Parus atricapillus. Feisty little critters. I've been bitten by them when extracting them from mist nets. For a small bird they can rally bite hard. Fearless as well.
300mm with a D2x is equivalent to 450mm. Usually I shoot a D3 but when I need that little extra mag., I slap the 300mm on my D2x. Still sharp as a tack. Nice work.