As for the question as to why Crugith has not been so faithful– we think she must’ve picked up some sort of injury, disease or parasite that has prevented her from reaching the desired level of fitness. There are no breeding birds in Cornwall so it is impossible that she is breeding there – she’s just sitting out the summer because she could not attain the condition required to return to Siberia. She is still alive as her movement appears to be normal, but there must be some underlying health issue.
It's worth noting that of the birds featured in the maps below, Remy, Amy, Rebecca, Lanyon, Wensum are all females. St. Brendan and Olwen are both of an unknown sex so could also be females. So the site fidelity we are seeing is definitely true of females but we don’t have any data for males – at least not in 2014. Last year, however, we had Monkey - a male who travelled to exactly the same site in 2013 as he did in 2012.