Hi Blackbird, it was a general comment since acorns, ash keys, alder catkins etc. in too large quantities can all be a problem (I think they concentrate chemicals in them, certainly for example when you're trying to use tannin to fix dyes you're advised to use the alder catkins rather than the leaves if possible).
But as far as I know, hawthorn berries are not a problem and I don't think either that sheep or goats would eat very many of them. My goats decimate my hawthorns for leaves but leave the berries mainly alone.
Most field hedges are largely hawthorn and sheep don't seem to have problem.