Nightranger
Senior Moment
I've seen them on sloes in the past too. I guess the obvious conclusion is that Waxwings will turn to pretty much anything once the rowan berries are gone (which I assume they will mostly are by now). I wonder whether cotoneaster comes out top in this tiny sample because they are so popular in towns and gardens where the birds are most likely to be noticed?
Odd (or maybe not) you should mention this because I have always had the distinct impression that waxwings tend to go for a much wider range of berries than our resident birds. I have always put this down to the species having quite a wide geographical range and therefore more regularly coming into contact with the exotic bushes we tend to plant around supermarket car parks. This also seems to be (at least, partially) confirmed when migrant thrushes arrive, which tend to go for hawthorns quite quickly whereas our resident birds tend to leave hawthorns until everything else has gone.
To answer John O' Sullivan, I am not sure how birds determine whether a berry is edible or not but presumably they do so quite quickly and retain some memory too. It is quite possible the ones you have are cultivars of a more familiar species and the birds have learned that they are not very good in some way.