It was only a small drama, but it encapsulated many of the downsides – and occasional upsides – which the hard winter weather presents to our wildlife.
A fox was creeping along the shore of the frozen lake, a quarter of a mile away. I was watching it at the London Wetland Centre in Barnes, south-west London – the urban nature reserve run by the Wildfowl and Wetlands Trust – and I imagined it was starving, for as I gazed on it earlier this week, with a panoramic view from the reserve's three-storey Peacock Tower hide, I could see what it was stalking: a bittern.
It had in its sights one of Britain's rarest breeding birds – and also one of the fattest. The brown striped heron-relative (a few of which winter in Barnes every year) would make a decent meal for any carnivore, and because the lake was frozen, the bittern was standing on the ice far more visible than it would be normally, hidden among the reeds.
So the fox had the bittern in its sights, for the bittern was visible. But here's the rub: so was the fox. On the snowy lake edge, it stood out a mile (or at least, the quarter of a mile to my vantage point) and it had not got to within 20 yards of its prey when, with a contemptuous flap of the wings, the bittern took off (it was then mobbed by a mad flock of carrion crows, but that's another story).
The point is, a big freeze is generally pretty bad for wildlife – for almost all creatures, food is harder to get and shelter is at a premium. But there can be advantages for some, and one of them is that predators can be much more visible in ice and snow. It's not just foxes. At Barnes this week, there was also the stunning spectacle of a peregrine falcon sitting at the frozen lake's edge, visible to every other bird.
Walking around the snow-covered, 100-acre reserve with Jamie Wyver from the management team and Richard Bullock, the resident ecologist, it became clear, looking at the frozen marsh and icebound water channels and pools, that different living things have different strategies to survive a nasty blast of winter.
With the exception of the occasional, unwelcome fox, most of the reserve's mammals – water voles, hedgehogs and bats – were in hibernation, or at least, snug in deep burrows. It was the birds which seemed to be bearing the full brunt of the freeze, since, with the sole exception of a species of nightjar found asleep in a crevice in California 60 years ago, none of the 10,000 species of birds in the world is known to hibernate.
Faced with Siberian weather, birds have only two strategies: stick it out, or flee. Many choose the latter, and from all over Europe, head south and west; indeed, the four or five bitterns present at Barnes are thought to have come from the Netherlands. Barnes's own great crested grebes have gone, perhaps to the coast.
Smaller birds suffer most in a freeze, for they lose heat and energy more quickly. The Wetland Centre has three feeding stations which are replenished, but staff rely more on their planting policy: they have planted many seed-bearing and berry-bearing plants and shrubs and trees with birds in mind. The berries have all gone now, but we watched as a mixed flock of siskins and goldfinches flew into an alder tree and began feeding.
In the end, though, casualties are unavoidable: in the last really terrible winter, of 1963, Britain lost most of its kingfishers (though they quickly bounced back). The RSPB has already issued warnings about the number of barn owls found dead, unable to hunt mice and voles under the snow.
The staff at Barnes are more concerned with smaller things, such as their overwintering warblers: chiffchaffs and Cetti's warblers. They face a hard time, for in spite of a few advantages such as spotting predators, a really severe freeze brings mainly misery.
Cold comfort for some creatures
Winter wildlife: Advantages
Predators are more visible as camouflage works less well in snowy conditions. Icy weather kills plant pests such as slugs and snails, and hopefully, the larvae of the horse chestnut leaf-miner moth, which has been attacking conker trees over much of Britain since 2002, causing them to drop their leaves early.
Mice, voles and shrews can stay alive and well under the snow for weeks.
Fish under ice are safe from land predators (such as herons).
Winter wildlife: Downsides
Frozen water means that fish-eating birds such as kingfishers cannot find food and will starve.
Frozen ground means that many birds will be unable to probe for invertebrates such as earthworms, which are their main sources of food.
In really severe cold, small birds are likely to freeze to death, especially at night. Some species such as wrens will gather together, occasionally dozens at a time, in a single nestbox, to keep warm overnight. Josephine Forster
Cold facts: The big chill by numbers
£10.5m was spent on emergency road salt in 2009-10 by local councils.
1,489,730 tonnes of road salt was ordered by councils this year, down from over 1.5m tonnes last year.
2,000 flights have been cancelled by British Airways over the past week.
£40m amount of total loss to BA from disruption caused by the weather.
282 extra deaths per day were recorded in England and Wales between 3 and 10 December.
-19.6C The temperature recorded in Chesham, Buckinghamshire, on 19 December – the lowest in Britain.
£13bn The estimated total cost to the economy of the freezing weather that began 27 days ago.
21,000 calls to AA's breakdown service were made on 22 December – an average of 900 calls an hour.
7,000 pipes burst in homes in Yorkshire, twice as many as normal.
4m letters and packages have remained undelivered.
600,000 passengers were stranded at Heathrow between 17 and 21 December as snow closed the airport.
Compiled by Josephine Forster and Joseph King