Research on Bar-tailed Godwits

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In 2001 the Royal Netherlands Institute for Sea Research (Royal NIOZ) launched a study on the ecology of Bar-tailed Godwit. In addition to the mainly shellfish eating Red Knot, which we have been studying since the late 1980s, we wanted to learn more about the ecology of a species whose food consists mainly of bristle worms. Starting from May 2001 we have been colour-ringing Bar-tailed Godwits in the Dutch Wadden Sea and the Banc d'Arguin in Mauritania, one of the major wintering areas of the species in Western Africa.

Below text is available in Dutch - Nederlands versie beschikbaar als PDF. Klik hier.

Collaboration with local bird catchers

Every year, many birds are caught and ringed by Dutch ringers from different ringing groups: VRS Castricum on the Dutch North Sea coast, VRS Calidris on the island Schiermonnikoog, and the Vereniging van het Friesche Vogelvangersbelang (also called “wilsterflappers”). Each spring the latter group, especially Joop Jukema, Catharinus Monkel, Jaap Strikwerda and Bram van der Veen, catches hundreds of Bar-tailed Godwits in the meadows of the islands of Texel, Terschelling and Ameland. They catch with a traditional method that was used in the past to catch Golden Plovers for income (“wilsterflappen”). Waders are also caught by the NIOZ team with mist nets in the periods around new moon in the Wadden Sea and in Mauritania.

Colour ring combinations

The colour ring combination consists of four colour-rings and a flag (a ring with a kind of streamer). The flag colours that have been used since 2001 are shown in Figure 2: in chronological order, Yellow (Y), Red (R), Lime (L) and Black (N). There are two colour rings on each lower leg (tarsus) with a flag in one of eight possible positions (see Fig. 1). Each bird also carries a metal ring; this ring is not part of the colour combination.

Fig. 1. There are eight possible flag positions ("vlag positie")

Since 2014, black flags are used in combination with four colour rings. The colours used are black (shorthand N in accordance with international agreements for colour ringing), Red (R) , Yellow (Y), green (G) and Pale blue (P). A Bar-tailed Godwit with a red over a yellow ring on the left leg and with a black flag above two green rings on the lower right leg is quoted as N4RYGG . First, the position of the flag is noted and then the colour-rings from top to bottom, first for the left leg (of the bird) and then for the right leg.

Fig. 2. Colour-ring combinations that have been used by the NIOZ team for Bar-tailed Godwits in the East-Atlantic Flyway

Since the launch of black flags, we have used a unique flag position for each catching area. This means that you can recognize where a bird was ringed from the position of the flag. N1 is used on Texel , N2 in Mauritania, N3 on Terschelling, N4 in Castricum, N5 on Ameland and N6 are birds that are caught with mist nets on Griend or Schiermonnikoog. N7 and N8 are not yet used.

Your observations

Observations of Bar-tailed Godwits can be sent to describing the colour-ring combination, the observation date and location. Additional to date and location we would like to receive your notes about:

  1. type of terrain
  2. flock size

If possible please also add the following:

  1. ring density - the number of colour-ringed individuals and the total number of observed birds
  2. plumage description - summer or winter plumage; if possible expressed as a percentage
  3. sex of the bird - males are smaller than females and male breeding plumage is redder.
Fig .3. An example of a colour-ringed Bar-tailed Godwit, Y2BRYR. This bird is observed and drawn in the Samone Lagoon in Senegal on 24 January 2014 by John Wright

Ring loss – important to report!

In the early years of the study our colour-rings were made of a plastic named Darvic. However, currently we are using a different material that has the properties of Plexiglas. These rings do not have eternal life and their life-span is often shorter than the lifespan of a Bar-tailed Godwit. This means as a result of ring loss incomplete combinations exist. Furthermore some rings may be strongly faded as a result of ultraviolet light. Importantly, we would like to receive your observations of birds with incomplete colour-ring combinations, so we can estimate the amount of ring loss. For birds with incomplete ring combinations, it is important for us to know the sex. It is easy to see the sexes between the birds in size (males are smaller than females) and plumage, especially in spring when the males get a reddish colour.

Study results

Our study of individually marked Bar-tailed Godwits has provided us with lots of information on their feeding ecology and spatial use of the Wadden Sea and beyond. For example we discovered that in winter only longer-billed females remained in the Dutch Wadden Sea, while the shorter-billed individuals moved away to an estuary with  a  more  benign  climate  such  as  the  Wash in Engeland (Duijns S, van Gils JA, Smart J, Piersma T. 2015 Phenotype-limited distributions: short-billed birds move away during times that prey bury deeply. R.Soc. open sci. 2: 150073: DOI)

In combination with the extensive benthic sampling program SIBES, which started in 2008 at NIOZ, we have an increasingly better understanding of the food landscapes for birds. In the SIBES program the annual distribution of benthic life is sampled on almost all the tidal mud flats in the Dutch Wadden Sea- covering over 4500 sampling stations. Here is a VIDEO about the field work.

In 2014, Sjoerd Duijns completed his PhD-thesis on the feeding ecology of Bar-tailed Godwits under the title Sex-specific foraging; the distributional ecology of a polychaete-eating shorebird. His thesis is available on the website of the Wadden Academy: Waddenacademie (a direct the link to the PDF is here).

In October 2018 we published a paper on our decades-long Bar-tailed Godwit research in the East Atlantic Flyway. During the last decades Bar-tailed Godwits experienced changes in the tundra phenology. We detected a chain of effects suggesting that conditions in the temperate zone (that is, the Wadden Sea) determine the ability of Bar-tailed Godwits to cope with the climate-related changes in the Arctic (read more here).

For more information on our work you can also visit the website the NIOZ at or Metawad at