Animal capture and tagging
The Israeli population of Griffon vultures is considered regionally
critically endangered (Mayrose et al. 2017). Therefore, the
Israeli Nature Protection Authority (INPA) operates a large-scale
management program that includes routine captures of free-ranging
individuals with walk-in traps and release of captive-bred or imported
Griffons. Captured birds (~100 annually, including
frequent re-traps) are banded and marked with patagial tags for field
identification and a subset are equipped with tracking devices. During
September-November 2020 we tagged 47 Griffons with Ornitela OrniTrack
GPS-GSM tags (50g, Fig. 2C), in a leg-loop harness configuration
(Anderson et al. 2020). These tags record the location, speed,
and altitude of individuals every 10 minutes only during the day to
preserve battery power, because vultures are diurnal. The high temporal
resolution and spatial accuracy (errors of a few meters compared to vast
movements across tens of kilometers daily) allow us to determine the
social interactions of vultures in different situations based on spatial
proximity, as detailed below.
We restricted our examination of vulture social interactions to the
breeding season (from December 2020 to June 2021), because that is when
vultures remain local in Israel and the close surroundings (mostly
within southern Israel and adjacent Jordan). We avoided the non-breading
season (summer and fall) because that is when vultures tend to go on
long-range forays, spreading beyond their local home range throughout
the entire Mediterranean (e.g. from Sudan trough Saudi Arabia to Turkey
(Spiegel et al. 2015)). During these forays they may interact
mainly untagged individuals, preventing reliable representation of their
social interactions. In contrast, during the breeding season they remain
locally and we are likely to capture most social interactions from the
movement data recorded by the tags because a very high proportion of the
population is tagged. In our analysis we only used data from individuals
whose tags provided locations for more than a third of the breeding
season (i.e., for >71 days) and who remained within the
local geographic region of the study population in southern Israel
(i.e., within 400 km of where they were tagged) throughout the breeding
season. We used these temporal and geographic restrictions to increase
the likelihood of capturing the majority of social interactions in the
population. After applying these temporal and geographic filtering, we
remained with rich movement data for 29 vultures, which are
approximately 15% of the Israeli vulture population and
~20% of the population in southern Israel.