Photogrpahing birds is not always easy. They are very small subjects, always moving and rather shy. You will start by choosing a camera setup taking this into account: a fast speed (several photographers advise to stick to 1/1000s), probably a high sensitivity to compensate for the lack of light under the trees.
But when you need to come near, you need to use other tehcniques. A long prime tele lens will not always be enough. You must be ready to come to the birds themselves and here comes the understanding and the knowledge of your subject.
Recently, Fabrice Hénon, a colleague of mine invited me to accompany him while he tried his new Canon equipment during a short lunch break in the St Germain forest (West of Paris). He accepted to show me a place where the birds let photographers approach because they are fed often and they are used to seeing cameras at short range.
Here is the result of my Sony shooting a blue tit, great tits, a male chaffinch and a European robin.
Don’t you think that the last bird is weird?