I hope some of you will have some advice for me 
I have 2 labradors who have always been outside dogs, recently one has started barking constantly through the day. It can be triggered by vehicles, visitors etc but more often than not I can't see a trigger for it. His routine hasn't changed, he gets loads of exercise and attention and when he's not with me he's with our other dog - so he's not on his own at any time.
Is there any way to deal with it?

I have 2 labradors who have always been outside dogs, recently one has started barking constantly through the day. It can be triggered by vehicles, visitors etc but more often than not I can't see a trigger for it. His routine hasn't changed, he gets loads of exercise and attention and when he's not with me he's with our other dog - so he's not on his own at any time.
Is there any way to deal with it?