I have a 6-yo sheltie and a 1-yo black lab (aka pain in the rear), and they bark at a multitude of things. Mostly the sheltie (Skye is her name, in case anyone cares, lol). She barks at her own shadow, the shadow of anything or anyone she *might* see, birds--boy, she loves to bark at the birds, and she tries to herd them, it's hilarious! She barks at the lawn mower while chasing it as fast as she can without actually getting close to it, cars, anything unfamiliar to her in the house, like when we were getting ready to have our first ds, we had the carseat, highchair, etc., in boxes in the corner of the family room, she barked at those things for three weeks everytime she went in the room! She's hilarious!
DH's lab doesn't bark much, unless someone rings the doorbell, walks or drives by the window (thank goodness we live on a quiet dead-end street), but their favorite thing to bark at....THE MAILMAN!!! They hate him! Skye chases him, runs at full speed and jumps at the front door if they hear him (we have a mail slot inside our house, the mail drops into a metal box and makes a noise, not to mention the handle on the outside of the slot slams shut). Poor Duke (lab) just stands there and barks, probably wondering what the heck Skye's problem is.
Funny story, Skye was outside one day, a couple years ago when she was an only dog, she was around back. The mailman walked across our neighbor's yard to our house, and she came running and barking like crazy from the back, and he pulled out his mace and was getting ready to spray her (he couldn't see her yet), and when she ran around the corner of the house, he dropped the mace and started laughing so hard, I thought he was going to wet his pants!!! I opened up the front door, and he said, "Is THAT the dog that constantly barks and growls at me from inside your house?" and I told him yes, and he said, "I can't believe something that little makes that much noise! I used to be afraid of her, but I'm not now, she's adorable!" He's always nice to her, and if she's out when he comes (not often, I try to avoid that unless he's early or really late and I don't realize it) he stays and pets her until I realize it (usually only a minute or so from all the barking) and bring her in, because she chases him and he's afraid she's going to get hurt.
I'll stop now!
Steph