Fission-fusion dynamics over large distances in raven non-breeders

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.

Figure 1

Co-occurrence of every combination of two individuals (A + B) at a single food source (Austrian study site). The percentage of days each raven was present is always shown for one individual of each combination on the x-axis (IDs A) and for the other one on the y-axis (IDs B). The z-axis represents the percentage of co-occurrence of these individuals. The different colors show the combinations of classified presence patterns: locals (loc), frequent visitors (freq) and rare visitors (rare). Only ravens that were present at least 10 times are included (N = 126).

Figure 2

Fission-fusion dynamics of GPS-tagged ravens. (a) The three study areas are marked with a diamond for AUT, a square for ITA and a triangle for FRA. All individuals with GPS-tags have been captured and released in AUT or in FRA (small triangles indicate release points for translocated individuals). The minimum convex polygon (100%) of each individual is represented in different colors to roughly cover their space use. The dashed squares indicate the areas of (b,c), both of which show the GPS-locations as points (5,004 for FRA and 1,668 for AUT) of fusion events at different sites. The colors of the points indicate different combinations of ravens. The maps were created using QGIS 2.18.1, an Open Source Geospatial Foundation Project: http://qgis.osgeo.org, QGIS Development Team, 2016.