Like Dove says . . . I have seen the birds hopping along the ground and then hop into my greenhouse. They don't seem to fly in and only take flight when scared which sends them in the opposite direction of the door, thus panic. I go around the outside to the end opposite the door and scare them back the other way.
I find the easiest way is as has been mentioned - walking round the outside of the GH and directing the birds towards the open door. There is always the odd one tho. I have a raised bed at the end of my GH and there is a small gap between the timber boards and the glass. When birds (usually inexperienced youngsters ) drop down in the space and are unable to flutter sufficiently to make a direct exit, I do scoop them up by hand and let them go outside. They sometimes sit on my hand for a minute until they get their bearings but no harm done.
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There is always the odd one tho. I have a raised bed at the end of my GH and there is a small gap between the timber boards and the glass. When birds (usually inexperienced youngsters ) drop down in the space and are unable to flutter sufficiently to make a direct exit, I do scoop them up by hand and let them go outside. They sometimes sit on my hand for a minute until they get their bearings but no harm done.