I tried to use the black hen carcass. No takers after the pups were gone. They've already tasted chicken. I'd use it if it were me. 

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I'm interested in what others will have to say on this.
Usually, when a dog kills, there are teeth marks and feathers everywhere. You won't just have dead birds with no signs...unless the dog isn't hunting but playing. He may have chased and chased and chased until they had heart attacks...which could seriously be a possibility.
And I could believe that from a silly labradoodle (which is a lab mix...I've raised lots of Labs in my day...and they do love a silly chase).
But I agree that doesn't account for the 7 birds unless they ran away and are still hiding or died in the bushes somewhere.
There are other possibilities...a coon or bobcat or something.
So sorry for your loss. That is heart breaking.
LofMc
With the chickens taken here by foxes the first hit was 9 chickens taken from inside the barn during the night; just feathers, no blood, no bodies. A wing showed up a year later under a tree. I saw the pups playing with what looked like a carcass a few days later. The next morning the black hen's carcass was beside the house. Dog killed one fox pup, which was close to the carcass...I trapped the other.
Next hit, next year: 9 chickens taken brought daylight about 7:30 at night within a 3/4 hour timespan. Again no blood, no bodies...just feathers. Caught the foxes watching us from across the road...waiting to take more. Fox pups and subordinate female dispatched...end of issues.
I should note that we had guineas and a dog in the yard when they hit in broad daylight. They are super stealthy.