There's a general rule that if a tree is covered in berries, the birds probably don't eat them so it's best not to eat them. I know holly berries are toxic
That's a terrible rule @LinoleumBlownapart
The trees and bushes that hold their fruit the longest tend to be the least appetising, but that's all part of their evolved dispersal strategy. The whole purpose of fruit is to be eaten by something, and the seeds dispersed.
Holly is eaten by birds, it's why I get so many holly seedlings under the fence where they drop out the birds sitting on the fence. Birds eat elderberries, the local pigeons even eat the young shoots of elder. Yew berries, widely regarded as very poisonous (but in fact it's only the seed, the fruit itself isn't) is eaten with relish by blackbirds in our garden. Even deadly nightshade seeds are dispersed by birds eating the berries.
If a tree is laden with berries it only means that the birds have a plentiful supply of a preferential food somewhere else.
The toxicity of many plants is very different depending whether you're a bird or mammal, and in many cases it's down to how they are eaten. Chewing (as most mammals do) risks breaking the seed (very bad from the plants point of view) and releasing any toxins inside whereas swallowing whole (like birds do) preserves the seed which passes straight through after the fleshy fruit is digested.