Abstract
The influence of fission-fusion dynamics, i.e., temporal variation in group size and composition, on social complexity has been studied in large-brained mammals that rely on social bonds. Little is known about birds, even though some species like ravens have recently received attention for their socio-cognitive skills and use of social bonds. While raven breeders defend territories year-round, non-breeders roam through large areas and form groups at food sources or night roosts. We here examined the fission-fusion patterns of non-breeding ravens over years, investigating whether birds meet repeatedly either at the same or at different locations. We combined four large datasets: presence-absence observations from two study sites (Austria, Italy) and GPS-tracking of ravens across two study areas (Austria, France). As expected, we found a highly dynamic system in which individuals with long phases of temporary settlement had a high probability of meeting others. Although GPS-tagged ravens spread out over thousands of square kilometres, we found repeated associations between almost half of the possible combinations at different locations. Such a system makes repeated interactions between individuals at different sites possible and likely. High fission-fusion dynamics may thus not hinder but shape the social complexity of ravens and, possibly, other long-term bonded birds.
Highlights
Life in structured social groups poses challenges[1, 2], that affect individuals’ fitness[3, 4] and may drive brain evolution[5, 6]
We investigate the question whether bird flocks can be characterized by fission-fusion dynamics that are comparable with those of some mammals, i.e., given individuals repeatedly meet at different sites
At the Austrian study site (AUT), 185 out of 256 marked ravens were observed at least once co-feeding with the zoo animals during the study period of 4.5 years (1091 observation days)
Summary
Life in structured social groups poses challenges[1, 2], that affect individuals’ fitness[3, 4] and may drive brain evolution[5, 6]. Individuals thereby engage in various affiliative interactions, which may result in the formation of social bonds, which is advantageous in conflicts[26,27,28]: bonded birds tend to support each other in fights[29, 30], they win more conflicts than non-bonded birds and obtain high dominance status[26, 27]; bonding partners engage in forms of post-conflict affiliation[31, 32], indicating sophisticated relationship repair and support mechanisms Captive ravens remember their relationship valence to former group members up to three years after being separated from them[33] and they notice dominance reversals among conspecifics within and outside of their social group[34]. We investigated whether factors like age class, sex or breeding status affect this tendency Since all these factors are linked with dominance status[26], we expected them to influence how consistently individuals use a certain food source and repeatedly meet other ravens
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