It is 10 p.m. Darkness smothers Skomer tonight. It falls like something almost tangible. The first shearwaters arrive back on land. Over the light from the moth trap - the only external light on the island - we see shearwaters as white streaks, shearwaters as meteors disappearing through the lit-up square of sky. We hear their calls from inside the hostel, that barely stifled cackle louder this time. This is what we've come for.

This is the main spectacle. We pull coats on, locate our red-light torches and slip out under the blanket of darkness and down the toad path, picking our way around the crawlers who slow to a stop, not relinquishing their ground. My sensory world has changed. Last night vision was split between the torch-lit ground and the bright sky. Tonight there is only the ground. Colours are again stripped back in the red light to just pale and dark. The dark is so much more. The shearwaters are so much louder, braying, as if mocking my slow, cautious steps into their world.

I almost pass the first shearwater. Its dark back on the dark vegetation is not distinct enough in the weak wash of red. I don't spot it until I hear it - its clumsy shuffle, stumble through the bracken, rustle. A walk that relies on resting on its chest, pointing with its bill and bludgeoning its way through barriers. They make their way by force of will. So do I. It takes twice as long as it should to stumble my way through the darkness to the North Haven, picking a route winding past the toads, stumbling on the rocks, my neck bent and eyes fixed on the ground just in front of my feet. The North Haven is a steep-sided cove, a semicircle of rock that acts like an amphitheatre, amplifying the shearwater's calls.

They have another call, one that is unlike the evil cackle. It sounds more like a cockerel gone wrong, as if powered by failing batteries or struck by an asthma attack mid cock-a-doodle-do. The sound is persistent, periodically erupting from different areas of the bracken-clad hillside as the birds return. One is the male's call; the other is the female's. In the darkness these calls are laden with the information that other species get through vision or behaviour. I turn my torch off. The three tankers in the bay are lit up but not enough to distinguish sea from sky and they appear to be lights floating in a seamless darkness. A distant gas terminal glows.

A lighthouse blinks its message from the mainland, a warning of rocks or the welcome of home ground again. The shearwaters flash in front of the lights like apparitions, glimpses of ghosts for my eyes to grasp at in the otherwise absolute dark. The calls, male and female, overlap, forming a single, cackling cacophony, sparking into life across different parts of the cove as the birds return. It is not often that spectacles in nature are defined by the inability to see them. It is not often in birdwatching that you need to watch with your ears instead. Of all the main avian spectacles - auk cliffs, starling roosts, any great aggregation of life - it is not often a supernatural otherworldliness that comes to mind. It is the elusiveness of these night-dwelling birds, their unusualness, that characterises them, right up until the moment they crash land and blunder up the path towards you. Neither of us are suited to this.

Them to land or me to the night. I turn my torch on. The weak puddle of red light picks out more shuffling shearwaters. I walk back to the farm. This is the shearwater's world. Elusive and transient and hard to get to know. And surreal. As I get back to the farmhouse, I see Manx shearwaters on the front and back steps of the visitor centre, sitting like lords of the island and all that their wings encompass.