Here's the updated graph of the ratio of how many black fantails people are finding relative to pied birds throughout New Zealand.
The ratio of black to pied is supposed to increase as you travel further south. That's what we're seeing travelling from Wellington (-41º) down to Canterbury and Westland (-43º) but, oddly, the ratio is then swinging back towards more pied birds in Otago and Southland.
With more observations, we can be more certain about whether or not this is true. With more observations, we can also start looking for more detailed patterns, such as whether there are more black phase birds in more forested areas of New Zealand. Lots of observations made over many years will show us how black phase fantails are faring in the face of global warming.