It turns out I wasn't far wrong, the yolks are larger and richer with a higher fat and nutrient content. To boot, duck eggs also have more protein in the white, which gives cakes a bit more structure and a higher rise. They have the added bonus of a longer shelf life as the shells are much thicker. For a complete nutritional comparison take a look at this website.
I'm very fussy about eggs and will only use eggs from poultry that are truly free ranging and are preferably raised organically (see Ingredients are the Key). I'm lucky enough to have several outlets for local organic eggs where I live and I've seen the birds scratching about more than happily on the farm where most of the ones I buy come from. Bright yellow or orange yolks are what you need to look for - these are usually a sign that poultry have access to fresh grass. For further information about the benefits of eating products from grass-fed animals try reading Jo Robinson's Pasture Perfect.
If you can't get access to good duck eggs, just substitute large hens eggs instead. If you find the concept of duck eggs novel and challenging, how about trying goose eggs? This picture comes courtesy of Walter Jeffries of Sugar Mountain Farm in Vermont. Goose eggs are meant to be the real business when it comes to baking - I haven't tried them yet!