bird
The Italian word 'uccello' is commonly used to refer to a bird of any kind. It is a masculine noun in Italian, therefore it usually goes with 'il' which is a definite article 'the' in English. For example, 'Il uccello canta' means 'The bird sings'.
I see a bird flying in the sky.
This sentence is simply observing a bird in flight. The word 'uccello' is used here to mean a bird, one of its most common meanings.
The bird has a nest high up in the tree.
In this sentence, 'uccello' is again used in its most common meaning of 'bird', and it is in reference to where a bird typically makes its home, which is a nest high up in a tree.
The bird's singing echoes in the woods.
This sentence uses 'uccello' to refer to a bird, especially noting one characteristic of birds which is singing. It could be used when walking through a forest and hearing the bird singing, the bird's singing typically echoes around which adds to the magical charm of being in a forest.