An odd pair of tits

That title should get my hits up a bit! But seriously though has anyone else noticed pairs of mixed tit species going around together? We have a perfectly bog standard pair of Blue Tits using an old dilapidated nest box that has seen better days on the end of the garage block. They have been collecting nesting material for some time now. We also have a splendid pair of Great Tits that come regularly to the feeders and confound us with their multitude of different songs and calls.

Blue Tit gathering nesting material

However we also have a pairing of a Great Tit and a Blue Tit. these two birds are inseparable, where one goes the other is sure to follow. They will forage together through the Ribes bush, fly off together into the surrounding trees and appear again, always together. We've not seen any kind of pairing or mating behaviour to be honest, they just seem to be good mates, very odd.

The odd couple

This reminds me of the unusual pair of tits at Hanson Educational Study Centre in Newport Pagnell. We have often been there to look for the Willow Tit, which again almost invariably appears with a Marsh Tit. Last year we wondered if they had formed a pair and mused as to what we would call the offspring, would they be Wish Tits or Mallow Tits?
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Marsh, Willow, Wish or Mallow Tit?

Here's the other one!