Constructing social networks from spatial data
To examine social interactions in different situations we focused on
three social situations: co-flying, diurnal interactions on the ground
(e.g., while co-feeding), and nocturnal interactions on the ground
(namely while co-roosting). All social interactions were inferred from
movement data obtained from the GPS tags, based on movement speed and
spatial proximity. For each social situation, we created an undirected
weighted interaction network (Croft et al. 2008; Wey et
al. 2008). In each network, nodes denote individually tagged vultures
and edges connect vultures that interacted in a particular social
situation, based on the details below. Edge weights represented the
association strength between each pair of vultures measured as simple
ratio index (SRI) which divides the time two vultures were observed
together (as defined below) by the total time they could have interacted
- i.e., times in which they both had a GPS location recorded.
Co-flying : Vultures were considered flying if they were moving
faster than 5 m/s (Spiegel et al. 2013b). Owing to their vision,
vultures can see each other from afar (Pennycuick 1971; Spiegel et
al. 2013a). We therefore deemed individuals flying within 1000m of each
other during a 10-min window to have a co-flight interaction (Fig. 2).
Nocturnal ground interactions : Vultures nest and roost on high
cliffs and tend to aggregate at communal roosts. Individuals were
considered interacting on the ground during the night if they
co-occurred at the same roost overnight. Roosts were spatially defined
as polygons on a map (see example roost polygon in Fig. 2 and (Harelet al. 2017)) shaped according to geographic features, such as
dry streams or cliffs, where vultures are known to roost. The area of
the roost polygons was 783.5 ± 1751.3 m2 (mean ± sd),
which falls well within the perceptual range of vultures. To account for
poor reception within the canyons serving as roosts, and for the
vultures’ diurnal activity, we associated vultures to their nightly
roost polygons using either their last position of the day or their
first position on the following morning; if neither of these two
locations fell within a roost polygon we used the average Euclidean
distance between these two positions to assign a vulture to a roost.
Vultures occasionally roost outside of communal roosts, so if this
average position did not fall within a roost we did not assign those
locations to any roost.
Diurnal ground interactions : Vultures were considered interacting
on the ground during the day if they were not flying (i.e., ground speed
slower than 5 m/s), and their locations were within 50m of each other
during a 10min time window. We excluded any interactions inside known
roosts during the day. Therefore, these diurnal ground interactions
likely represent interactions while feeding and joint sunbathing at
feeding sites.
Aggregate networks : To combine interactions from all social
situations in a single aggregate network, we summed the edge weights
from all three social situations (co-flight, diurnal, and nocturnal
ground interactions) for each pair of individuals.