which berries do you mean?

You've put Japonica underneath your question - this is sometimes used as a name for Chaenomeles (a form of quince) but isn't actually a plant name as such, just a component of many plant names, as in Chaenomeles japonica, Anemone japonica, etc., it just means 'from Japan', so we don't really know which plant you're asking about with the uneaten berries. Photograph? Or do you have another name for the plant?

could be skimmia.....
often berries are eaten in preference order. A bit like us; grapes then pears, banana, orange and lastly apples, in my case anyway.

thats true -- like in my garden the red berries go before the orange ones

Some berries are too sour or thats what you told me last year when I said my Cotoneaster berries never get eaten.

yes, maybe they need longer to ripen so that the're sweeter ( a bit like plums)

Birds often leave berries of plants in the autumn and early winter as there are plenty of other good things to eat. As the winter goes on and food gets short, they will eat things which aren't quite so nice.
I guess that's why they do it, as there are very few berries which are actually poisonous to them.

Plus there is probably a specific bird (or birds) in Japan that eat these berries with gay abandon unlike species here that will be able to detect something about it that they either don't like or are unfamiliar with.