I'd like to print modifications for my bird feeder, both to patch over the hail damage from last summer and to try to deter the neighborhood squirrels. I have an FDM printer (and experience with nylon, ABS, and PLA, though don't restrict answers to those if there's something else that's better), what kind of filament would stand up best to daily exposure to sun, rain, snow, etc?
PET(G) is a strong contender. It is very strong and water-resistant, and as such is often used to make pop bottles.
PLA has a reputation for being "biodegradable" and therefore it is often discouraged to use PLA outside and/or in contact with water. However, PLA only biodegrades under very specific conditions which it won't generally be exposed to so it can be used (though, as a harder and less flexible material it is more likely to be damaged by hail).
ABS and Nylon are good choices as well. Basically, any plastic you have on hand will last for years, even in an outside application.
I would think that ABS is a good choice if the application isn't in direct sunlight. ABS is what sprinkler lines are made from and those obviously do well under ground, though I have seen ABS that is left in the sun get super brittle over time and snap easily.
PLA also will slowly melt in direct sunlight. I have seen this one firsthand, having left a print on my windowsill and watching it slowly morph with the weight of objects on top of it.
PETG is good in terms of easier printing with a good strength, but I cannot say that it's particularly good for outdoor use, depending on your use-case. It can be food-safe though!
It would seem to me like Nylon is your best bet, since this is super strong stuff, and is used as gears for things that exist outside, like boats and motorcycle parts, etc. I don't have specific proof that this is the best, but depending on what exactly you're trying to accomplish, it's likely the best choice.
You can always spray paint some kind of coating on the resulting print that will increase the UV protection. Something like Krylon Preserve It Aerosol Spray perhaps?
I have several items printed in PLA that have been outdoors for about a year now. Apart from some discoloration I don't see any structural damage (yet?). Some of them are at the south side of a building so they get maximum sunlight.
Of course Belgian summers can not be compared to Arizona summers!
I think ABS would be your best bet. It's not biodegradable and realtivley easy to print.
But you can use more or less any material if you use some form of coating on it. I would always go for coated PLA instead of other materials just because PLA is the easiest to print and it's nontoxic.
Most plastic resins decompose under UV light from the sun. The chains breakdown, and the plastic becomes a powder.
From examining plastic parts for outdoor use, they typically have a stable pigment built into the plastic. The pigment prevents the UV from penetrating into the object because the pigment stops or reflects the radiation. A good choice for the pigment is titanium oxide. TiO is stable under UV light and heat, so it will not bleach and lost effectiveness. As the plastic degrades in the outermost layer, the TiO remain effective until it loses all support from the plastic and is removed from the object.
TiO is a bright white pigment, so the temperature of the object is lower than it would be if it were compounded with carbon black.
Because TiO is inexpensive, I would expect it to be the pigment in most bright white filaments. Not "natural" white, but true, bright white.
So to answer your question, I would pick the whitest version of whatever filament you would otherwise favor for the outdoors application.