Ibis or Ibises?
Is it Ibis or Ibises? The answer seems to be - it depends! There almost always seems to be two possible plurals for birds, the 'ordinary' plural and the collective plural. The 'ordinary' plural (Ibises) is for a number ie 'three Ibises at Marloes Mere'. The collective plural dispenses with the 's' (Ibis) but presumably should need a collective noun, so: 'a flock of Ibis at Carew'. That would appear to be the technical side of it.
There's a difficulty though (isn't there always?). The difficulty is 'Usage'. Sometimes it sounds completely wrong to use the normal plural even when it is technically correct. Dunlin(s) and Whimbrel(s) are good examples. It feels really strange writing 'seven Dunlins' in fact it almost never happens as far as I can see. Usage has taken over. Is 'Snipe' an example of long term usage or is it an irregular plural? Either way 'Snipes' is just weird.
Where are we now?
It seems normal to write 'three Dunlin' but what about 'three Great Northern Diver'? Yuck! It seems we haven't got that far yet. Maybe if you need the adjectives preceding 'Diver' then it changes things? Well perhaps not that simple, I can't imagine many people writing 'three Robin in my garden' either. It seems the English Language will make its own slightly chaotic path through Bird Plurals but at least we can all be a bit right some of the time.