In my fifty-something years of birding I've witnessed an incredible expansion of the ranges of almost all the herons: Little, Great White and Cattle Egrets are now abundant and are becoming regular breeders. Bitterns, Purple Herons and Little Bitterns are moving in the same direction. None of these represent a challenge to any year-lister, whereas twenty years ago all six were very problematical. The same is true of other large wading species like Ibises, Cranes and Spoonbills.
Whatever you believe to be the reason for the northward expansion of their ranges (and climate change must be at least a factor) it's interesting to note that two species are 'bucking the trend': Night Heron and Squacco Heron are just as hard to connect with as ever. Since I more or less retired from long-distance twitching twenty years ago, I haven't encountered either of these charming little herons in the UK. Fingers crossed that I can remedy the situation this Spring!