfully applied to a bird family, the corvids ( Bond et al. The underlying driving forces behind such abilities are thought to be the same in the different vertebrate taxa. Higher cognitive abilities can be found in many animal species and are especially renowned in some families of birds and mammals. This, along with temporary stability in group composition, provides the opportunity for social relationships to develop, and enables the existence of socialised subgroups within free-flying Raven aggregations. Birds may spend time and/or interact affilliatively with multiple individuals during the day. These findings suggest that Raven aggregations are not as unstructured as previously thought. Affiliative behaviours, such as sitting within reaching distance, allo-preening and social play, were observed particularly in the small subgroups. Aggression was high during crowd foraging but low outside of a feeding context. Birds only flocked for feeding, mobbing and roosting, and spent the rest of the day in loose aggregations, characterised by temporary small subgroups of 2–5 individuals. The composition of marked Ravens in the local population changed slowly but constantly, although often remaining stable for several weeks. We here focus on the daily fission–fusion dynamics, individual spacing, and the influence of spacing on the birds’ agonistic and affiliative behaviour. Over 2 years, we observed a free-flying population of Ravens that visits a game park in the northern Alps.
In captivity, Common Raven Corvus corax, nonbreeder groups show strong social bonds and complex tactical manoeuvring, whereas wild non-breeders are thought to resemble anonymous aggregations. In birds, however, data are primarily derived from captive animals, which strongly contrast with free-flying birds in terms of the number of interaction partners as well as available space. A complex social life serves as one of the main driving forces behind the evolution of higher cognitive abilities in vertebrates.
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