Trugs, bird houses, dovecotes, etc are all designed to be used outdoors and subject to everything Mother Nature can throw at them. Bare timber left in the weather develops a grey patina; it happens to all timber although some take longer to go grey than others. It is part oxidation (like rust on steel) and part as a result of exposure to UV. There are other elements involved but those two are the main ones.
For many people that ‘grey’ look adds character to the piece; bird houses and trugs are often left to go grey and crack to give them an ‘aged’ look. The great thing about timber is that it can always be restored to how it was when made. Generally a light sand will remove the patina and then it is just a matter of applying protective oil or wax.
More about caring for bird houses here.