it is definitely a grey heron ... but remember that in the book richard adams is creating the
Rabbits own language ( hence a hrududu is a car for example) therefore the
Rabbits would not have reffered to the bird as a grey heron but as a great grey fisher bird - so if you want to remain true to the spirit of the book then it might be better to translate these words into their romanian equivalents rather than using the species names.
also a lot of the animals encountered are given their own names in the
Rabbits language - lapine- the
Fox for example is a homba which is a made up word and thus probably can be left as is in the translation
by the way i have just found this website
http://www.langmaker.com/featured/lapine.htm which gives a list of the "lapine" words and their english equivalents which you may find useful