When we first started selling eggs it took a while to get people to stop to buy. Now we have a regular set of customers and at peak times such as Easter and the summer we cannot keep up with demand. The worst time is Jan-Mar when the hens are laying for Scotland but people don't realise that - seem to think eggs are a summer thing, but in fact they begin to tail off then.
I think you might have a point about the duck eggs. Advertise the hen eggs as the main thing and just put out a box of duck eggs at a time.
Make a point of chatting to walkers as they pass by and while they might not buy that time, they may well next time.
Do you have an honesty box, with sufficient change?
Do you offer carrier bags (old supermarket ones) as walkers may well not have a way to carry the eggs home?
We put out only 2 or 3 boxes at a time and make a point of telling people how fresh they are. We also encourage new customers to come and see the birds and how free range they are.
I will try to get a pic of our sign - but I don't think many people actually notice it. We take it down in the autumn when we don't get enough eggs to have a surplus to sell, but people still stop, expecting eggs and are surprised when we point out that the sign is down

Another thing is that we sell our eggs in nice clean boxes, hand written with our name and the type of eggs plus price. That way when they get home they have a record of where they bought the lovely brown, blue and tinted eggs and will come back again - hopefully bringing back the boxes as we re-use them if clean.