Flagging up potential problems

Any device that is added to a bird (or other animal) has the potential to affect the way it behaves. Even something as simple as a metal ring could increase risk, if it is fitted incorrectly or if fishing line gets caught around it, for instance.

In a 2020 paper in Bird Study, Thomas Mondain-Monval and colleagues report on the way that differently mounted geolocators affect Common Sandpipers. These devices were being used to help understand the migratory behaviour of the species, part of a Lancaster University PhD project that aims to explain a rapid decline in breeding numbers in England.

Safety first

Any researcher who uses rings, colour-rings, tags or tracking devices to study waders needs to ask (and answer) the following four questions:

  1. Is there a good reason to use the device? What’s the question and will the results be analysed and published?
  2. Is the device being fitted as safely as possible? Has it been used on similar species and what happened?
  3. How do the birds react to the device? If trying something new, perhaps the device can be trialled on captive birds.
  4. Are there any differences between birds wearing different types of rings or devices? Write up your results so as to help future researchers.

Thomas and his colleagues have followed this process through to its conclusion by writing a paper that is published in Bird Study. In it, they compare return rates for Common Sandpipers wearing rings and geolocators and detail a number of injuries that could potentially be linked to the geolocators.

Ringing a Common Sandpiper, before adding a colour ring and a flag

Common Sandpipers

The latest Breeding Bird Survey data suggest that numbers of breeding Common Sandpiper dropped by 40% in England and 24% in Scotland between 1995 and 2018. Over the longer period covered by the three BTO-led breeding Atlases (1968-1972, 1988-1991, 2008-2011) there have been losses from the edge of the species’ range, suggesting that decreases were already under way before the start of BBS recording period (see map). Common Sandpiper was added to the Birds of Conservation concern amber list in 2009. There are insufficient data from the BTO Nest Record Scheme to work out whether declines may be linked to breeding success.

Common Sandpipers in the River Lune study area nest close to running water

The European population of Common Sandpiper has seen a widespread, moderate decline since 1980, indicating that there may be large-scale drivers of losses. Is something going wrong in the non-breeding grounds? Previous geolocator studies have shown that Common Sandpipers rely upon a series of stopover sites on migration (see Not-so-Common Sandpipers) and it is possible that these are declining in quantity and/or quality.

As part of his Lancaster University PhD, Thomas Mondain-Monval’s PhD took a two-pronged approach to an investigation of migration routes. He added geolocators to flags on birds in both England and Senegal. The fact that different tags were used in the two countries enabled him to compare the tag effects on study birds. He was also able to compare tagged birds to a sample of colour-ringed birds.

The study systems

UK fieldwork was carried out in the River Lune catchment area in Cumbria, a northern county of England, as part of a detailed study of 24 breeding pairs. Unmarked adults were caught each year and fitted with a BTO metal ring, a yellow colour-ring (engraved with two unique black characters) and a plain red ring or flag. Similar colour rings were used on Common Sandpipers that were caught on their wintering grounds in Djoudj National Bird Sanctuary, Senegal.

Red flags were used on birds that carried geolocators as these provided space to affix the device. This sample consisted of 22 individuals in the UK and 10 individuals in Senegal. The control samples of birds with colour- rings but no geolocators were 28 individuals in the UK and 6 individuals in Senegal. Dimensions of flags and geolocators are provided in the paper, together with information on methods of attachment. The combined mass of the geolocator, flag and glue was 1.1 g for the birds ringed in England and 1.0 g for the Senegal birds, which is about 2% of body mass of the 50 g Common Sandpipers. The Senegal tags were slightly lighter but a little longer. See paper for details.

Mist nets, drop traps and whoosh nets were used to catch Common Sandpipers that were wintering in the Djoudj National Bird Sanctuary in Senegal

The Common Sandpipers in the UK were observed at least weekly throughout the breeding season. Tagged birds wintering in Senegal remained site-faithful and were observed opportunistically, usually weekly for up to five weeks following capture. It is unusual for researchers to be able to monitor the behaviour of tagged birds as closely as was the case here. When it became apparent that two birds belonging to the breeding study had started to limp, attempts were made to catch the birds. One bird was retrapped and the orientation of the geolocator was changed from parallel to the leg to along the line of the tag. This bird stopped limping and the parallel orientation was not used again.

Flag & geolocator effects

Common Sandpiper in Senegal, wearing a flag-mounted geolocator

The key measures of success that are usually monitored by researchers indicated no difference between birds with and without geolocators:  

  1. For the English, breeding population, there were no significant differences between the return rates or return dates of birds with geolocators and those without.
  2. There were no significant differences in hatching success or fledging success between birds with and without geolocators in either 2017 or 2018, although sample sizes were small.
  3. There was no significant difference in condition between birds with and without geolocators.

Most researchers who deploy geolocators on waders are using them to collect a year’s worth of data from their study birds. Typically, a bird is caught on its nest in one year and then caught again a year later – which might sound easy but isn’t! The fact that Thomas was also studying his small population in detail provided extra opportunities to collect information which should be helpful to others. Although there were no detectable effects of geolocators, as assessed using the metrics described above, a small number of individuals tagged in the UK experienced injuries:

  1. One of the birds that had been fitted with a parallel-mounted geolocator sustained an injury to its lower leg, possibly due to a constriction of blood flow. The bird was still able to continue with its breeding attempt.
  2. On their recapture in 2018, two of the seven birds carrying parallel-mounted geolocators were noted to have bruising on the tarsus, apparently caused by the geolocator hitting the lower leg whilst the bird was walking.
  3. In five cases, individuals had a slightly swollen tibia or had lost some skin underneath the leg flag. This occurred irrespective of tag orientation and appeared to be caused by the internal diameter being marginally too small for the individual, although no rubbing was noted and all flags rotated freely at the time of fitting.
  4. In Senegal, no injuries were seen on any of the tagged birds. These birds were wearing similar flags but carried lighter geolocators than the English birds.

The research team concluded that injuries to the legs of some of the study birds were caused by carrying geolocators. They suggest that they were probably due to a combination of geolocator size and weight, and the short tibias of Common Sandpipers. Mounting long geolocators parallel to the leg on species with short tibias may impede leg movement. When the team switched to thinner and lighter tags for their work in Senegal there were no problems.

It is good that Thomas and his colleagues have published the information about the issues associated with the original tagging method that they used, so that others can learn from their experiences. Had they simply reported return rates and measures of reproductive success their results would have suggested that geolocators had no negative effect on these Common Sandpipers. It would have been easy to miss out the extra detail about the small risk of leg injury.

Bird ringers are always aiming to improve catching, handling and tagging techniques. Within the UK, the use of flag-mounted or harness-mounted geolocators requires project-by-project approval from the BTO’s Special Marks Technical Panel. Annual reporting enables the SMTP to update guidance for other researchers.

Refining the way that flags are used

Flags have been used on waders for over forty years and only occasionally have birds seemed discomforted by being asked to wear them. This seems more likely to happen if the flag is applied to the upper part of the leg. When occasional individuals are observed leg-flicking it may be because the ring sits awkwardly on the tibia-tarsal joint. The flick is thought to rotate the flag into a more comfortable position. Nigel Clark, who affixed his first flag to a Dunlin in 1978 suggests the following remedies:

  1. It goes without saying that the edges of all colour-rings and flags should be sanded to remove sharp edges.
  2. Flags are heavier than colour-rings and this means that they sit more firmly on the tibia-tarsal joint, at a point which is wider in diameter than that of the rest of the tibia. When making flags the internal diameter may need to be slightly larger than that used for colour-rings on the same species.
  3. The addition of a geolocator further increases the mass of the flag. When there were concerns about flag-mounted geolocators in North America, Ron Porter solved the problem by making sure that there was a colour-ring underneath the flag. The ring rotates easily, acting as a ‘washer’ between the tibia-tarsal joint and the flag.
  4. When a wader is very thin, as it may be after a long flight, the diameter of the leg can sometimes be less than expected for the species. In Spoon-billed Sandpipers, where there is only space for one ring on the tibia, flagged birds have occasionally been seen leg-flicking. When the leg-flags were modified, to reduce the internal diameter, things improved.

If there is a paper that describes or expands upon the above list, I shall be delighted to add a reference.

To learn more

This blog focuses on a 2020 paper in Bird Study, the journal of the British Trust for Ornithology:

The effects of geolocators on return rates, condition and breeding success in Common Sandpipers. Thomas O. Mondain-Monval, Richard du Feu and Stuart P. Sharp

Three previous WaderTales blogs have discussed issues relating to flags and geolocators:


WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.

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