The question of how to care for birds that are going to be mounted is asked quite often, so, I have decided to post a thread (as a sticky) that can be used for future reference.
First of all, no matter how great a taxidermist is, the quality of a mount will first and foremost be determined by the quality of the bird and the manner in which it was handled before being taken to the taxidermist. That being said, when selecting a specimen for taxidermy, I advise people to select mature, late season birds with the least damage possible. Obviously, a kid's first bird, a dog's first/last retrieve, etc can't be chosen and you have to make the best of it. Late season, mature birds make the best mounts because they usually do not have pin feathers (which fall out during mounting) and have the best colors.
Now on to the field care...
When you shoot a bird that is a candidate for taxidermy, I generally recommend retrieving the bird yourself if possible instead of allowing a dog to retrieve it. However, I mount numerous birds that are retrieved by dogs each year. Some dogs will basically ruin a bird and some birds you can't tell they've been in a dog's mouth. If the bird is still alive, the best way to dispatch it that I have found is to open the bird's mouth and stick a small pocket knife through the roof of the mouth into the duck's brain. This causes instant death and I feel is much better than drowning or squeezing a bird as some other recommend. Just make sure that you don't get rough and stick the blade through the top of the head. DO NOT wring the bird's neck.
When carrying the bird, always try to carry it by the feet instead of by the neck. Don't use a game strap if possible. Basically, you want to handle the bird just like you would handle the mount.
When you get the bird back home or back to camp, rinse off any blood/dirt/etc with COLD water. Be gentle with the plumage as you rinse it. You will hear all kinds of advise about putting the bird in panty hose and stuff like that, but you really don't need to do that. At this point, all you need to do is to gently place the bird in a plastic garbage bag with the head folded against the body (not stuck straight out. With the bird at the bottom of the bag, roll the bird up in the bag so that the air is forced out the top. If you have a bird with a long tail (oldsquaw, pintails, etc) take care not to bend the tail. Place this inside another garbage bag, get the air out, and seal it up. Freeze the bird until you can get it to your taxidermist. Birds frozen this way should keep at least a year in the freezer if you have taken care to get the air out of the bags before freezing. If you plan to take the bird to your taxidermist the same day or the next day, you can bag them up and place them in the fridge. I would keep a bird in the fridge no longer than 48 hrs.