There have been some inquiries about the origin on our cranes on social media recently so here is an e-mail from Norman Sills, our ex-Site Manager which sheds some light on the subject:
The only evidence (that our birds are wild birds) that we have is from a DNA sample taken from Lakenheath Fen pair B (Little & Large) by Andy Stanbury in 2011. That showed that either or both of the birds had descendants in western Russia or eastern Finland. The sample was matched to DNA profiling done by guys in Germany.
There is also circumstantial evidence, namely the increase in occurrence of cranes in the UK in spring. So, in 1998, cranes were seen at only three and six non-Broads locations in the UK in April and May respectively (mainly Scotland). From 2004 to 2006 the number of non-Broads locations was between eight and 19 per month (April/May) and then 17/29 locations in April/May 2007. Over the whole spring/summer period of 2007 - 31 March to 23 July - cranes were seen at 64 non-Broads locations in the UK, from northern Shetland to southern Cornwall and from west Wales to Kent. The weather from about 22 March to 6 May that year was persistently from the NE or E with anti-cyclonic conditions over the Baltic and southern North Sea. Thus birds heading north on Europe’s western seaboard had favourable conditions for drifting westwards. Jesper Tofft (‘Mr Crane’ of Denmark) has told me he’s seen cranes heading due west from the Danish coast in spring (ie towards Bewick-on-Tweed/Newcastle-on-Tyne. (These latter details are in articles I wrote in Suffolk Birds 2007 and Suffolk Birds 2009.)
Andy Stanbury wrote The changing status of the Common Crane in the UK (British Birds 104, August 2011) in which he said that evidence was stronger for Broads birds having colonised other sites in the UK, but that was written before we received the DNA results from Germany.
So, it’s quite clear that continental cranes come over to the whole of the UK in spring when conditions are favourable. Also, the European population has expanded greatly (eg in Estonia, 300 prs in 1970 and 5,400 to 6,200 prs in 1997 -2001; in Denmark 4 pairs in 1990 to 60 prs in 2006 ) … and as they are migratory it’s difficult to believe that all of the young produced remained to breed in continental Europe.
There’s plenty more to speculate on but, basically, the jury is out until we can gather more DNA or isotope evidence from UK-nesting cranes.