Mass migrations
Birds such as Rosy Starling, Waxwing and Nutcracker are prone to big movements. Josh Jones looks at the history and possible reasons for these irruptions.
This summer saw an amazing arrival of Rosy Starlings to Britain, with recordbreaking numbers of this smart species observed, while boreal birds such as Waxwing and Nutcracker are also prone to periodic big movements. Josh Jones looks at the history and ecology of such irruptions.
Nature may not be an exact science, but many aspects of watching birds offer a degree of predictability. Resident species can be found in the same spots year round. Summer migrants will appear in spring, even if their arrival dates may vary from year to year. Even vagrants can be anticipated – to a degree – by watching the weather and keeping a close eye on the pressure charts.
One ornithological phenomenon that remains poorly understood – at least to the point that it is usually unforeseen and may at first seem entirely random – is the irruption of a species: when a population suddenly forms, or rapidly increases in number, outside the known range.
The lines are blurred between what constitutes an irruption or an influx. To a degree, they are the same thing, although birders will tend to refer to one or the other depending on the species involved. The drivers behind these mass displacements of birds may also vary, which can lead to slightly differing colloquialisms.
Ornithologists broadly agree that irruptions in birds are generally driven by food shortages. The ‘classic’ irruptive species that many birders will immediately think of are those from high northern latitudes which do not normally migrate far but, in some years, invade southwards in vast numbers. But the phenomenon is far from restricted to boreal regions.
These extraordinary, transient shifts in population aren’t solely driven by food. Extreme or unique climatic conditions can also cause abrupt arrivals of birds, and that is where the term ‘influx’ comes into play among birders. In many cases, though, weather and food availability are inextricably linked and the use of one term or the other is trivial, given the reality is often a complex, interwoven combination of both, in addition to many other contributing factors that are – and most likely always will be – unknown to humans.
What we do know is that avian study is at unprecedented levels. Birds are recorded and monitored around the world with an ever-greater degree of accuracy, and documenting these special events is now a sure thing. There are fascinating comparisons to be drawn with the irruptions of yesteryear: their scale, timing and indeed the species involved. There can be little doubt that the world’s warming climate is driving changes in range and migratory behaviours, while people have also impacted other irruptive species through factors such as habitat loss and hunting, among others.
Rosy outlook
A current example of a species rapidly shifting its range is Rosy Starling. In
June 2021, the third major influx of this species in four years unfolded across western Europe, with recordbreaking numbers logged in Britain by the end of the month – beating the previous record, set the previous year. In France, the 2018 influx was the biggest documented, while the species bred there for the first time in 2020.
Prior to this run, the biggest influx had been in 2002. At this point, such westward pulses of birds in late spring were considered occasional – once a decade or so – and presumed to be driven by a lack of food on their Central Asian breeding grounds, hot weather across Europe or a combination of the two. Whatever the case, it was by no means a near-annual event.
What has driven this sudden upturn in regularity? It’s possible that food supplies are becoming more hit and miss in Central Asia, forcing birds further west more often. Or, alternatively, is it a genuine range expansion, driven by an increasing population? It could be climate driven, with Europe becoming increasingly warmer and more suitable to the starlings. The answer is unclear, but things seem to be changing fast and it might well be that this species starts breeding in Britain in the not-toodistant future.
In stark contrast, the decline of Pallas’s Sandgrouse has been extraordinary. In the 19th century, huge irruptions of this species were a relatively regular occurrence during European summers, with particularly big British arrivals in 1863-64 and especially 1888-89.
Arguably, this must be the most outstanding ornithological event of bygone days. Scanning through the list of records, references such as “Northumberland: Ross, 30-40, midMay to late June; 150-200, early August, all gone by October” or “Norfolk: about 1,100-1,200 at various localities, including 180 at Morston; 186 shot, 13 May-31 October” seem scarcely believable. Furthermore, some pairs even nested in southern English counties.
Just what must Britain have looked like, with flocks of sandgrouse flying around? The simple fact is we’ll almost certainly never know: no photos or footage exist and the only relics of the time are words, statistics and the odd stuffed museum specimen.
So, where did it go wrong for Pallas’s Sandgrouse? An influx of around 90 birds to Britain in 1908 was barely given any coverage after the much larger irruptions in the preceding decades; it was considered the norm. Yet it proved the last sizeable arrival of its kind.
Since then, there have been only seven individuals in four different years, and none after the famous Shetland bird of May 1990. This sudden drop is reflected across Europe.
The logical explanation for the species’ demise is the conversion of steppe habitat in the western part of its range to agriculture throughout the 20th century – but would this alone cause such a rapid drop-off or is something else at play? Pallas’s Sandgrouse is classified as Least Concern and remains numerous across a vast area, but presumably populations are now a fraction of what they were 150 years ago. The mystery remains unsolved.
Boreal breeders
Mention the word ‘irruption’ and most birders’ thoughts will immediately head to boreal forests, where a boomand-bust cycle operates, driven by food availability and causing periodic mass southward movements of a wide range of birds.
Finches form a mainstay of such fluctuations. In years of bumper pine crops, for example, crossbills have successful breeding seasons and populations swell. If the following year sees a failure of the same food source, a southward exodus commences.
British birders will be familiar with how populations of Common Crossbill vary year on year, but it is two rarer species – Two-barred and Parrot Crossbills – for which the aura of intrigue is heightened. Although the latter breeds in the Scottish Highlands, it remains very rare across the rest of Britain and only occasionally arrives from the Continent, usually in October. Two-barred, meanwhile, is very much on the up in terms of British occurrences. With huge arrivals in 2008, 2013 and 2019, the last the largest on record with almost 200 recorded in Shetland alone, the species seems to be occurring ever more frequently.
The impacts of climate change are becoming apparent worldwide, but it is the boreal forests that are already among the most affected ecosystems – and it is here that some of the most violent warming is expected by 2100. Such rapid heating will no doubt have a cataclysmic impact on how these forests operate, with food availability likely to become increasingly erratic. Perhaps the increasing regularity of
Two-barred Crossbill irruptions since the turn of the century is an indicator of an already-changing landscape.
Though crossbill influxes seem to be increasing, big arrivals of redpolls do not. If anything, both Common ( flammea) and Coues’s Arctic ( exilipes) Redpolls are becoming relatively scarcer on
British shores in winter, this reflecting anecdotal evidence across Scandinavia which suggests that Coues’s is occurring with decreased frequency south of its breeding range. The record-breaking influx of winter 1995-96 – a year that produced more than 430 Coues’s and many thousands of Common Redpolls – seems a distant memory, with nothing like it having occurred since.
While redpolls are impacted by the varying availability of birch mast, there is a growing suspicion that rising temperatures and resultant milder winters are playing their part, allowing birds to remain at more northerly latitudes throughout the year. If that is the case, then we can expect ‘northern’ redpolls – and especially Coues’s – to become scarcer as we progress through the 21st century.
If 1995 was the year for redpolls, then 1968 was the time to catch up with a Nutcracker in Britain. An astonishing 315 birds were recorded that autumn, beginning to arrive from early August and peaking between 21 August and 11 September. That year produced the perfect storm for a mass arrival: a failure of the cone crop from its favoured Swiss Pine across Russia had birds moving into Europe as early as June, with an exceptional area of high pressure that delivered sustained calm conditions over northern Europe during late August, in conjunction with unseasonally cold weather throughout western Russia and into Central Europe, providing additional stimuli for birds to flood west.
When a British birder thinks of winter, they think of Waxwings. Arguably our best-known irruptive visitor, this species consumes fruit rather than nuts or seeds, but its appearance nonetheless strongly coincides with food availability. The size of annual arrivals, the bulk of which occur in late October and November, tend to ebb and flow, but some of the biggest invasions on record have occurred since the turn of the century – the last of which was in winter 2016-17.
The world’s other waxwings are prone to irruptive behaviour, too: Cedar Waxwing of North America is highly variable in its number across the US in winter, although there’s no strong link between irruptions and European records. Japanese Waxwing has not been definitively recorded as a wild bird in Europe, but its similar wandering tendencies must surely mean that any future record should be taken seriously. Pine Grosbeak is a much rarer bird out of range in Europe, but irruptions
can and do occur – the huge southward push of tens of thousands of birds in autumn 2019 will live long in the memory of many Continental birders, even if not a single bird reached Britain. These chunky finches seem averse to lengthy sea crossings.
But such oceanic wanderings aren’t necessarily enough to put others off: a Red-breasted Nuthatch, an irruptive North American species, arrived in Norfolk in autumn 1989, coinciding with a huge dispersal across the US. We’ve also had two Evening Grosbeaks from across the Atlantic – surely a Pine Siskin is overdue?
Owls of anticipation
In these northerly latitudes, birds of prey are also susceptible to periodic southward invasions, with several owls particularly prone to fluctuations in availability of prey. In Europe, the species most associated with this phenomenon is Northern Hawk-Owl. Every few winters, swollen numbers of this species flood south through Scandinavia, with one or two usually ending up in Denmark, Germany or The Netherlands.
This regular experience means that Northern Hawk-Owl is widely seen as one of the most overdue birds to reoccur on British shores. Not since September 1983 has this species made landfall, when an individual spent a few days in Shetland. While its appearance here seems far from certain, what can be guaranteed is plenty of hype the next time hawk-owls start moving south. Tengmalm’s Owl is also associated with these irruptive movements, although unlike the previous species it has recently made landfall in Britain.
Perhaps the most spectacular of the irruptive owls is Snowy Owl – and the story behind its periodic appearances in north-west Europe is fascinating. While a limited population does persist in northern Scandinavia, it is considered too small to drive these arrivals and it seems that most – if not all – of the Snowy Owls that appear in Britain, Ireland and surrounding nations originate from North America.
Here, ‘Snowies’ will stream south in some years in search of voles, often pitching up in unusual places. They can pose as collision risks on airfields, whose open, rough habitats provide suitable hunting grounds, while in early 2021 hordes gathered to watch a bird that took up temporary residence in New York’s Central Park. A series of European records at or close to ports, along with documented sightings on transatlantic container ships, suggest this theory has plenty of merit.
What the future holds
Over the past 150 years of welldocumented ornithological study, we have been able to chart the changing fortunes of many irruptive species. Some, such as Pallas’s Sandgrouse, seem consigned to the history books, with their extraordinary invasions unlikely to be repeated. Others, meanwhile, are much more contemporary – the rise of Rosy Starling in western Europe feels like a story with plenty left to run. As humans continue to disrupt the delicate equilibrium of life on Earth, it seems probable that irruptive behaviours will become increasingly prominent in the decades to come, with more extreme movements driven by climate breakdown and resultant food shortages. It may also be that new phenomena develop and come to light – was the astonishing and largely unexplained Siberian Accentor influx of autumn 2016 a one-off, or was it the first episode of many? Only time will tell.
Such events may well provide shortterm exhilaration for birders, but the reality is that in many cases such signs are not good, either for the birds or our wider planet, and this should be at the forefront of our minds as we head further into what is set to be a volatile 21st century. ■
References
• Brown, A. 2007. One hundred years of notable avian events in British Birds. British Birds 100: 214-243.
• Gauthier, S, Bernier, P, Kuuluvainen, T, Shvidenko, A Z, and Schepaschenko, D G. 2015. Boreal forest health and global change. Science 349 (6250): 819 DOI: 10.1126/ science.aaa9092.
• Hollyer, J N. 1970. The invasion of Nutcrackers in autumn 1968. British Birds 63: 353-379. • Riddington, R, Votier, S, and Steele, J. 2000. The influx of redpolls into Western Europe, 1995/96. British Birds 93: 59-67.