You walk along thea path in the sunlit forest.

Suddenly, amid fallen mossy twigs, you notice an inconspicuous brownish bird the size of a sparrow, with a reddish breast and ruffled feathers.

It resembles a robin, but has a more crimson breast.

Although such tiny creatures naturally fear humans, the bird does not hasten to fly away from the ground.

And it has a good reason for this: it is pecking at a fleshy, wriggling fleshy worm.

This order sounds slightly more natural.

But then something unforeseen happens: it pecks the worm again and hauls it in through its beak, gradually devouring the gyrating worm.

After all is done, the worm is not done and dusted: with a violent body movement, it managets one its end protruding half an inch from the beak and once more wriggling.to wriggle half an inch out of the bird's beak.

What you had written makes sense, but this flows a bit better.

This time, however, it does not stand a chance: the ravenous “robin” in a matter of seconds draws inside, the ravenous “robin” draws the remnant of the worm inside.

Unfortunately, the bird’s plumage makes it impossible to find out when the worm’s death agony ends inside the bird’s gizzard.

At last, you, and not the bird, are the one who first leaves the crime scene.

You walk along thea path in the sunlit forest.

I would say “a path” instead of “the path” since the path has not been specified previously.

Although such tiny creatures naturally fear humans, the bird does not hasten to fly away from the ground.

I don't think the verb “hasten” applies here. I think “The bird does not (ready itself/prepare) to fly away” is more what you are trying to say. “Hasten” would apply if the verb has already happened or is happening, but since the bird does not fly away I don't think you would use it.

But then something unforeseen happens: it pecks the worm again and hauls it in througwith its beak, gradually devouring the gyrating worm.

“It picks the worm again and (grabs it with/snatches it with/pulls it with) its beak” would sound better to me. Typically “haul” is used when the object has a lot of weight. Ex: I hauled my furniture to the moving van.

After all is done, the worm is not done and dusted: with a violent body movement it gets one of its ends protruding half an inch from the beak and once more wriggling.

“After all is done, the worm is not finished: with a violent body movement one of its ends protrudes half an inch from the beak and is wriggling once more.”

Original sentence is grammatically correct, but the sentence above sounds a little bit more natural to me.

This time, however, it does not stand a chance: the ravenous “robin” in a matter of seconds, the ravenous “robin” draws inside the remnant of the worm.

I am not sure what you mean by “draws inside.”

Unfortunately, the bird’s plumage makes it impossible to find out when the worm’s death dies and its agony ends inside the bird’s gizzard.

Death and agony are both nouns so they cannot be put together in that way. You could say “agonizing death.”

At last, you and, not the bird, are the one who first leaves the crime scene.