Mammals
Being the start of winter, December is a month in which many mammals venture out less, with some choosing to opt out of the next couple of months altogether. Bats, dormice and hedgehogs are the only British mammals that truly hibernate. If you're wonder what makes hibernation "true", it's all to do with lowering body temperature and reducing biochemical processes (read more here). Hedgehogs build thick-walled nests called hibernacula in which to spend the winter. Hibernation is not, however, the constant state that many imagine and a hedgehog will wake up periodically, although in most cases it'll wake up, move around in the hibernaculum for a while, and then resettle. It is not uncommon for hedgehogs to venture out, though, particularly if it gets very cold, or even for them to move to a new hibernaculum for the rest of the winter.
With hibernation in mind, please check any piles of garden rubbish before having a bonfire, because these spots make ideal nesting venues for hedgehogs; an untold number are killed in bonfires every year. Furthermore, please keep an eye out for hedgehogs visiting your garden this month. Every autumn, late litters of hoglets are born and these youngsters don't have sufficient time to reach 700g, the minimum weight many hedgehog carers consider is required to see them through hibernation. If you find a hedgehog in your garden, particularly if it is small or out during the day, please get in touch with the British Hedgehog Preservation Society (Tel: 01584 890801) to find your local carer. You can also check out my Helping Hedgehogs article for more information and advice.
Hedgehogs perhaps take things to extremes by skipping winter altogether, but many mammals are less active than usual. Badgers, for example, venture out of the sett much less often during the winter than other seasons, although they don't hibernate. Similarly, the rutting action has died down among deer and the males are now rather more sociable as they spend the winter feeding together in bachelor groups. Roe deer will soon start to cast and re-grow their antlers and can often be seen feeding out in the open in large groups during December.
The fact that some mammals are slowing down doesn't mean that all are. Indeed, we're now arriving at the busiest time of year for our foxes -- busiest in the sense of the greatest fluidity in the UK fox population, with animals moving around more than in any other season. Winter is the fox breeding season and this month you'll start seeing foxes moving around in pairs. The dog fox doesn't leave his vixen's side as he waits for her to come into season. Once she has finished her oestrus and he has mated with her he's off, ranging widely to look for other receptive females. During this period, foxes are very vocal as both sexes call to attract mates and the males fight as they trespass on each other's territory trying to get at vixens. The lack of leaves on the trees and bushes at this time of year means there's little to muffle the sound, and calls can travel a considerable distance, particularly on cold, clear nights.
The "teddy bear" or "vampire" deer, more decorously known as Chinese water deer, are also rutting now, and if you live in eastern England you may be fortunate enough to see the courtship of these fascinating little cervids. This species escaped (along with some deliberate releases) from captivity during the 1940s and had established itself within the wilds of Bedfordshire within two decades. Water deer can now be found widely throughout Bedfordshire, Buckinghamshire, Oxfordshire, and East Anglia, particularly the fenland and farmland of Cambridgeshire, Norfolk and Suffolk, Woburn and Aylesbury. At this time of year, the normally chilled-out water deer bucks dig small scrapes in the ground into which they urinate or defecate; these latrines serve as territory markers. Bucks try to attract does to their territories and then follow them while emitting a "squeaking" call. When approaching a doe the buck will hold his head low while twisting it, causing a slapping noise to be generated by the ears.
Most matings will occur in the buck's own territory, and there are reports of males trying to constrain does by encircling them. Studies in China suggest that bucks may move from their normal ranges and establish small territories in areas that females are known to frequent, while data from Whipsnade suggest that females tend to clump together during the rut. A territory doesn't guarantee a mating opportunity for a buck, but not having one makes breeding much less likely. In areas where numbers are low bucks may obtain a territory and mate in their first year, but most won't breed until at least their second.
Interloping males are chased away and occasionally two evenly matched bucks may fight. As with all deer species, aggressive encounters start with two animals walking side by side, each assessing the other's size and general condition (parallel walking). If neither backs down, the two will fight. Water deer lack antlers; instead, they dance around each other, each trying to slash the other with their elongated upper canines, which may grow to 6 cm (almost 2.5 inches). Fights can be serious, and deaths do occur, although water deer fur is comparatively loose and falls out easily, which can make fights seem more violent than they actually are. Cold weather seems necessary to bring Chinese water deer does into season, so mild autumns and winters can reduce rutting activity. This species makes an interesting barking noise when disturbed that, to my ear at least, sounds fox-like (audio clip below). For more detail about this species' breeding biology, please check out the Chinese water deer section of this site.
A pair of Chinese water deer (Hydropotes inermis) calling at one another across a track on a Buckinghamshire farm. Contact barks are most often heard during the summer. - Credit: Marc Baldwin
Birds
Our lakes and estuaries are busy at this time of year with flocks of waders and ducks, many having arrived from the Arctic or northern Europe to overwinter in the milder climes of the UK. Keep an eye out for gadwall, teal, lapwings, snipe, and large flocks of dunlin. Estuaries and reservoirs are also good spots for swan-watching, with large aggregations of mute swans to be found as well as some immigrant yellow-beaked Bewick and whooper swans. Some 190,000 Canada geese and 200,000 greylag geese come to Britain for winter, along with an estimated 7,000 avocets, Brownsea Island and Poole Harbour being among the best spots to watch them.
In the fields, keep an eye out for large mixed flocks of buntings and finches, including reed buntings, yellowhammers and chaffinches as well as the odd hawfinch and brambling. Winter also sees flocks of snow buntings venturing along the coastline as they spread out from their normal haunts of the Cairngorm slopes.
Raptors and owls are also prominent at this time of year. The hen harrier is a winter visitor to the West Country, southern and eastern England, central and western Wales and western Ireland. There are some resident birds, but our population being bolstered by immigrants from the continent during late autumn improves your chances of spotting one. This species can be found inhabiting lowland farmland, heathland, coastal marshes, fenland and river valleys. Here in the New Forest, the birds roost among gorse and heather stands before moving off to feed in nearby farmland. Marsh harriers, short-eared owls and long-eared owls are also more likely to be spotted during the winter.
In good owling years, Britain's resident short-eared population can swell by as many as 50,000 individuals coming in from Europe. "Shorties" aren't the only strigid to overwinter here. Britain's 4,000 pairs of barn owls are joined by as many as 25,000 migrants from Europe during the winter. The lack of foliage on the trees at this time of year makes spotting other owl species, particularly long-eared and tawny owls, a bit easier. Tawny owls are frequently heard through winter and now's the time people start posting on Facebook asking if anyone can identify the owl that's keeping them awake at night. With this in mind, I will put the tawny calls up again this month to help folks out. Click to listen to a clip of a calling tawny male, tawny female, and a female tawny making the 'hoo-hooo' type call as evidence that it's not as simple as you might think to tell the sexes apart based solely on their calls. The reason tawnies are so vocal at this time of year is that they're re-establishing both pair-bonds and territories in preparation for the breeding season.
Kee-wick call of female Tawny owl (Strix aluco). - Credit: Marc Baldwin
Hoo-hoooo call of male Tawny owl (Strix aluco) - Credit: Marc Baldwin
Hoo-hooo call of female Tawny owl (Strix aluco). Note this is more "warbly" in nature than that of the male. - Credit: Marc Baldwin
Our resident populations of garden birds like robins, starlings and blackbirds are also augmented with migrants from continental Europe during the winter, and one of my local nature reserves has been host to almost 4,000 starlings in previous years. Garden birds may become more noticeable this month, particularly if the weather's very cold and/or snow arrives. Indeed, bouts of very cold weather will often see less common garden visitors (such as redwings, fieldfares and waxwings) turning up to raid feeders and windfallen fruit
Among our most conspicuous garden birds this month is the robin, which is far less tolerant of others as temperatures drop. Despite their demure appearance, robins are territorial birds and can be highly aggressive, with an estimated 10% killed each year in fights with other robins. Male birds maintain the same territory throughout the year, while females tend to have separate nesting (summer) and feeding (winter) territories. Territories are marked by singing and posturing to rivals; if these actions fail to dissuade intruders, fighting may ensue, following highly stereotyped behaviours. In the event that singing doesn't dissuade an intruder from entering the territory, and the two birds meet, a fight may ensue.
The resident bird will begin by ruffling its feathers, craning its head and dropping its wings; the bird then strikes at the intruder with single blows from the feet and wings. If the intruder doesn't back down, both birds may roll around kicking and wing-beating each other. They may also jump and flutter up in front of each other, striking out with their legs. Each bird attempts to pin its rival to the ground in what may be a prolonged and violent clash; fights have been recorded to last anywhere from a few seconds to well over an hour. Tracy Ruck filmed two robins fighting in London's Greenwich Park in February 2009 -- the 40 second clip is on her YouTube page and nicely illustrates the ferocity.
David Lack studied robins on his patch in London between 1934 and 1938 and we owe much of what we now know about their territorial behaviour to his meticulous observations. Lack found that there were specific triggers for specific actions: a bird flying away would elicit a chase, a song would trigger a response song, a robin-shaped model would elicit an attack, and a red breast (whether on a robin or not) would cause posturing by the territory holder. These triggers weren't absolute, however, and a territory holder would sometimes sing at a silent robin, strike at a red breast, etc. Robins were only triggered to attack while in their own territory, though, suggesting the trigger to attack was not simply any other robin, but another robin (excluding their mate, which males seemed to recognise individually) within a specific area. Again, this observation was not absolute and Lack did record that, on occasion, a male without a territory would attack and evict a territory holder. Lack noted that, when posturing, the position of the red breast is relative to the position of the intruder so that the largest possible area of orange is displayed. The aim of all this aggression is to secure a territory that will both attract a female in spring and provide sufficient food resources to raise a brood of chicks.
Reptiles and amphibians
If it's cold, you're unlikely to see any but the hardiest of herps out and about this month. Wall lizards tend to fit this bill, being seen on the south coast even in snowy weather if there's a decent patch of sunshine in which to bask. Frogs, toads and newts will spend the winter in torpor. Being cold-blooded animals, reptiles and amphibians are what we call "facultative hibernators" -- when the air temperature drops too low, their muscles simply cannot function because the rate of chemical reactions in their tissues drops too low. Consequently, most spend the winter under logs, among dense vegetation or in leaf litter where they try to avoid the frost and wait for temperatures to rise in the spring.
Some frogs and toads may spend winter hibernating at the bottom of ponds. The low temperature of the water during the winter has two benefits. Firstly, it's pretty stable, meaning that they are less likely to rouse mid-winter than when hibernating on land. Secondly, the combination of the cold water (which holds more oxygen than warm water) and their lower metabolism in cold conditions means that they can stay submerged for the whole winter, surviving on the gasses that diffuse through their skin, without the risk of drowning. In the summer, the water's oxygen content is lower and their metabolism too high to be sustained by diffusion alone. Hence, they drown if prevented from surfacing.
Invertebrates
As with the herps, in cold conditions we're unlikely to see much invertebrate activity. There may still be a few small clouds of midges around, despite the cold, but most bees, butterflies and beetles will either have died by now or be in their winter torpor. Butterflies and ladybirds often choose outbuildings or communal stairwells in which to overwinter -- the temperature stays quite stable and it's free from frost. Hibernating insects such as butterflies are a marvel of biological adaptation, able to survive temperatures well below freezing during torpor. These insects have large quantities of sodium, potassium and chloride ions in their body, which increases the concentration of their body fluids, lowering the freezing point. In some insect species, glycerol is produced which can prevent the insect freezing even if the ambient temperature drops to -50C (-58F).
People often question what happens to spiders during the winter, having seen so many during the autumn. Most simply die. Those that do survive the ravages of winter will either do so in a comfortable nook in our houses or outbuildings (some that live in our centrally-heated homes may continue to be active if there's sufficient food) or in torpor in the leaflitter.
Plants and fungi
A walk in the woods to help settle your Christmas lunch is a great idea, and while looking for the usual mammal and bird fauna, keep an eye out for some of the under-appreciated flora around at this time of year. Woodland mosses and lichens can be beautifully elegant and, particularly when tinged with frost, make good photographic subjects. Lichens are a fused partnership of fungus and algae. The algae photosynthesises to produce sugars (food) which the fungus absorbs, while the fungal cell protects the alga from the environment in a charming symbiosis. Also around at this time of year is the seasonally-appropriate scarlet elf-cup fungus which grows from rotten wood and is widespread although not abundant in Britain. The fruiting body has a diameter of a few centimetres with a pale pinky-orange outside and a bright red inside to the cup. The velvety-soft Jew's ear fungus is also around.
With most trees bare at this time of year, keep an eye out for clumps of the evergreen parasitic plant mistletoe among the branches; it's very obvious and in berry this month. For many of us no house is properly decorated, no office party complete, without a sprig of mistletoe hanging from a doorway or ceiling. In the Middle Ages, though, this poisonous parasitic plant was hung above doors to prevent witches and ghosts from entering. More widely, and largely as a result of Celtic influence, mistletoe is considered a symbol of fertility and a meeting point under which couples kiss at Christmas.
Mistletoe is poisonous to humans, consumption of the fruit leading to gastrointestinal problems, because it deactivates part of our cells' protein-making machinery. Many species of animal have, nonetheless, evolved mechanisms to cope with mistletoe's toxic effect with consumption by mistle thrushes, fieldfares and blackcaps being the main means by which the plant is spread in the UK. The birds eat the berry, passing the seed in their droppings. When the seed hits a branch its gloopy coating, called viscin, fixes it firmly as it dries. Germination is then triggered by milder weather in the spring. About April-time, the seed sends out a special root called a hypocotyl that punctures the tree's bark and allows the seed to tap into its circulatory system. The adult plant's dependency on the host species for water and minerals, while still maintaining some low-level photosynthetic capability through its own leaves, makes mistletoe a hemiparasitic plant.
Mistletoe is dioecious -- in other words, a bush is either male or female, unlike most plants that have male and female parts. Flowering usually occurs between February and April, the inconspicuous flowers producing a sweet-smelling nectar that attracts a variety of insects to aid pollination.