I have three Exchequer Leghorns (did have six, but 3 were boys and had to go) - they are 22 weeks old and flying machines, even with one wing clipped. My 6 foot fences have now had to be extended by a further 4 feet after I saw two of them quite happily hop onto the top of the fence - and when your land borders a fast flowing river and then farmland, it's not a good idea!
They can also be quite bullish, squaring up to even the older birds but I wouldn't get rid of them. As Wildwitchy says, they are completely mental but since I hatched them they are very friendly and perch on my arm.