Raggy, what makes ducks' eggs better for sponge cake than chickens' eggs?

Cake is not my thing, so this query originates with Himself, who fancies ducks' eggs boiled for his breakfast and is willing to spend serious coin to get them. Himself also loves cake and would like more of it in his life.

I draw my cake line at fruitcake in the fall, and several forms of gingerbread. I admire people who can produce sponge cake at all, let alone on impulse, and achieve the desired result without fuss and fretting.