Birdwatcher Hannes Margusson wrote very aptly last year already:

“A little while ago I saw the willow tit at the bird feeder. At first there was a sound from the computer, "di-dää-dää, di-dää-dää". When I opened the webcam window the guy was soon there. Black underchin patch with blurred edges, pale panels on wings, all nicely shown.“

The willow tit and the marsh tit look similar to the point of confusion, but the call that Hannes described is a sure sign of the presence of willow tits.

 

The winter population of willow tits is estimated at five times that of the marsh tit, or half a million, with variations - years are not twins.