I start feeding the birds as autumn heads towards winter. Sesame seeds seem to be enjoyed by just about every kind of bird but I have to put them in a caged feeder round here. Squirrels and parakeets empty an open feeder in a day or two and the squirrels and pigeons hoover up anything on the ground or on a bird table.
 The smaller birds like Tits and Goldfinches have no difficulties getting in and out. As they peck away a few nibbles fall to the floor which attract ground feeding birds like Robins who get in there quick ahead of pigeons and squirrels.

 [Correction- what I meant to say is sunflower seeds though I gather sesame seeds are also good for small birds]