disyllabic and trisyllabic great tits

[helinnmae]

when a writer differentiates between a disyllabic and a trisyllabic great tit I assume they are talking about the same bird, only that they sing differently at different times? so sometime a two syllable song and sometimes a three syllable song. Is that correct? or does it refer to two differnt types of great tit?

[David Riddell]

My understanding is that although there are 15 described subspecies of great tits (and some more in a couple of closely related species that have recently been split off), there is little geographic variation in their calls. On the other hand, the species is known to produce at least 40 different calls or songs. So I suspect this writer is referring to one type of bird giving two distinct songs.
https://en.wikipedia.org/wiki/Great_tit#Voice