Can anyone give me some idea on what's wrong with my young Silver Birch please?

It was planted (by my house builders) 6 months ago - it's in a bit of an exposed area subjected to occasional strong westerlies - it's about 8-10 feet high and it survived the winter with new leaves opening up this spring. However over the past week I've noticed an increasing number of yellow leaves (almost like autumn - already) - Is this normal, and if not does anyone have any ideas as to whats wrong with it? - I've regularly watered it; it's also securely staked.
Any advice would be appreciated.

Thanks

ROb