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I am amending my yard with plants attractive for birds, and I noticed Prunus padus as one of such plants. However, on multiple sites I found info that it is poisonous for humans if the fruit is consumed and have sour taste, to such extent that it can cause death.

On the other hand, it is said that birds love its fruit. I am confused. If it can be poisonous for humans, it could very well be toxic for birds too.

Can you enlighten me regarding this plant toxicity?

https://en.m.wikipedia.org/wiki/Prunus_padus

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Since you're concerned about the potential toxicity of the prunus padus to birds, I'll address that.

The fruit from the tree is not only safe for birds, but is actually a healthy part of their diet. It supplies them with needed vitamins and nutrients. Sources generally agree that, even though they're edible, the seeds are undigestible for birds. Therefore, they expel them, where they become a benefit to various types of mammals.

The fruit which is roughly the size of a pea, although edible, has a very bitter taste and contains a poisonous stone but birds such as the Robin, Redwing, Blackbird and Fieldfare, as the trees name suggests, love them, providing them with a valuable source of vitamin C and other minerals. The seed is not digested by the birds and is therefore distributed far and wide through their droppings. Apart from being an important food source for birds, many small mammals benefit from any of the fallen fruit. Source

Also,

Like wild cherry, the spring flowers provide an early source of nectar and pollen for bees, while the cherries are eaten by birds including the blackbird and song thrush, as well as mammals such as the badger, wood mouse, yellow necked mouse and dormouse. Source.

Birds are also an important part of helping the tree thrive in your garden.

The insects such as beetles and bees that pollinate bird cherry benefit from the nutrients in the flowers' nectar, while at the same time providing the essential service of pollination to the tree. Despite their bitter taste, the fruits of bird cherry are eaten by birds, especially robins (Erithacus rubecula), starlings (Sturnus vulgaris) and members of the thrush family (Turdus spp.), and are relished as some of the earliest fruit to be available in the summer. While the birds benefit from this food, the tree also gains, by having its seeds dispersed in the birds' droppings. Source

You didn't mention whether or not you have livestock, either in your garden or nearby. If so, please don't plant these trees. They're highly toxic to farm animals, especially goats.

The foliage is eaten by caterpillars of many species of moth, including the orchard ermine, brimstone and short cloaked moth, however it is toxic to livestock, particularly goats. Source

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An interesting question which is perhaps better on biology.se than here.

But here's my guess. Prunus padus has actually been used in herbal medicine for centuries, and although it does contain cyanide, it is in quite small amounts. The more cyanide it contains the fruit becomes more bitter making it unpalatable.

The seed and leaves contain hydrogen cyanide, a poison that gives almonds their characteristic flavour. This toxin is readily detected by its bitter taste. Usually present in too small a quantity to do any harm, any very bitter seed or fruit should not be eaten. In small quantities, hydrogen cyanide has been shown to stimulate respiration and improve digestion, it is also claimed to be of benefit in the treatment of cancer. In excess, however, it can cause respiratory failure and even death.

PFAFsource

Now the toxin is only harmful to some mammals

The toxic principles of the Prunus family are the cyanogenic glycosides prunasin, prulaurasin and amygdalin (2, 3). These cyanogenic glycosides are typically stored in the vacuole of the plant cell where they do not harm the normal metabolic processes of the plant. However, if the vacuole membrane is broken due to damage or stresses (e.g., chewing, crushing, wilting, freezing), certain enzymes in the plant cell and micro-organisms in the ruminal stomach hydrolyze the glycosides into hydrogen cyanide (HCN) (2,4). Horses and pigs are less susceptible to poisoning since they have acidic stomachs, which inactivate these enzymes, preventing hydrolysis of the glycosides (4).

And it may be that birds with a stomach pH down to 0.2 also inactivates the β‐glucosidases in the fruit which thus prevents the release of the hydrogen cyanide, and they also are unable to digest the seed which contains the main concentration of cyanide. However, interestingly the seeds need to pass through the digestive tracts of birds in order to break dormancy.

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This plant (Prunus padus) is definitely toxic to humans, well, the leaves and berries, because they contain hydrogen cyanide, a substance which is toxic to us; birds, however, are unaffected by the toxin simply because their physiology is different. In order for a substance to be toxic to a particular creature, it has to be capable of interacting with a substance in the creature; if that interaction is not there, then the berries are not toxic.

More information here http://www.thepoisongarden.co.uk/faqs.htm

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