Isn't a big part of the problem that for nearly all of duration of life, all animals had their own excretion patterns? If a monkey eats a banana, and 2 days later excretes nutrients in forms it cannot use... then typically the distance between plant and and dropping is not that far. Plants also spread throughout the landscape.

Birds may perform a larger diffusion work part of the time (random droppings while flying), yet a large part of the time it would eat some fruits from one type of tree, then fly to another type of tree, and leave droppings that represent a mixture of the nutrients the different types of trees had access to, but for a large part what came from trees returns to trees even if it's different types of trees. So by producing one type of fruit the tree can trade it's nutrients through birds-as-traders with other trees. Humans ban birds from trees because we want to keep the fruits to ourselves.

Perhaps we are simply wasting too much of our faeces into rivers and the sea? and generally disturbing the natural transport patterns (hidden labour) performed by different types of animals.

Migratory birds might trade nutrients over huge distances, which may sound ridiculous since they can't hold their droppings that long, but their bodies in general also contain nutrients, and the bird could die at either end of the migratory path.