Do satellite tags hinder our woodcock?

One of our kind sponsors recently asked whether the satellite tags we use to track our woodcock ever hinder them in terms of flight, feeding or courtship.

Here was our response:

With regards to the potential impacts of the tags upon the birds: these are clearly questions we are constantly asking ourselves.

Obviously the welfare of the bird is paramount and the continued use of the tags depends entirely upon their safe use.

Additionally, it is important to us that the tags are comfortable and have no impact upon the bird as we are seeking to record normal behaviour that is unaffected by the presence of the device.

Tags are under 3% of the birds body weight. The 3% figure is a benchmark used by most ornithologist when considering the added weight of a tracking device. It is widely agreed that at this level, the tag's impact will be minimal.

Nevertheless we need to be certain that the tag (both its weight and means of attachment) poses no detrimental effect upon the bird. This is understandably hard to measure.

However, we have been able to re-capture three of our tagged birds in subsequent winters - these bird's tags had stopped transmitting, probably as the result of 'battery issues' and so were removed and re-used.

Having a bird in the hand, that had carried the tag for a year or more, allowed us to check for physical signs of wear or damage resulting from the tag. In all cases, there was no evidence of this.

Additionally, the birds were all still healthy weights, similar to those when first caught, indicating that the tag was not impacting upon fitness or the ability to feed.

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5 things we’ve learnt from our woodcock satellite research

With satellite technology improving all the time we've been able to conduct our woodcock research over the last three years by attaching small satellite tags to each of our birds.

The information provided by the tags has provided some fascinating insights including the following:

1. Departure times are typically mid-March to early April from winter sites and late September to late October from breeding sites.

2. There is a mixing of migrants at wintering sites – birds tagged at the same winter site often flew to different breeding locations.

3. Some birds fly extraordinary distances – three birds have travelled distances of 6,180-7,100km (3,860-4,440 miles) to breeding sites in Siberia. One male, tagged as an adult in 2012, is estimated to have flown at least 38,000km (23,750 miles) during his lifetime!

4. Journey times to reach breeding sites can vary from three weeks (northern Norway) to eight weeks (central Russia).

5. Evidence from a few of our woodcock followed for more than one year indicate they use exactly the same winter and breeding sites each year.

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Why aren't we seeing much new location data?

Many woodcock sponsors have noticed a recent lack of activity from the tagged birds, and would be forgiven for thinking that there is a problem with the Woodcock Watch website. We can assure that this is not the case; it’s simply that no data is being transmitted at present.

We currently believe that the problem relates to the tags’ batteries. The tags are solar-powered and need to be recharged by the sun. To fully recharge a battery from flat will take about 4 hours of bright sunlight. It seems that the tags are not receiving this. Day length is short and sunlight intensity low in the northern locations where the birds have spent the autumn. Light intensity on a sunny winter’s day is only one ninth of that experienced on a day in June. On top of this, the woodcock’s nocturnal, secretive nature means that it spends most of the day hidden beneath dense vegetation. This does not mean the tags are permanently out of action though; assuming that some of the birds are still alive, their tags will begin to transmit again as soon as they recharge. This point has been proven by Olwen, who has transmitted good-quality data this week despite having been out of touch for over two months. He has now reappeared in Lincolnshire, where he is alive and well, having made his way from Western Russia.

Olwen's journey

It’s important to remember that this research is not tried and tested; this is new technology which has not been trialled on woodcock. Most current satellite tagging studies performed in the UK relate to diurnal summer visitors migrating between Britain and Africa. This is a far cry from the nocturnal woodcock which is migrating north to Russia and Scandinavia. Because this is such a new area of research there will undoubtedly be surprises and unforeseen problems. Whilst this may seem a disappointment, we must keep in mind the quality of the revelatory spring data we have gathered using these tags which more than make up for the problems we experience in autumn.

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Autumn Progress

In early October the first few migrant woodcock begin to arrive.  The vast majority, however, will not be here until November or even later; in an average year birds are still arriving through December. Last week I heard the first reports of migrants seen on the east coast but our satellite-tagged birds are showing little in the way of migratory behaviour.

The GWCT’s Woodcock Watch project tracks the migrations of British wintering woodcock across Europe; over the last three years the project has followed the fates of thirty-nine individuals using state of the art tracking technology.

In February and March, woodcock are caught and fitted with lightweight tags which are able to provide a near real-time relay of their outward spring migration. The devices are equipped with tiny solar-panels, and providing they receive enough sunlight, they should remain charged to track their returning journeys in the autumn and in some cases, in subsequent years. In 2014, we tracked twenty-five woodcock; seventeen of which were caught this spring and eight of which were tagged in previous years but whose transmitters were still active. All but one tag provided useable data and twenty of our birds completed a spring migration.

Eleven of these migrated to breeding sites in Western Russia. There were three woodcock that chose relatively southern locations in the area west of Moscow and east of the Belarussian border, whilst another eight settled in the more northerly region east of Finland. One of these birds, James, was amongst the most northerly breeders Woodcock Watch has tracked, visiting a site close to Arkhangelsk on Russia’s White Sea. Other tagged woodcock this year headed for Finland, Latvia and Norway.

The birds we have been able to track in multiple years have demonstrated that woodcock remain loyal to the same breeding site year on year. Rebecca, in particular, has been a striking case in point and has returned annually to the same wood in Russia since being tagged in February 2012. In just the three years we have monitored her, she has covered a total distance of 15,000 km from Wales to Russia and back two and a half times. We have yet to conclusively prove it, but we believe that birds like Rebecca are returning to breed at sites close to where they hatched as chicks; research suggests this to be so in most other wading bird species.

Nastasia

We are still receiving data from many of the tagged woodcock and transmissions have remained encouragingly frequent this year, but so far most are only showing small scale movements on the summering grounds. Instinctively, the woodcock will be anticipating the long flight ahead of them and are probably busy trying to find sufficient food to reach an optimum physical condition.

The only bird that appears to have begun her autumn migration is Nastasia. She was tagged in County Cork, Ireland, in March 2014. During the first two weeks of April she made her way to Western Russia via Belgium, Germany and Poland, ultimately settling in a site not far from St. Petersburg. Here she remained until early October. On the 18th October her tag sent fresh data showing her to be in Lativa, having flown the first 600 km of her westward migration.

It remains to be seen when the rest of the birds will depart. The weather in Finland and Western Russia is still relatively mild, but this appears to be changing. If the temperature continues to drop, we would expect this to prompt migration in those woodcock that have yet to leave. Typically, the average woodcock migration consists of a long, sustained flight over a relatively short time period, followed by a break of a week to ten days’ rest before the next long flight. Migrating in stages keeps the birds ahead of the oncoming cold weather and gives them an opportunity to feed and recover en route. This could mean it is another month or more before they are back in the UK and Ireland.

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Sunny weather leads to glut of new location data

We've received a rush of new location data including information from birds who we haven't heard from in a while. The sunnier weather has obviously recharged the satellite tags so we can gladly share the following location updates:

Rebecca

Tagged in 2012, Rebecca is in Wales for a 3rd Winter having travelled to western Russia.

St. Brendan

Last heard from in October, St. Brendan has sent data from his winter site in Cork, Ireland having returned from Latvia.

Olwen

Having left East Yorkshire on the 3rd March, Olwen was in Denmark by the 5th. Now she's on the Sorve Peninsula in Estonia!

Crugith

More data has been received from Crugith's location close to her catching site in Cornwall after her return from Siberia.

Location Update - Was it Crugith?

During our trip to Cornwall last week, we spotted a woodcock with a satellite tag. Andrew Hoodless got within 10 feet of the bird and could clearly see the tag, but wasn’t quite able to catch the bird. Hence, infuriatingly, we weren’t able to confirm the bird’s identity, but it was in the same field where Crugith was caught in February 2013 and we believe it is most likely to be this bird.

We received good data on Crugith’s autumn migration until she reached Germany on 9th November, so it is possible that she has been back in Cornwall for a while. The sighting, along with a recent transmission from Amy in Ireland, confirms our suspicions that a reasonable proportion of our 2013 batch of birds are still alive but their tags are not transmitting owing to the gloomy, wet winter weather.

The reliance on solar recharging of the smallest satellite tags is known to be a limitation in winter, but there is a chance that some of the tags may start transmitting again in late March/April when the birds head off on spring migration.

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A report from our Cornish woodcock trip

We’re pleased to report that, despite the stormy conditions, our Cornish trip was a success with 80 woodcock caught over 5 nights (plus 4 golden plover, 5 common snipe and 5 jack snipe). This included 4 recaptures of birds ringed in February 2013.

Numbers of woodcock seen were lower than in previous winters, at about 80% of average density, most likely explained by the milder weather this winter. Numbers in Cornwall tend to be at their highest in the very coldest winters when birds from across the UK and Europe take advantage of Cornwall’s relatively mild climate. Nevertheless, there were still plenty of birds to be caught and we’re happy with the total for the week.

Last week’s trip was about building up a sample of ringed woodcock and recapturing birds from previous winters in order to estimate wintering site fidelity. A second trip at the end of February will focus on the deployment of satellite tags and geolocators.

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Geolocators vs Satellite Tags

Since 2012, the GWCT Wetlands team has tagged 24 woodcock with satellite transmitters. The movements of these birds have been transmitted back to us in near real time, and as well as a large amount of valuable data, they have provided a great deal of interest to the thousands who have viewed their migrations on the Woodcock Watch website.

But these few satellite-tagged stars of the show are only a small proportion of the woodcock caught and tagged. Though they may provide an exciting and easily-accessible insight into the movements of our birds, there is no escaping the fact that satellite tags are expensive, costing around £3000 each. It’s with this in mind that we have looked at alternative methods of gaining data on woodcock migration routes, and found one, in the form of the geolocator.

Geolocators are tags that are even smaller and lighter than satellite transmitters. They are also a fraction of the cost - we can buy around twenty geolocators for the price of a single satellite tag. They work by logging light levels, from which time of sunrise, sunset and day length can be determined.

These data can be used to calculate an approximate longitude and latitude for the bird twice a day. Geolocators are accurate to 60-150 km and whilst this is far less accurate than a satellite tag, it is an acceptable margin of error for a bird that may migrate 2000–7000 km.
Geolocators do present one difficulty however. The tags store information on the birds’ whereabouts but do not transmit it – the tags must be retrieved and the data downloaded. The chance of recapturing the same woodcock from one year to the next – considering the several thousand miles it may have travelled in the meantime – probably seems like finding a needle in a haystack.

Amazingly, however, it is possible, and this is thanks to the fact that woodcock tend to be very site faithful. Woodcock will return to the same wintering sites year on year and on some occasions even the same field. We have deliberately deployed these geolocators at sites where we have previous knowledge of site fidelity from ringing and are able invest time in recapture efforts each winter.

As well as recapturing live birds, we have also had several tags returned to us by those that have shot woodcock wearing them. It is with this in mind that we share the following information so that those who shoot woodcock are able to recognise the tags and return them to us.

• The tags are very small, approximately 8 x 20 mm.
• The tags may be almost entirely covered in feathers as they sit flat on the back. The light sensor should be visible, protruding from the feathers on a small stalk about 20 mm long. It is possible that this stalk could break off but the tag itself may still contain useful data so please check beneath the feathers.
• The tags are mounted to the lower back and held using leg loop harnesses. These pass under both legs and hold the tag above the preen gland and between the wings in an area that does not impede preening or flight. A tag can be easily removed by stretching the elasticated loops back over the legs.
• The tags have been deployed in small numbers in Norfolk and in north-east Scotland. Tags have been deployed in large numbers in Cornwall, and, with the help of the Woodcock Network, in Mid-Wales. People who shoot in these areas are far more likely to encounter geolocators.

To date, just over 100 geolocators have been deployed since 2010 and 18 recovered. People who recover a geolocator can return it to Andrew Hoodless, GWCT, Burgate Manor,
Fordingbridge, Hampshire, SP6 1EF or email: woodcock@gwct.org.uk for more information.
Both geolocators and satellite tags are valuable tools in monitoring woodcock migration, each with their own benefits and disadvantages. Whilst satellite tags are able to provide us with very accurate and easily-accessed data, the geolocators are a cheaper way to increase sample size. The success of our geolocator study hinges on achieving reasonable return rates and every tag that we are able to recover will make a big difference.

Tracking woodcock

So that we don’t impair the birds’ flight, we have utilised small satellite tags that are dependent on solar charging to replenish their batteries. Consequently, low light levels caused some gaps in data transmission in winter, but we have been able to capture locations accurate to between 150 metres and one kilometre.

In tracking these enigmatic waders, we are able to gain a better understanding of the 750,000-1,200,000 migrants which join the considerably smaller resident population in Britain in winter.

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