Allotments 4 All

Last (first) year, I had no problems growing onions.  BUT, I went down to the new plot today and found that lots of onions I planted a couple of weeks ago had been pulled up and 'gnawed' :(.  Garlic hasn't been touched though.

I was prepared for birds to pull them out, but this has confused me. 
Whodunnit ?  and what should I do ??

It could be a rat, rabit or vole but rabits and vole only tend to eat off the tops.