by Mike Dilger
Amongst Britain’s summer visitors there seems a distinct pattern, with those species migrating the furthest south generally delaying the majority of their moult until reaching their wintering grounds. Like the Cuckoo, this delayed moult will also occur with the Swallows currently entrenching their positions in and around South Africa’s Western Cape. Despite the Swallows’ moult having started around the beginning of October, or even earlier in a few cases, the entire process of replacing around 1,500 feathers may well take as long as five or six months. This protracted process could mean that the large outer primaries and tail feathers in many Swallows might still be actively growing upon their arrival back in Britain. For the Bewick’s Swans currently bedding in to their wintering grounds, the moult of the adults’ main flight feathers will already have been completed before they migrated south, leaving the exchange of the best part of 25,000 body feathers to occur at a more leisurely rate right the way through to February.
During their marathon flight to Britain the Bewick’s Swans will have lost considerable condition, with newly-arrived males weighing in at little more than 5kg and the females around a kilo lighter. In addition to sorting out their hierarchy within the flock, piling the lost weight back on will also be undertaken as a matter of some urgency. Certainly around the WWT Slimbridge reserve in Gloucestershire the swans are thought to initially prefer feeding on the rich improved pastures around the Severn Estuary. However, for those birds returning to the Ouse Washes in Cambridgeshire, or WWT Martin Mere up in Lancashire, the habitat of choice for putting the pounds back on will be a whole variety of arable fields. In those areas where feeding amongst arable crops is favoured, the numbers of Bewick’s Swans tend to be initially highest on the stubble fields where the birds are presumably mopping up any spilt grain. But as soon as this resource starts to run out they will then move on to polishing off any waste from the remains of root crops left in the fields after harvesting. So nutritious are these varied foodstuffs, that scarcely a month after having returned back the males should have been able to quickly increase their weight back to a fighting fit 7kg, while the females will end up a touch lighter, at around a kilo less.
Tipping the scales at a mere 55g, or just under 1% of the weight of an average Bewick’s Swan, the Waxwing’s appetite could be considered even more prodigious in proportion to its size, which will primarily be due to the relatively poor nutritional value of many berries. Even in invasion years, such as the last one experienced in 2010, there still seem to be surprisingly few birds reported south or west of a line drawn between the Isle of Man and London by the middle of November. This means that birdwatchers in cities such as Brighton and Bristol may just have to show a touch more patience yet before the first of these delightful invaders finally begin turning up on their doorsteps. Following the movement of close to 500 Waxwings that were individually colour-ringed by the Grampian and Orkney Ringing Groups in 2010, it appeared that many of the birds quickly relocated in southern England were individuals that had patently leap-frogged those colour-ringed Waxwings which had already moved down into northern England from their original ringing locations. Presumably, as the best food sources become steadily cleared out in a rolling wave across the country, ‘getting ahead’ is the smart way to ensure access to fresh, untapped supplies.
While the Waxwings’ winter diet is laid out for all to see, trying to elucidate exactly what food Puffins eat when dispersed out to sea is decidedly more difficult. What little information there is on their North Sea diet comes from 68 dead birds found washed up on beaches. In two-thirds of these, remains of gadoid fish (the fish family containing Cod, Herring and Pollack), Sprat, Goby, Three-spined and Fifteen-spined Sticklebacks and Snake Pipefish were all found. In just under half, the jaws of polychaete marine worms were found, and from one dead Puffin a squid beak was recovered. These remains could have been a slightly misleading representation of their true diet, however, as bones of fish, plates of pipefish, spines of sticklebacks and jaws of marine worms will undoubtedly persist in the stomachs of the birds longer than small, soft-bodied prey items. More accurate information on the Puffin’s winter diet may have come from birds which were unfortunately snared by fishing long-lines between Norway and Iceland. On analysing the stomach contents of 11 accidentally hooked birds, researchers found that they had been feeding on Glacier Lanternfish, squid and polychaete worms. While it is striking that marine invertebrates undoubtedly form at least part of many Puffins’ winter diet, in addition to a whole variety of fish species, educated guesswork would suggest that when the birds are foraging below the surface, there may well be more of an opportunistic element to what they are catching rather than the precise targeting of any one species.
Thanks to recent work carried out by Peregrine experts Ed Drewitt and Nick Dixon, the diet of urban Peregrines throughout the year is now known to be far more diverse than just Feral Pigeons. By collecting and identifying the prey remains from Peregrines in the cities of Bristol, Bath and Exeter between 1998 and 2007, they were able to identify an astonishing total of 98 species. Varying in size from the diminutive Goldcrest to the substantial Mallard, this emphasises not only the huge diversity of prey now taken, but also the opportunistic nature of the modern urban Peregrine. In their study Feral Pigeons were still identified as the most important prey species, comprising 42% of all items taken throughout the year, but certainly in November, a massive spike was recorded in the numbers of Common Snipe, Woodcock, Fieldfare and particularly Redwing that were taken. All primarily continental breeding species that visit Britain in large numbers during the winter, these birds are almost certainly being singled out while on passage at night.
Despite Robins eating a diet largely based on insects and arthropods between spring and autumn, vegetable food in the form of fruits, seeds and nuts will inevitably become an increasingly important component as the temperature drops. During early winter, fruits taken from both gardens and the wild will include currants, apples, blackberries, haws, rose hips, elderberries and Yew berries. Having learnt to also take advantage of any food we put out will undoubtedly boost their chance of survival, but in those years with a harsh winter their mortality may still be extremely high. From studies it seems that it is the length of the cold spell rather than the intensity that seems to cause the Robins most problems. Weighing in at around 15 or 16g in the summer, a fat layer put down before the weather turns cold will see the Robins a decidedly chunkier 22 or 23g during the height of winter, which can then be quickly metabolised to help tide them over any short periods with little or no food. However, an analysis of ringing recoveries during harsh winters has shown that most Robins are usually recovered dead in the second week of a cold spell, doubtless birds with their fuel tanks already having been drained dry after the first seven days.
Either unwilling or unable to move out of their immediate territory, as the temperature begins to plummet, Tawny Owls will need to rely on both an intimate knowledge of their local patch and resourcefulness to see them through the winter. Those Tawny Owls holding territories in more rural locations will undoubtedly rely mostly on Wood Mice and Bank Voles throughout the year, while urban pairs may take a larger proportion of birds. However, if push comes to shove, these adaptable and opportunistic birds will also consider taking more unusual prey. Grey Squirrels, for example, have occasionally been recorded at a variety of locations, Edible Dormice have also been documented being taken by pairs holding territories in the Chilterns and even the remains of fish have been found in some Tawnies’ diets. With a far wider prey spectrum than all the other British owls, it seems that, in essence, if it’s edible then the Tawny will eat it!
Unlike our mostly sedentary Blue Tits, which will frequently stay within their home ranges for the duration of the winter, migration amongst continental Blue Tits is a far more commonplace activity, as birds nesting at higher latitudes are forced further south to escape the severe north European winter. In fact the only significant movement of any British-breeding Blue T
its will be from those birds nesting in more northerly or upland sites, which will have little choice other than to accept retreating to more benign locations or adjacent low-lying areas as conditions deteriorate. Being such a popular species for research, with close to four million individuals having been ringed, the huge dataset managed by the BTO indicates that females tend to disperse further afield than males. One possible reason for this differential movement between the sexes may be down to an element of competition, with the larger, more dominant (and frankly unchivalrous) males effectively ousting the females from the best neighborhoods.
Likewise with Kingfishers, any birds which bred at higher altitudes may well have already moved towards the lowlands and coast where new winter territories will then need to be established. Kingfishers have not traditionally been as extensively ringed as many other species, but the scant data available suggest that most northern Kingfishers will still be reluctant to move any more than 15km from their breeding territories. However, with nine Kingfishers initially ringed in Britain and then subsequently found abroad, there does seem some evidence of a migration south and west, with a few individuals even recorded to have taken advantage of the milder winters experienced in countries such as France or Spain.
Unable to cope with any prolonged cold snap, the option of making a quick bolt for mainland Europe’s Atlantic coast as soon as the weather turns unpleasant appears a much more commonly exercised option for the Lapwing than for the Kingfisher. Out of 845 Lapwings ringed in Britain that were subsequently recovered abroad, an astonishing 714 were reported from France, Spain and Portugal. This high total suggests that the warming effect of the Gulf Stream along Europe’s western seaboards should offer frost-free feeding, which will more than compensate for the effort and inherent risk of undertaking the journey in the first place.
Late November
With autumn drawing to a close it seems the timing of the Kingfisher’s breeding season back in the summer will play a large part in dictating when the adults manage to complete their annual moult. Those birds, for example, that concluded their breeding cycles by mid-August should by now be looking particularly smart as they see their moult reaching completion. However, any individuals still in mid-moult due to a late-fledging brood may be left with little option other than to suspend this energetically demanding process to a time when their food supply improves. With the larger, slower-growing primaries usually being retained, these feathers will then be the first to be replaced upon recommencement of the moult cycle after the following year’s breeding season. This suspension will have the knock-on effect of handicapping some Kingfishers due to the moult of one wing being out of synch with the other and even an unbalanced tail too, all factors which could limit their ability to see out the winter. Any surviving juveniles still going strong by this stage will also have completed a partial moult of their entire body and certainly most, if not all, of their tail feathers. Ringers report some youngsters’ moults can even drag on into December in those fledging from particularly late broods, but the all-important flight feathers, which initially emerged just a couple of weeks after hatching, will now not be replaced until after their first breeding season.
Having started their annual moult back in their taiga forest breeding grounds as long ago as early August, those adult Waxwings reaching Britain should also be looking bright and fresh as the last of their new set of feathers reaches full length. Despite undergoing a partial moult, those surviving youngsters from the summer’s broods will not develop their full adult plumage until well into the next year. The retention of their original flight feathers for at least the next eight months will mean these first-year birds will not have been able to develop either the pronounced yellow on the outer webs of their primaries, or as many prominent red waxy tips on their secondaries. These waxy tips are not only the feature that gives Waxwings their name but are also the badge of status for any birds reaching adulthood. Despite the oldest known Waxwing having survived to the grand old age of 13, it is highly likely that the average bird’s life expectancy will be distinctly less. With over 3,100 birds ringed across Britain, only seven Waxwings have been recorded returning here in subsequent winters, and with mass invasions being irregular at best, this would suggest that most individuals may not reach Britain any more than just once in their lifetime. This will make their trip all the more remarkable as they take in a part of northern Europe that many of the flock will have never seen before.
Like the juvenile Waxwings, retention of the young Lapwings’ flight feathers until after their first breeding season will mean that when observed in late autumn an experienced eye should still be able to pick out the immature birds among the mixed-aged flocks currently dotted around the British countryside. As Britain has been subjected to a recent run of comparatively mild winters, the Lapwing seem to have responded with a shift towards wintering in the traditionally colder, but possibly more productive feeding grounds in eastern England. Feeding on the rich arable fields, which form such a dominant part of the agricultural sector in this part of the world, the Lapwings’ mobile nature should enable them to quickly respond if freezing temperatures suddenly make feeding conditions decidedly more difficult. Moving south and west, the Lapwing will then often target those habitats less prone to frosts, such as pasture and grazing marsh.
For young and inexperienced Blue Tits in particular, summer and autumn can be a period of peak mortality as they run the dual risk of predation or starvation. However, for those youngsters surviving this brutal examination, and along the way acquiring both a thorough knowledge of their home range and far better insulated body plumage, late autumn should finally see their chances of reaching next spring begin to improve considerably.
Likewise for any juvenile Bewick’s Swans which successfully fledged from their predator-riddled and climatically unpredictable breeding sites up on the Arctic tundra, the British wintering grounds will represent a place of comparative sanctuary. The young Bewick’s Swans’ survival prospects will also be enhanced considerably by the constant company of their supremely experienced parents, many of which will know the lie of the land at their winter quarters intimately.
Extensive research carried out at Slimbridge Wetland Centre in Gloucestershire has revealed that Bewick’s Swans are surprisingly long-lived birds, which not only need a number of years to pair and breed, but also have a low reproductive output each year. Most of the swans only breed successfully for the first time when between four and six years of age, but this can often occur far later, such as the male swans ‘Victory’ and ‘Money’, which were not recorded with cygnets until ten and eleven years of age, respectively. The Bewick’s Swan is of course strongly monogamous by nature, with experience often being the vital ingredient dictating reproductive success, but even amongst long-standing pairs, those seasons when young are produced can frequently be little more than erratic at the very best. Of 27 pairs followed at Slimbridge which were known to have been an item for at least ten consecutive years, only seven were found to have consistently and successfully bred, with young present, on average, at least one year in two. In fact, the best breeding rates were achieved by just two pairs – ‘Dougie’ with ‘Estralita’ and ‘France’ with ‘Valois’ – which both successfully managed to breed in nine out of 13 winters during the study period. Not only will breeding success vary widely between pairs, but also of course between years, with poor breeding seasons seeing only around 4% of the flock represented by juveniles, as opposed to about 20% of the flock being cygnets when conditions up in the Arctic have been far more favourable and predators less effective.
By contrast to the sociable nature of Bewick’s Swan families, any Robin holding a winter territory will have no sentimental thoughts whatsoever towards any of its other family members as it works hard to maintain an exclusive territory for one. Certainly in David Lack’s pioneering study on Robins in Dartington, Devon, the majority of individuals which established winter territories in his survey area were male, with o
nly a quarter of females reckoned to have stayed throughout the entire year. He believed the majority of his female Robins, along with a smaller proportion of first-year and older males, disappeared for a large part of the winter. They would then arrive back early the following year, to either pair up with a resident male or, in the case of the migratory males, to try and carve out a territory for themselves. Lack was subsequently proved correct in his assumption, as ringing has since revealed not only a substantial number of British-bred Robins opting to relocate elsewhere in Britain, but also a smaller figure preferring to spend their winter on the continent. The three countries accounting for most British-ringed Robins are France, Spain and the Netherlands, all countries influenced by the warming effects of the Gulf Stream along the Atlantic seaboard. Ringing in Spain has additionally shown that some Robins of unknown origin will return to the same Spanish territory in successive winters, raising the intriguing possibility that some Robins may spend their lives switching between a breeding territory in Britain and a winter pied-à-terre on the continent.
