Find something about bird sized around the house and put your lens on the camera, then simply move closer until you can fill the frame with the amount of "bird" that you roughly want and see if the lens will focus on it. If it will you can then check the distance - now you'll know how close you have to be to get the shot you want.
At which point you might have to investigate some alternative setups depending on your situation and the birds. A hide, a remote camera release, a lure. You're already using a lure (food) however you might have to change the nature of it to get the shot you want. Birds at a bird feeder are more likely to land and get their heads down which hides them up. There are loads of little tricks people use to help their situation - placing food into little nooks and holds in logs is one, the bird does still dip their head down but you've got those few moments when they bob it up to get a shot.
Getting close might require a hide and many hours spend inside (leaving it up as long as you can between shoots so that the birds are used too it). Another is a remote release; putting the camera on a tripod - setting the focus to manual focusing and pre-focusing on the area you want and using a lure once again. Then you've got to trip the camera when the bird is in the right place - a wireless remote release can do this though its very much "spray and pray" shooting.
In general for the shot you want you want to move closer to the subject. You might need an extension tube, though the kind of lens you're using tend to be pretty good at close focusing. Extensions tubes DO come into play with this kind of photography; but they are more commonly seen on higher end lenses such as the 400mm and 500mm primes which, whilst they deliver far superior image quality, can have quite long minimum focusing distances (several feet) which makes them very hard to get up close to smaller birds; without use of an extension tube. Extension tubes fit between lens and camera and reduce the minimum AND maximum focusing distances in order to let you focus closer.
If you do find you need extension tubes get the kind that have metal contacts in them so that you retain lens control. The ultra cheap extension tubes in the £/$5 ish price range will lack these and thus you won't have any control over the lens (there's no way for the camera to talk to the lens). Kenko makes a very affordable good quality set of 3 extension tubes with metal contacts.