Woodcock Journeys

Swift Report - Winter 2022

The Woodcock (Scolopax rusticola) occurs in Britain as both a resident breeding species and a migrant winter visitor. It is estimated that around 55,000 pairs of Woodcock breed in Britain, but their distribution is patchy, with the highest numbers occurring in northern England and Scotland.

Cornwall, like Devon and south west Wales, has practically no breeding population, and there are comparatively few historical records of breeding from these areas. Cornwall’s climate and landscape does, however, make it particularly suitable for wintering Woodcock. Because they predominantly feed on earthworms found by probing soft ground, Woodcock are reliant on mild, wet winters without extended periods of permanent frost. They make daily flights from woodland to feed on grassland, heath and arable land at night, and Cornwall’s grazed pastures offer high earthworm densities and a favourable sward height for foraging.

Extensive forested areas are not necessarily required in winter as most feeding occurs at night, and a small patch of dense scrub or woodland will suffice as a diurnal roost. In very cold conditions, even resident British Woodcock, which do not normally migrate, may temporarily travel to Cornwall to escape hard frosts further north.

The Game and Wildlife Conservation Trust (GWCT) has been conducting scientific research on Woodcock since the 1970s but only recently, with the development of new tracking technology, have we been able to study the species’ migration in detail. Thanks to the ongoing support of local landowners, and the particularly high numbers of Woodcock the county holds, a large proportion of this research has taken place in Cornwall.

A broad impression of our wintering Woodcocks’ origins was already known from ringing and isotope analysis, But the use of tracking technology has afforded us a much greater understanding of the Woodcock’s migration routes, the timing of migration, the environmental cues that drive migration and the factors that influence migration strategy e.g. the location, frequency and duration of stop-overs. Tracking data are helping us understand how and why Woodcock numbers in Britain vary from winter to winter, whether relating to annual variation in productivity or survival on migration, or flexibility in the migratory route (e.g. short-stopping).

Between 2012 and 2018, we deployed 65 satellite tags across Britain and Ireland, ten of which were deployed in Cornwall. Overall, spring departure dates for tagged Woodcock ranged from 3rd March to 13th April and yearly averages varied by up to two weeks depending on spring temperatures. Woodcock returned to eight different countries (in order of frequency: Russia, Sweden, Norway, Finland, Latvia, Belarus, Denmark) and we observed rough patterns in their origins, with birds from Scotland, northern England and Ireland more likely to originate from Scandinavia than those wintering elsewhere in Britain. Cornish woodcock were most likely to be of Russian origin: six out of ten tagged birds returned to Russia, one to Belarus, and three to Sweden. Two Cornish-Russian birds made impressive migrations of over 6000 km to western Siberia (Krasnoyarsk province). Only one other Woodcock, tagged in Norfolk, made an equivalent migration. Migration routes could be categorised as belonging to one of two general groups – Scandinavian Woodcock that migrated straight over the North Sea into northern Britain and Ireland, and those that take an overland route from Finland, the Baltic States or Russia, passing through Poland, Germany and France to arrive in southern England via the Channel.

One drawback of the satellite transmitters was the amount of power required to transmit location data. Despite their solar recharge, battery-life was not always as long as hoped. Some individuals transmitted over multiple years, but often the frequency of transmissions dwindled over time and were generally more intermittent in autumn. We think this is because Woodcock use very dense cover during their moult, restricting the amount of sunlight that reaches the tag in late summer. To counter this, we have now started using GPS loggers, more efficient tags which store rather than transmit data. This reduces battery consumption and means that even with limited recharge, the loggers are still capable of recording consistent location data for up to two years. These tags are helping address some of our remaining unanswered questions, particularly regarding the timing and strategy of autumn migration. Because they are loggers (as opposed to transmitters), it does mean that the tag has to be recovered to access the data, but past experience has demonstrated that we could recatch a large enough sample to make this viable.

This past experience came from the long-term ringing study we have been conducting on the Lizard in Cornwall since 2010, and this familiar area became our chosen site for the deployment of GPS loggers. It is one of two long-running Woodcock ringing projects that the GWCT conducts in England, the other based on an estate in Hampshire. These projects aim to ring a minimum of one hundred new Woodcock at each site annually, and in recent years, might generate up to 30 – 50 recaptures of previously-marked birds. In spite of the availability of high-tech tracking devices, ringing still presents a very valuable tool for ornithologists, particularly for the study of survival and population dynamics.

By marking a cohort of birds annually and recording re-encounters with these birds over subsequent years we can, with the aid of some complex statistical models, make estimates of our capture rate and external recovery rates, and from these calculate the annual survival rates of marked Woodcock and their likelihood of returning to the study site each year. The act of marking a sample each winter also provides us with interesting information on the populations’ age ratios (i.e. first-year birds: ‘adults’), and thus some measure of annual productivity.

The traditional view of ringing as a tool for studying birds’ movements has largely been superseded by tracking technology, but ringing still offers some interesting, albeit anecdotal, insights here too.  Since starting my work with GWCT back in 2011, I have ringed approximately 1,000 Woodcock, but during that period have only ever caught ‘re-trap’ Woodcock ringed by myself or my colleague who works on the same study site. It has long been a personal ambition to remove a Woodcock from the net and find an unfamiliar foreign ring – perhaps inscribed with a Finnish or Latvian address.

This year, during our annual trip to the Lizard, I made my first foreign recovery, although I didn’t immediately recognise it as such. It was only when I began to trace the origin of an unfamiliar ring number that I was reminded that Ireland operates on the same ringing scheme as the UK. It subsequently came to light that this Woodcock, captured in Cornwall in early February 2022, had been ringed in County Cork by James O’Neill, a fellow Woodcock researcher at University College, Cork. He had ringed the bird in December 2020 and, because it had been part of his PhD radio-tracking study, knew that it had remained in Ireland for the entirety of the 2020/21 winter. Finding this bird in Cornwall in early February was a little puzzling, especially as most of satellite-tracking work has shown that most (but not all) Woodcock are faithful to their wintering sites year on year. It may be that this Woodcock is one of the few that is less site-faithful, perhaps stopping short of Ireland in equally suitable Cornwall.

Alternatively, and I think more likely, the Irish-ringed bird had returned to Ireland for the winter of 2021/22, only to be caught by me on its return journey. This is early when compared to the mid-March departure dates recorded by our satellite tracking study but seems to make sense given the mild spring. As further proof, when I returned to the Lizard for a second week of Woodcock catching in early March, I found numbers to be less than half of those observed in February, suggesting many birds had left early.

Chris Heward – (Acting Head of Wetland Research at the Game & Wildlife Conservation Trust) 

With the numbers of Woodcock ringed by myself and colleagues in south-west Ireland each winter, we are well used to getting interesting recoveries as a result, ranging from distances of a hundred metres from where the bird was ringed, to many thousands of miles away on the breeding grounds to the east. None of these recoveries have been live – Woodcock is a popular game bird in both Ireland and Russia, and unfortunately some birds have a habit of fatally hitting windows in cities during migration!                      

So, it was a very pleasant surprise to hear that one of our radio-tacked Woodcock from the 2020/2021 season, ringed as a juvenile on our study site west of Cork City, had been re-trapped by Chris in Cornwall recently. Although more questions are raised by its apparent change in location, it represents another fascinating detail of the lives of these secretive birds that both the GWCT team in Britain and we in Cork have been working hard to understand. Perhaps we will catch it back in Ireland again next winter.

James O’Neill (PHD student, University College, Cork)

 

 

 

 

 

The spring migrations (March-June) of ten Woodcock tagged in Cornwall between 2012-2016. Where multiple spring migrations were recorded for a single bird, only the first year’s migration is shown.