Was it really just the other morning that I opened my eyes to behold a thick frost on the ground around me beside Lake Kussharo in the Akan National Park of eastern Hokkaido? It already seems an age ago.

At that early hour, a glowing tinge of orange was barely lightening the sky just above the caldera rim to the east. Silhouetted in the pre-dawn glow, a pair of Whooper swans moved gently across the silvery, mirrorlike surface of the lake in front of me. I watched the orange glow deepen and spread, creeping in a broadening swath above the horizon, then I hauled myself from the warmth of my sleeping bag.

There was good reason (or so I thought) to leave the primeval warmth of that womblike cocoon, since just a few hundred meters away was a rotenburo. And is there any better way to greet a frosty dawn than from the piping hot waters of a favorite outdoor hot spring? So I duly stripped and ventured in -- only to get the shock of my life: this pool at the southern end of the lake had often been almost too hot to bear; on this occasion, though, I found myself knee-deep in tepid water and casting around for a warm spot in which to settle down. (Little did I know then that a recent earthquake had left the lake with lowered water levels, and the rotenburo with mostly cool water).

I speedily found a warmer corner and, as I wafted hot water across myself, began to contemplate my disappointment and my next move. I would be unable to recapture the thrill of the dawn light from the warmth of my sleeping bag, even if I returned to it; yet warmth down one side and cold water down the other was hardly my idea of a luxurious, winter's-dawn soak.

But then, as I was plucking up courage to exit the tepid pool for the much colder morning air, a wren chattered noisily nearby. It was still barely light, and as it was so rare to have the place to myself, I thought that if there were birds to be watched, then perhaps I'd stay a little longer.

Dawn was hastening by then, but each time I thought of leaving, another species called or appeared, as if to taunt me. I squirmed deeper into the gravel at the bottom of the pool, seeking out a hot spot, and watched and listened as bird after bird appeared in the trees around.

An hour later, feeling part boiled and part frozen, I exited the pool having recorded an astonishing 21 species, ranging from those Whooper swans, goldeneye and goosander, to white-backed, great spotted and Japanese pygmy woodpeckers. Among the various finches I had noted, a brambling was a new entrant on my bath list, but a flock of hawfinches close overhead was a more inspiring sight. Particularly exciting, too, was when all the birds fell silent as a sparrowhawk zoomed past.

Some people might consider my penchant for birding in the buff to be a bizarre fetish, but really there is no better way to appreciate the delightful sights and sounds of wildlife than from a hot spring, Japanese style, cozzie-less, of course. I've bathed my way the length and breadth of Japan in hot springs from the humble to the grandiose -- but in none do I revel more than in a rotenburo beneath the heavens, where bathing and wildlife come in one and the same package.

I discovered such delights more than 20 years ago during my first winter visit to the Shiretoko Peninsula in northern Hokkaido, and my real bathing story begins there, where the brown dipper I scored from a riverside pool called Kuma-no-yu (literally "hot water of the bears") was soon overshadowed, literally, by a passing Steller's sea eagle. We birders love to list, but instantly I gave up my tedious life, country and year lists in favor of just one: my bath list. I have even studied my favorite birds -- Whooper swans -- from the rotenburo.

The Bathing Bird Club (BBC, no less) is elite, exclusive and open only to the most dedicated. As such, it so far has only one other official member, my Russian pal Sergey Frolov in Petropavlovsk- Kamchatsky, with whom bird-bathing can get no better. A session in a hot pool on one of the Kuril Islands gave us a wood sandpiper, a peregrine falcon, several harlequin ducks and, amazingly, a star of stars, the enigmatic and extremely local whiskered auklet. It was bathing birding bliss!

These outdoor bathing sessions are not only for birders, however, and in more than 20 years' immersion in this hobby I've lost count of the shooting stars and moon moths I've seen. Mammal sightings have been scarcer, and so more memorable, such as the sika deer that trotted past me one wintery dawn (while in that same Wakoto pool in the days when it was pleasantly hot), and the red fox that came to peer at me in another nice hot pool.

Jigokudani, though, undoubtedly tops the list for a mammal-watching soak. There, in the mountains of Nagano Prefecture beside a river and a country lodge, is my very favorite pool. I have taken to its piping waters both by day and late at night when moon-gazing, scudding snow clouds and shooting stars are the most likely entertainment. But my favorite time is early in the morning, when I have soaked and searched for one mammal only: the endemic Japanese serow.

This stocky, goat-antelope is a grizzled, frosty gray and -- through the rising steam from the bath -- is hard to spot on the snow-covered mountain slopes above the river. Morning after morning, with my binoculars at hand, I have scanned the slopes looking for the slightest movement. I emerge from the heat of the pool from time to time to cool off, despite the subzero temperatures and deep snow, and only return to the therapeutic mineral waters when the cold begins to bite rather sharply at my nether regions.

One chill dawn, I had just slipped back into the steaming pool, and was beginning to delightfully warm up again, when I realized that a distant dark shadow had moved. I grabbed for my binoculars . . . and there was a serow! I wanted to shout out very loudly, "At last it's on my bath list!" Then, as the initial thrill waned, I became aware of movement behind me and of the subtle sound of someone else slipping into the bath. No doubt they thought me daft as I stared intently at the snow-covered hillside through my binoculars while soaking, but they said nothing and I carried on.

The long-sought, distant serow seemed settled now and was nibbling at some exposed twigs, so I took the opportunity to make my fellow bather's or bathers' acquaintance, turned -- and with the whispered words "There s a serow, just over there" -- poised on my lips, I found myself face-to-face not with a fellow naturalist, nor even an early-rising traveler, but with a large male Japanese macaque enjoying his morning soak. Was I now on his bath list?