The website at http://www.chemistry.co.nz/new_zealand.htm says that the Kiwi Bird is "canny, strong and stroppy, but it's vanishing from New Zealand at a rate of 5.8 percent every year." I had to look "stroppy" up and found multiple definitions, not all of them the same. It sounds like slang but I don't think it is. It is certainly rarely used in the USA.