The photocells are used to monitor the reintroduced deer and they supply photos, which are useful to understand their health status.
Links:
[1] http://www.onedeertwoislands.eu/sites/default/files/foto-trappola2015_1.jpg
[2] http://www.onedeertwoislands.eu/en/project/actions/action-d2-periodic-control-reintroduced-deers-health-status