# Bewick's Swan tracking and what satellite tags have changed GPS tags revealed the staging network between Arctic Russia and northwest Europe, and why the wintering range is shifting east. Source: https://glassandfeather.com/posts/bewicks-swan-tracking-and-what-satellite-tags-have-changed/ Published: 2026-10-04 --- The Bewick's Swan population trend remains one of the most troubling stories in European ornithology. ## The birds with names Study of Bewick's Swans at their wintering grounds predates satellite telemetry by decades. What tracking has added is the middle of the journey — a stretch that was effectively invisible before transmitters small enough for a swan became available. The named birds that appear in WWT updates — Bean, Crispin, Beech, and others fitted with GPS-GSM tags in recent winters — are not celebrities for the sake of public engagement, though they serve that purpose well enough. Each one is a data point that accumulates value over years. Multiple seasons of data begin to reveal how an individual responds to variation: a cold snap in the Baltic, an early freeze on a staging lake, a failed breeding attempt that changes the timing of departure. Taken together, and combined with the longer dataset from neck-collar resightings, the tracks from tagged birds have built a picture of the migration system that no amount of observation at either end could have produced. ## The route itself Bewick's Swans breed in the Russian Arctic and migrate southwest to winter in northwest Europe. The route from the breeding grounds to the wintering areas passes through a network of staging sites. Some birds move through the Gulf of Finland and into Estonian coastal wetlands. A proportion cross the Baltic to stage in Denmark or northern Germany before continuing west. The migration is not a single flyway with neat boundaries. Individual birds use different corridors, and the same bird may shift its route between years. The data from tagged individuals, overlaid on decades of neck-collar resightings, shows a network of staging sites rather than a single pipeline, with some nodes used heavily and others used opportunistically. The timing is reasonably predictable. Departure from the breeding grounds occurs from late September into early October, with numbers at wintering sites building through November and into December. In years with early cold snaps across the continent, arrivals can be earlier; in mild autumns, some birds linger on the continent longer. ## A wintering range that has shifted One of the clearest findings from the long-term dataset — tracking and collar data combined — is that the wintering range has contracted eastward. In the 1970s and 1980s, Britain held a substantial share of the northwest European wintering population. Over the past two decades, the centre of gravity has moved. Sites in Zeeland, Friesland, and the IJsselmeer area draw birds that formerly continued to Britain. British wintering numbers have dropped markedly. The Wetland Bird Survey (WeBS) counts tell this story in aggregate. The shift is not simply a matter of new cohorts choosing different sites. Some of the same individuals have moved. The reasons are debated but likely involve a combination of factors. Bewick's Swans are not exclusively wetland feeders in winter; they use agricultural fields extensively, and the food landscape matters. The practical consequence for British birders is fewer Bewick's Swans at traditional sites and less certainty about numbers in any given winter. The birds haven't vanished from Britain, but the flocks are smaller than they once were. ## The population problem The eastward shift in wintering distribution would be interesting but manageable if the overall population were stable, but it is not. The northwest European population of Bewick's Swan has declined significantly since the mid-1990s. The tracking data contributes to understanding why, though it doesn't provide a single clean answer. Several threads emerge from the combined dataset of satellite tracks, collar resightings, breeding-ground surveys, and demographic modelling. Counts of juveniles in wintering flocks — a standard demographic indicator for swans — have shown poor breeding success in many recent years. Disturbance, habitat degradation, or reduced food availability at key staging areas could have downstream effects on productivity that wouldn't be visible from counts at either end of the flyway. Mortality outside the breeding season is a concern. Lead poisoning from spent ammunition and fishing weights has been documented in Bewick's Swans, and power-line collisions are a known cause of mortality along the migration route. Illegal killing along the migration route was documented as recently as the 2010s. Satellite-tracked birds were found to have been shot in several countries along the flyway, which brought a level of specificity to a problem that had been suspected but hard to quantify from collar returns alone. None of these factors operates in isolation, and the tracking programme's value lies partly in connecting them. A bird that is poisoned by lead at a staging site doesn't breed. A bird that is disturbed repeatedly at a key stopover arrives late. A bird that encounters a power line on a foggy November night over the Netherlands is simply gone. Tracking has been essential in showing that the annual cycle extends well beyond the two endpoints where humans traditionally counted swans. ## Where to see them in Britain For birders planning to catch up with Bewick's Swans in winter, the key sites remain the ones that have held birds for decades, though expectations should be calibrated to the reduced numbers now reaching Britain. Slimbridge has been the spiritual home of Bewick's Swan study in Britain for decades. The tradition of individual identification by bill pattern continues. Numbers have fallen from the hundreds that were routine in the 1980s and 1990s, but Slimbridge remains the most reliable site for close, prolonged views of Bewick's Swans in Britain. The Ouse Washes have long been a major site for wintering swans. Both Bewick's and Whooper Swans use WWT Welney, and sorting through the flock is a useful exercise in separating the two species at rest and in flight. Welney's numbers are also reduced from their peak, but the site remains important, with the first birds usually noted in late October to mid-November. The RSPB reserves along the Ouse Washes more broadly also hold wintering Bewick's Swans, though flooding patterns affect accessibility and distribution. In wet winters, the birds may be spread across a wide area of the washes; in drier conditions, they concentrate on the remaining wet fields and pools. **Martin Mere, Lancashire.** WWT Martin Mere holds wintering Bewick's Swans. Worth checking, but not a certainty for Bewick's. **Nene Washes, Cambridgeshire.** Another Fenland site that has held Bewick's Swans in winter. Small numbers of Bewick's Swans occasionally use wetland sites in the Somerset Levels and around the upper Severn Estuary, though these are not reliable in the way that the WWT reserves are. The second half of October is the time to start checking, and the WeBS alerts and WWT arrival announcements are worth following. ## Neck collars and citizen science The wider picture of Bewick's Swan migration depends heavily on neck-collar resightings, and this is where ordinary birders contribute directly. Bewick's Swans have been marked with large, readable neck collars since the 1990s. These collars are designed to be read at distance through a scope, and the resighting data feeds into the same international database as the satellite tracks. The collar-resighting network depends on observers across the flyway reporting what they see. In Britain, this means scanning through any flock of Bewick's Swans with a scope and noting collar codes. Sightings should be reported to the WWT or through the colour-ring reporting website (cr-birding.org), which routes the data to the researchers managing the marking programme. Each resighting adds a data point. An individual with a collar fitted in the Netherlands and resighted in Britain confirms the cross-Channel link for that bird. This is one of the areas where birding and conservation science overlap most directly. Reading a neck collar takes a scope, reasonable light, and patience — the birds don't always present their necks conveniently — but it requires no special training beyond what a birder heading to a swan flock would already carry. ## What the tags have changed Before satellite tracking, the Bewick's Swan migration was understood in outline: Russian Arctic to northwest Europe, with some staging in between. The tags have shown that staging sites in the Baltic and White Sea are not interchangeable but are used by different subsets of the population. They have not, so far, reversed the decline. The population trend remains downward. For a birder standing at Slimbridge on a cold November evening, watching a small flock of Bewick's Swans drop onto the floodlit pool, the tracking data adds a layer that wasn't available a generation ago. That bird with the yellow collar (the one currently facing away from you, naturally) left the tundra five or six weeks ago, stopped in Estonia, crossed the North Sea, and chose this particular field in Gloucestershire for reasons that are partly traditional and partly contingent. The collar code, if you can read it, links to a history that may stretch back a decade. The flock is smaller than it would have been in the 1990s, and the reasons for that are clearer now than they were then, even if the solutions remain frustratingly slow. The tags transmit positions. The collars, read by birders with scopes and cold hands, fill in the rest. Between them, they've turned a migration that was known only at its endpoints into a story with a middle, and it's the middle where the problems — and possibly the answers — are concentrated. --- ## Sources References drawn on for this article. - [(PDF) Decline of the Bewick's Swan Cygnus columbianus ...](https://www.researchgate.net/publication/399552596_Decline_of_the_Bewick's_Swan_Cygnus_columbianus_bewickii_population_wintering_in_NortheastNorthwest_Europe) - [Bewick's Swan Facts | Cygnus Columbianus bewickii](https://www.rspb.org.uk/birds-and-wildlife/bewicks-swan) - [The changing survival of Bewick’s Swans - British Ornithologists' Union](https://bou.org.uk/blog-wood-bewicks-swan-survival) - [Decline of the Bewick’s Swan Cygnus columbianus bewickii population wintering in Northeast/Northwest Europe | Wildfowl](https://tidsskrift.dk/Wildfowl/article/view/162735) - [Dynasties, divorce, and 10,000 swans – 60 years of WWT’s unique Bill Pattern Study | WWT](https://www.wwt.org.uk/about-us/news/dynasties-divorce-and-10000-swans-60-years-of-wwts-unique-bill-pattern-study) - [Identification of individual Bewick’s swans by bill patterns | Springer Nature Link (formerly SpringerLink)](https://link.springer.com/content/pdf/10.1007/978-1-349-03711-7_17.pdf) - [WWT Slimbridge](https://en.wikipedia.org/wiki/WWT_Slimbridge) - [Ouse Washes](https://en.wikipedia.org/wiki/Ouse_Washes) - [Whooper swan](https://en.wikipedia.org/wiki/Whooper_swan) - [WWT Martin Mere](https://en.wikipedia.org/wiki/WWT_Martin_Mere) - [Nene Washes](https://en.wikipedia.org/wiki/Nene_Washes) - [Cambridgeshire](https://en.wikipedia.org/wiki/Cambridgeshire)