I've been keeping ducks for a couple of years, and therefore I cook with duck eggs, since they're effectively free. They're Indian runner ducks, which aren't very large, and the eggs are pretty much the same size as a large hen's egg, so I just switch them one-for-one, although the duck eggs have a bit more yolk and less white than a hen's egg. And they're better than hen's eggs for most purposes, with the exception of toad in the hole.  I use the Delia Smith recipe with double quantities of batter, and it always comes out big and fluffy with hen's eggs, but flat and stodgy with duck eggs, and takes 50% longer to brown properly. What's going wrong, and is there any way to fix this other than switching to hen's eggs, which I would have to buy specially? Use three whites and one yolk? Use different quantities of the other ingredients? 
I have ducks and have been experimenting with making Yorkshire Puddings using their eggs. I always used to use 4 hen eggs, 140g plain flour, 200ml milk + seasoning, and that always worked well. When I substituted the hen eggs with duck eggs they never raised very well. After a little experimenting I found the following works well... Pre-heat oven to 180deg Put a little olive oil (not extra Virgin) into a Yorkshire pud tray - 12 Yorkshire’s Mix together.. 2 large duck eggs 90g plain flour 110ml whole milk 50ml water Salt & pepper Bake for 10 minutes, then open oven door for 2 seconds to release moist air & close again. Then bake for a further 6 minutes. Done Hope this works for you. I use an electric fan oven for this.  