Why use one way glass to look at the birds?
Go out and meet them.
We have just retired and moved from London to the country and are surrounded by wildlife. We regularly feed three families of blackbirds, three sparrows, two robins and a yellow hammer, all of which will come within three feet of us without a problem. (apart from the yellow hammer)
When we first fed the blackbirds they obviously had young in the nest as they picked up food and carried it back.
When the young started to fly they accompanied their parents to our garden where the parents fed them until they were old enough to feed themselves.
Time consuming I know but far better than peering through glass.