I would like to provide a place outside for my ducks to relax during the day. I'm going to use either straw or hay any suggestions for which one would be best for them? Is there a difference between the two?
In addition to the great link @peepquack provided, straw is more hollow and allows more air to be trapped to help keep warmth underneath the bird/animal in addition to keeping them dry.
Do be careful as the article mentioned "It is not moist like hay and is unlikely to mold"; it can in fact mold. Therefore you will want to remove and replace it if it gets wet. Also when pulling flakes to spread it around look out for any mold trapped within the layers. I toss any portions that look moldy into the garden or compost pile if I find any when breaking it apart. If you find mold it's generally isolated in layers so you don't have to worry about the whole bale being bad. But since mold is very bad for birds just be watchful and remove any if seen.
Will definitely begin molding if it gets wet so keep it dry...dry...dry. Birds can and will eat it so you need to provide grit to help them digest it. It does not supply sufficient insolation from the ground.
Go with the straw for bedding. It's cheaper anyway.