"Appleseeds contain amygdalin, which is broken down by stomach acid to release poisonous cyanide. Seeds contain up to 3% amygdalin by weight, and amygdalin is about 6% cyanide by weight. An appleseed is about 5 milligrams, so it contains about 10 micrograms of cyanide. The lethal dose of cyanide is 50-200 milligrams, so you'd need to eat, rapidly, and completely release the cyanide from 625 apples to be at risk" stolen from yahoo answers so no idea how accurate it is, but sounds about right, hydrogen cyanide is broken down fast so it all has to be eaten practicaly in one go. now scale that down to chicken size and assume they have the same suceptability as a human (probably not true) and you get the seeds from 16 apples for a 4lb chicken. or 164 apples for my dog. not overly likely.
we ourselves also die;
but one thing never dies
the fame of him who has earned it
I've always given my chickens the dropped apples each fall, by the bucketful. About 5 gallons at a time, just rough chopped with a shovel. I've been doing this for more than 30 years and my chickens routinely live to between 10-12 years old, so I wouldn't worry about it!