Pallets would be a lot better than straw or hay, which you'd need to waste a LOT of and will be HORRIBLE to have to remove when the water goes down. Alternatively do you have any sheets of plywood that could be put up on cinderblocks or buckets or whatever, and bedding put on them?
(e.t.a - the point isn't to cover the ENTIRE barn floor with pallets, the point is just to give the chickens a high and dry area to congregate. So ANY amount of pallets is a big improvement over all-wet!)
Try real hard also to get some runoff drainage going, especially if this is snowmelt. Unless your barn is built in a very inappropriate location (i.e. a pondlike dip) you should be able to find some snowbank or 'ice path' somewhere to cut thru with a shovel to let the water drain away better, and/or to intercept *incoming* meltwater so it goes elsewhere. Also make sure you haven't got damaged gutters/downspouts adding to the problem.
In some cases you can find (or make, e.g. if your floor is brick pavers and you can chip a few loose) a low enough spot to run a utility pump. Just going out there every hour or so and firing it off til it slurps dry can sometimes make a suprrisingly big difference in quality of life to the critters in the barn.
Good luck, have fun,
Pat, with a slightly-below-ground-level barn with *two* actual sump pumps in it, located in almost hte wettest part of our property, we did NOT put it htere ourselves needless to say!, and thus able to sympathize fully -- but after 8 years of slow but steady trenching, ditch management and so forth I have *finally* got it figured out to where this is the first year it did not flood at all even in the slightest at all, ha!