I Got Tainted

After becoming an Animagus, Gavin noticed some changes in his body.

Changes that carried over to his human form.

He discovered them entirely by accident.

He had just left the Room of Requirement and was walking down a corridor, still thinking about his research, when he rounded a corner and sensed something odd in the air to his left.

It probably wasn't wind—

The corridor had no windows, and the torch flames weren't flickering—

The density and direction of the air itself had shifted ever so slightly, as though someone approaching from that direction had pushed air ahead of them, sending it against his skin seconds before they arrived.

Then Snape stepped out from around the corner, his black robes sweeping in an arc behind him.

Snape glanced at him without a word and walked straight past, soon disappearing down the other side of the corridor.

Gavin said nothing either.

But he immediately began testing it.

The results were more pronounced than he'd expected.

His body could now faintly sense the movement of air. His skin and bones picked up the slightest signals when the airflow changed.

In seagull form, that sensation was instinctive. Apparently, some of it remained even after he turned back into a human.

It made him think of Professor McGonagall.

He had always assumed her silent footsteps were some occupational quirk peculiar to a Transfiguration professor—

Strict, precise, leaving no trace.

Now he understood. It was probably a side effect of becoming an Animagus.

Professor McGonagall's Animagus form was a tabby cat.

Cats were classic digitigrades.

They walked on their toes, using their paw pads, arches, and highly developed leg fascia to absorb the vibrations of each step.

Professor McGonagall had very likely picked up a similar habit.

By the same token, her agility at her