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.