By Huntingdon
Strathclyde,
United Kingdom 
I have a large Yew tree outside my house. I understand that Yew berry fruit is not poisonous but the seed, (and leaves/bark), are.
I notice over the last few seasons, that squirrels and wood pigeons start eating the many dropped seeds from the ground during and after 1st qtr each year. That suggests they have lost their toxicity over the autumn and winter. Any ideas?
20 Apr, 2020
Answers
also just because some things are toxic to us doesn't mean it is toxic to all animals. the converse is also true.
Many plants produce toxins/irritants to help hurry the seed through the gut so it doesn't get damaged in transit, when it leaves the body the seed is already in a dollop of fertilizer.
welcome to GoY too.
20 Apr, 2020
The flesh of yew arils is very sweet making them attractive to animals. I've tried tham and once spoke to someone who said he ate several hundred every year though he spat the seeds out.
As Bamboo said, the seed has a casing that stops it being digested. Over evolutionary time, the yews that didn't have a sufficiently robust seed cover died out as did the creatures whose stomach acid was strong enough to break down the seed.
21 Apr, 2020
Birds eat the berries and excrete the seeds undamaged - squirrels will strip the aril from the outside of the seed and eat that,but they don't eat the seeds as such - they remain toxic.
20 Apr, 2020