Animal algebra: Can you answer this tricky question?

I've just spent a week holidaying in The Yorkshire Dales and there's a lot of pheasants, partridge and grouse about at the moment, they are clumsy creatures, running into the road at random.  We didn't kill any, but plenty of people had. 

We got talking about plurals of animals and GROUSE cropped up, but neither of us knew the answer.



  
Can anyone answer this animal algebra question?

1 Mouse + 1 Mouse = 2 MICE

1 Louse + 1 Louse = 2 LICE

1 Grouse + 1 Grouse = ?