As a generalisation, birds might dart around but do stop between the darting so a combo of prediction and luck ensures the keeper rate isn't atrocious! Biggest problem I have is the slightest twitch puts them out of frame
Whilst I wouldn't instinctively grab the 105 to take bird shots, there's nothing wrong with it - and the increase in possible aperture can give you an advantage (potentially negated a touch by the slow AF - I tend to use MF with the 105). Probably bored you with this but I trot out this shot every time there's discussion on focal lengths - was set up to take fungi and the Robin appeared
Sigma 105mm @ f/2.8, 1/300