by Mike Dilger
By mid-September those juvenile Peregrines that left their parents’ territory in the summer will also be continuing to move around the region, as they explore other territories, meet other Peregrines and hone their hunting skills. In Scotland a study of Peregrines by the Lothian and Borders Raptor Study Group showed that females tended to disperse much further, averaging around 80km from where they were reared, as opposed to their male siblings moving only around 48km. Certainly across northern Europe, continental Peregrines are much more migratory than their British counterparts, with many birds both moving to avoid the colder, shorter days and to follow their prey which is also keen to depart for warmer climes. As continental Peregrines relocate, the east coast of Britain can often see an influx of these wandering raptors from around this time, which have opted to take advantage of our relatively benign winter climate. Most of the Peregrines nesting high up in the Arctic are thought to overwinter in Spain or Africa, while those breeding birds from southern Scandinavia are more likely to move to Britain – a classic example of leap-frog migration, where birds nesting at the higher latitudes travel the furthest distance to spend the winter. The BTO has so far compiled records of 53 Peregrine Falcons ringed abroad and recovered in the UK, with 32 of these coming from Norway and Sweden. Just one example of this continental influx was of a Peregrine ringed as a nestling in Arvika, southern Sweden in 2008, which was then recovered injured in Somerset the following autumn.
As British-nesting Lapwings continue to relocate either within the UK and Ireland or simply migrating abroad themselves, the middle of the month should also mark the point when the Bewick’s Swan young finally fledge. Leaving the maritime tundra, the families will be keen to relocate to the relative sanctuary of the lakes, estuaries and coastal waters close by. As conditions continue to worsen, the daily average temperature on the tundra may only be 5°C at this time of year, and with each passing 24-hour period receiving seven minutes less daylight, feeding will soon be downright impossible even for birds as hardy as Bewick’s Swans. Immediately identifiable by their grey plumage and grey-pink bills, the youngsters will continue to stand out like sore thumbs from their snow-white parents for some time yet, and any pair managing to fledge at least two youngsters will by any definition have had a very successful breeding season. Due to both the number of predators and precarious nature of the weather on the breeding grounds, any young cygnet able leave the tundra behind will instantly see its chances of survival suddenly take an exponential leap. With their parents continuing to lead by example, the young will have the luxury of personal, on-tap tuition from mum and dad as they set out on the steep learning curve of acquiring first-hand knowledge of their migration route and the good stopover locations on the way to their wintering grounds. Building in number as they coalesce at a variety of key sites around the Pechora Delta, these locations will give the birds a final opportunity to feed before the continually deteriorating weather eventually persuades them to embark for north-west Europe.
Waxwings amassing in northern Sweden will often be one of the first clear indications of an irruption year in the making, but as large flocks continue to strip the finite food resource, their stay there will be little more than a temporary one as wave after wave of birds are pushed further south. Due to the remote nature of these temperate Swedish and Norwegian forests, the number of Waxwings on the move in invasion years is difficult to estimate, with the French ornithologists Fouarge and Vandevondele having estimated an absolute minimum of 10,000 birds to have congregated in Sweden by mid-September before the great British invasion of 2004 and 2005.
Having made mental notes as to the positions of local landmarks and good feeding areas they might need upon their return, many Swallows should by now be leaving in their droves as the declining number of insects suddenly makes Britain a decidedly unwelcome place for insectivorous birds. Heading off across the English Channel, this narrow marine waterbody will represent just the first of many obstacles in an epic journey will take around five or six weeks, and covering in the process close to 10,000km. After bidding the southern English coast farewell, the Swallows will reach the continent in less than an hour, before then pushing along the French coast towards the Bay of Biscay. Stopping off each night as dusk approaches, the Swallows will often be attracted to roosting sites that may already be occupied by birds, and which have been tried and tested by generations of hirundines over the years. Restricting their navigating and foraging to daylight hours, the Swallows will often migrate reasonably close to the ground where their food tends to accumulate in much higher concentrations. As the birds are quite capable of covering anywhere between 100 and 320km a day, it will not take them long to cross France, before then turning east along the northern edge of the Pyrenees en route to the Mediterranean coast. The distance travelled will of course depend hugely on the weather, with the Swallows’ progress often being held up by wet weather or unfavourable winds. Alternatively, they may also choose to have a lazy day at particularly rich feeding sites they encounter along the way, such as The Camargue on France’s Mediterranean coast, where the feeding may be just be too good to pass on quickly through.
With the Swallows streaming towards northern Spain, the Nightingales, by contrast, will be deserting the Iberian Peninsula in their droves as they make the short trip across the Mediterranean Sea heading for West Africa.
Nightingale OAD, retrospectively tracked by the BTO following the recovery of its geolocator, is thought to have left southern Portugal on 19 September 2009, only then to arrive in the small disputed territory of Western Sahara by the following day, a scarcely believable distance covered of around 1,300km. This movement of OAD in mid-September also tallies with ringing data of Nightingales collected from the Rock of Gibraltar, the British overseas territory on Spain’s southern coast, which reports a spike in Nightingales observed at this time of year. The route taken to the Western Sahara can only be surmised, but if the Nightingales were to follow the Moroccan coastline, after crossing the Strait of Gibraltar, this will mean they are able to circumvent both the Atlas Mountains and the Sahara Desert in one deft sweep.
Having conquered the Sahara Desert as long ago as late July, most British-breeding adult Cuckoos will still currently be feeding up in the wooded savannas of Chad and Nigeria as their biological young back in Britain will finally be setting off for Africa. With no lead to follow, the juveniles’ journey must be genetically hardwired, but as satellite technology has still not been miniaturised sufficiently for immature Cuckoos to be tracked, we can only presume that a similar route to that of their parents must be taken.
So little is also known about the dispersal of Puffins from their breeding grounds that their winter distribution must also be considered little more than educated guesswork. However, the limited research carried out suggests that those dispersing furthest are younger birds. Of the four Scottish-ringed Puffins recovered off eastern Canada during the last 50 years, for example, all were in their first year of life. Why young birds seem to have more wanderlust than their less adventurous parents can possibly be explained by the fact that most Puffins do not even attempt to breed until at least five years old. So these far-flung individuals may simply be youngsters keen to see some of the world and enjoy a relatively care-free adolescence until responsibility kicks in a few years down the line.
Late September
Having joined forces to form mixed species flocks with Great Tits, Goldcrests, Wrens and even Treecreepers, back in high summer, juvenile and adult Blue Tits will be keen to maximise the advantage that groups can offer, foraging right the way through the darkest and coldest days ahead. Containing a core number of resident Blue Tits familiar with the area, these roving, mixed species flocks will also include nomadic individuals from further afield, which may come and go as they see fit. Despite their mobile nature, these flocks will nevertheless remain within a comparatively small home range, with almost 90% of ringed Blue Tits recovered being found within 20km of where they were originally cau
ght.
Unlike the roaming Blue Tits, adult Kingfishers will remain rigidly attached to their territory throughout the winter, only possibly accepting any other kingfishers’ presence on their patch during the rare occasions when the weather becomes so severe that it will concentrate feeding to a very few locations. After parting ways with their curmudgeonly parents, the importance to juveniles of acquiring a territory cannot be overstated. Most juveniles are not thought to travel far, with few recorded moving more than 12km from their original nest site. During this dispersal period, young birds can occasionally be seen crossing over land as they get ushered out of territories already jealously guarded, or in the process of branching out suddenly turn up at surprising locations, such as garden ponds.
By late September, Robin territories will be becoming quickly delineated as the birds battle it out, initially with birdsong and then with aggression if newly acquired boundaries are not being respected. One of the many astonishing facts about Robins is that at least half of all the females are thought to regularly sing. This is a feature that is decidedly uncommon in the bird world; the only other British species where females are known to commonly join in the dawn chorus are Starling and Dipper. The fact that female Robins sing was already known before being definitively stated by Charles Darwin in 1871, but this unusual behaviour still didn’t change the great man’s opinion that bird song was primarily ‘for charming the female’. It was not until the 1920s that the naturalist William Henry Hudson argued that the reason why females sing as well as males is that both must hold separate territories in autumn and winter. Even though the singing females tend not to be either as vocal or possess quite the repertoire of their male counterparts, we now realise that Mr Hudson’s assertion was indeed the correct one.
By now, those juvenile Peregrines that began moving away from their parents’ territory in high summer should certainly have become much more proficient at hunting. Such is the immense pressure for young Peregrines to perfect the art of catching prey quickly that raptor expert Ed Drewitt believes that first-year mortality of juveniles could well be similar to that of relatively short-lived birds like Robins and Blackbirds. However, for those young that do manage to make it through the perilous bottleneck of their first year, a longer life expectancy can then be expected, with the current longevity record being that of a male Peregrine ringed as a nestling in Cumbria, which reached at least 17 years and two months.
The most commonly recorded technique for a Peregrine catching and killing prey is with their mightily impressive stoop dive. This method involves the falcon closing its wings to form an aerodynamic teardrop in order to descend at huge speeds as it aims to catch its prey by surprise. Recent research work suggests that Peregrines have learnt to attack in a spiral, which enables them to keep a constant eye on their prey, without frequently turning their head, which could increase drag to such an extent that the inevitable slowing up might result in their quarry potentially escaping. Even though this curved flight path is slightly longer than the straight line, the extra speed generated by significantly reducing the air resistance will more than compensate for the extra distance as they attempt to strike lucky. The Peregrines additionally possess a nictitating membrane, which helps keep their huge, sensitive eyes both dirt-free and moist during the descent. Unlike an eyelid, the translucent nature of the membrane will still enable the bird to detect light while the eye is covered, meaning their eyes can be cleaned without a temporary loss of sight at the crucial moment. The Peregrine’s aim is to strike the target so hard with its talons that the prey will instantly be killed, but should this not be the case, the hapless victim will usually be quickly dispatched with a bite to the neck, severing the spinal cord. This lethal bite is executed with a sharp triangular-shaped ridge on the outer edge of the upper mandible called the tomial tooth.
How fast the Peregrines travel during these spectacular dives is difficult to measure, but with speeds estimated to top 200mph, this should enable them to easily catch up with any unsuspecting prey, while also comfortably holding the record for the world’s fastest bird. Moving at such phenomenal speeds means the birds will also have to cope with incredibly high G-forces when pulling out of any dives or altering their flight path. In fact, researchers have calculated that the birds may experience forces as high as 28G during these manoeuvres, making light of the maximum G-force recorded on a roller coaster of 6.3G, or even the 9 to 12G fighter pilots will experience when executing turns at high speeds. The ways in which Peregrines are able to cope with this immense force without losing consciousness include the maintenance of a very high heart rate during the stoop and also the use of valves in the veins, which prevent the blood draining away from key areas like the brain at the crucial moment.
For young Tawny Owls, mastering the art of hunting doesn’t quite require them to subject their bodies to the extreme forces experienced by Peregrines, but their technique certainly requires no less skill. Unlike Barn and Short-eared Owls, which mostly hunt on the wing, Tawny Owls are the classic ‘perch and wait’ predators. Using a suitable branch as a base, the owls employ a combination of eyes and ears to constantly scan the ground below. Once movement has been picked up, they will then simply drop down on to the prey with the talons doing the damage. Thrust out in front, the talons are fully opened just prior to impact in order to maximise the ‘killing zone’, and on the few occasions when the momentum of the strike doesn’t kill the mouse or vole outright, the powerful follow-up clench of the talons will. The rough underside of their toes also combines with the incredibly strong grip to ensure that any ensnared prey not immediately killed by the strike will have no chance of escaping. Obviously for rural Tawnies which concentrate principally on mammal prey, in those years when mice and vole populations are at a cyclical high then a far larger proportion of young owls will survive. However, irrespective of the abundance of prey each year, a Finnish study of Tawny Owl mortality still showed the odds being stacked against young birds, with just 33% surviving to their first birthday. Those birds which do manage to survive their first year will then see their survival rates leap to 64% for every subsequent year. These enhanced survival statistics can easily be attributed to the greater experience of older birds and the advantage of an exclusively maintained territory.
While the Lapwings continue to arrive in their winter quarters, the Bewick’s Swans in an average year should by now be leaving any pre-migratory sites in and around the Pechora Delta of Arctic Russia as they head for north-west Europe. The swans are believed to take a relatively narrow migration flyway as they follow the north Russian coastline west to the White Sea, before then heading south-west across Karelia, via Lakes Onega and Ladoga, and on to the Baltic coast. Satellite tracking of the swans’ migration by Dutch scientists in 1998 and by the WWT in 2003 has also revealed that the autumn passage occurs much more rapidly than the spring migration, with Bewick’s commonly only taking one proper rest. For example, five birds carrying transmitters in 1998 were recorded flying virtually non-stop to the Baltic countries, a distance of anywhere between 1,800 and 2,300km, with only brief stops along the way, probably to drink. Three other swans tracked during the same migration also stopped for just a few days at the White Sea, making it probably a far less important stopover site in autumn than spring, with two of these birds then also briefly recorded resting up in the Gulf of Finland, just west of St Petersburg and off Estonia’s northern coast. Three other swans tracked by the WWT in 2003 were also recorded to fly directly from the Pechora Delta to the Baltic region, with the journey taking just two days. Possible reasons why this migration may occur more rapidly than their return journey include the birds being held up by icy conditions further north in spring. The swans also need more time to feed intensively in preparation for the rigours of the breeding season ahead. While the number of swans seen in Estonia during the autumn passage is much lower than recorded in spring, one site that seems particularly important at this time of year is Lake Peipus, a huge freshwater lak
e straddling the Estonian and Russian border. Covering over 3,500km, and ranked the fifth largest in Europe, this lake is now considered a remnant of a much larger body of water dating back to at least the last ice age. Averaging a depth of just seven metres ensures the lake quickly warms up but also rapidly cools down, with the result that it can be frozen for over five months of the year. This shallow nature not only produces particularly rich feeding opportunities but also plenty of suitable resting areas for the swans needing somewhere safe to recover from the first and longest leg of their journey. Sightings of ringed birds on the lake suggest that the swans may well spend up to a couple of weeks feeding away on the abundant pondweeds before then finally moving on to either the Netherlands or Britain.
As Waxwing numbers continue to build in northern Europe in an invasion year, resulting in a surge down to southern Sweden, the majority of British-breeding Cuckoos will still be some 5,000km away due south in either Chad or Nigeria. The moult is complex in Cuckoos and still not clearly understood, but while a partial moult of body feathers may occur as early as June and July, most, if not all, of the wing and tail feathers will not be replaced until well after the birds have arrived in Africa. This should mean that at this stage the birds won’t just be feeding to recover condition after their migration across the Sahara but also to power the growth of fresh feathers. The habitat that the Cuckoos have plumped for in Chad and Nigeria may well be very similar to that found in Manda National Park, a reserve close to the border with the Central African Republic, and near to where Chris the Cuckoo has stopped to feed on at least two of his southerly migrations. Consisting of woodland-savanna and grassy floodplain, with patchy stands of dense woodland and fringing vegetation, the Cuckoos here may also be rubbing shoulders with the occasional pack of African Wild Dogs and even wild Ostriches which are also resident in this part of Africa.
