Seeds from fruit trees can be toxic because they contain
amygdalin, which, upon decomposition in the digestive tract, can release HCN -- cyanide. Please note the important sentence in the wiki link:
This leads to an unpredictable and potentially lethal toxicity...
unpredictable because it depends on a bunch of other factors: was it taken on an empty stomach or a stomach full of food? If it was with other food, was it acidic food? What's the variability in the individual seeds? were the seeds whole or cut? were the seeds cooked or raw?
If a chicken ate a whole lot of apple seeds, which were cut open so that they could all be digested, and it had an acid stomach, and it didn't have any limestone or egg shell in its crop or digestive tract, and Jupiter was in the 7th house with Mars rising, you might end up with a poisoned chicken. But I'm with Su Ba, a couple apple pips here and there are no big deal.