By necessity I trust my bird dog around my flocks. He is charged with keeping hawks, foxes and coyotes out during day when I am not present and all sorts of varmints at night. He has access to brooder even when I am not home. He is also a very active dog but most of his energy is directed at predators and toys. ,
As a pup he killed a couple chicks. It took a little work to get him where he is now but well worth it.
I have always used dogs around poultry and feel they are needed in my free-range setting where fences do not contain poultry or exclude predators, despite fences being present.
With respect to chickens defending themselves, generally no. I have hens and roosters that can repell a submissive adult or pup but any dog greater than 10 lbs that is also determined can kill even the largest and most aggresive chicken. My chickens most reliable protection is flying up into trees or into barn where dog can not follow.. The chickens have to get up becuase they cannot outrun a dog that is even of reasonable health. Chickens are not long distance athletes, they are energy-effecient sprinters that are not to terribly fast either.