I've looked on Opencaching.org.uk, Opencaching.com and Terracaching and can't see any existing caches. However, the post does say 'placing'.
The island isn't showing as any restrictions on Magic Map either. However, by posting here, you are making the Groundspeak Reviewers and Opencaching.org.uk approvers aware that they may be a problem at nesting time. (Not sure if the OC.com or Terracaching people regularly read these forums).
Don't know who owns the island, but would hope that anyone wanting to place a cache there would seek permission anyway - and be advised on the best location (and if any restricted access at breeding times)
GAGB member since 2005
GAGB Committee member 2010 to 2016 (Chair 2012 to 2015)
UK Mega Event Chairman 2009 (Weston-super-Mare)