Close portraits of small birds

Wildlife photography has its share of challenges and when it comes to taking close portraits of small birds they will drive you crazy as they always seem to move on just as you get a clear view focused but before you can the shot !    My approach is to work from a reasonable distance without chasing after the subject using a 500mm  F4 lens and a 1.7 converter to achieve a focal length of 850 mm which will work down to a minimum focal distance of 3.5m, This will record the likes of the Blue Tits below full frame,  With a heavy camera and lens I do not consider hand holding for anything more than an ID shot,  I always work from a tripod with a gimble head that allows freedom of movement, critical composition and point of focus while taking the weight of the equipment for the duration of time that any one small bird might present itself at a close enough distance for a successful portrait,  I often find by being static that many birds will keep on moving towards me rather than chasing tails while on the move, sometimes they even end up closer than the minimum focal range but this should not matter for they do as you will already have had the opportunity to take your full frame image before they move out of focal range.