Project CHIRP
Onderzoek naar
achteruitgang Scholekster
IJs Waddenzee zorgt voor lokale verplaatsing gezenderde scholeksters
Successful winter catches 2018: More than 700 oystercatchers ringed!
Nature Today: Texelse graslanden trekken scholeksters van Vlieland
Welke Vliehors-scholeksters weten de weg naar Texel?
42 scholeksters gezenderd op de Vliehors
Aankondiging Scholeksterweekend 2018
Hoe snel en hoog trekken Noordelijke scholeksters?
Steeds meer zendervogels terug(gevonden) in de Waddenzee
Verslag CHIRP vrijwilligersdag 2017
Viermaal is scheepsrecht voor scholekster in Friesland
Update zendervogels Vliehors
Overleving en migratie van scholeksters in Nederland
CHIRP vrijwilligersdag: 19 augustus 2017!
Update: CHIRP-broedseizoen
Broedvogels op de Vliehors: Nieuwe zenders en storm
Wat deden de gezenderde scholeksters afgelopen winter?
Update CHIRP-scholekster waarnemingen
Ringaflezingen met BirdRing direct naar Wadertrack!
Zegt snavelkleur iets over conditie?
Gezenderde scholekster Wietske teruggevonden
Woerden valt niet in de smaak bij gezenderde scholekster
De wintervangsten zijn afgesloten!
Nieuwe kleurringcodes in Wadertrack
Vroege Vogels reportage over gezenderde scholeksters
Productie kleurringstrengen in volle gang
UvA-BiTS pagina online
Succesvolle eerste wintervangsten
Nieuwe kleurringcodes


The Eurasian oystercatcher has been the main focus of the CHIRP project. Hence, during last 3 months (December, January and February) we captured these birds in order to colour-band them and measure their body condition. We selected 9 locations from 3 provinces (Zeeland, Noord-Holland and Friesland) where oystercatchers spend the winter season. Depending on the location we used cannon net or mist nets. The first technique requires the knowledge of the exact area where a group of oystercatchers roosts during the high tide, whether with mist nets bird’s location may be not that precise but the wind shouldn’t be too strong. After each catch, oystercatchers are placed in special tents, where they remain until the ringing and measuring procedure. The process starts with ringing the oystercatcher (if it’s not ringed yet), then we take biometry measurements and blood samples to determine the sex of the bird, to analyze the hematocrit (volume of red blood cells) and the cholesterol levels. After that a tail feather is removed to allow us to study the stress that the oystercatcher has been through during the winter months. In the end the bird is placed in a special box with light where pictures of its eyes and bill are taken. Later on we release the oystercatchers hoping to find them again in the breeding areas. During this winter season we captured 701 oystercatchers, which was a great improvement compared to the 279 oystercatchers ringed during last year’s catches.

Interactive map with catch sites and pictures of 2018 winter catches

The first catch of the season happened on Vlieland, between 15th and 19th of December 2017. Mist nets were considered the best technique in this environment. In total there were caught 115 oystercatchers, but a GPS logger was placed on 42 of them which may influence their body condition and as a result the reproductive success. Therefore 73 oystercatchers are suitable for determining the effects of body condition in winter on the breeding success.

Ringed oystercatcher on Vlieland. - Geringde scholekster op Vlieland. - Picture by Henk-Jan van der Kolk

One month later I joined the catches in Prunjepolder, Oosterschelde, Delta. We departed from Wageningen on 18th January during the afternoon, but due to the bad weather the traffic was problematic, so the trip took way much time that we expected. After having a pleasant dinner at a restaurant I soon realized that it will be quite useful to learn Dutch, but it’s also easier to learn it when it’s the language that I hear the most. In the next morning we placed the cannon net while the tide was low. This net is very long so it takes 4/5 people to transport it in a straight way. There are 4 cannons that we buried in the ground; each of them has part of the net attached, so when it shoots the net goes with the cannons covering the birds that are resting. We expected to catch at 13:00 so we had time to have a cup of coffee and lunch at our place, then we waited the rest of the time nearby the location with some wind, rain and hail. At 17:00 Kees pushed the button and we first heard the cannons shooting and right after we heard the oystercatchers. Everyone ran to the field and realized that we just caught around 450 oystercatchers. Some of us started removing the birds from the net and placing in crates, others transported these crates from the field to the tents, placing them there. All this process is done as fast as possible to decrease the amount of stress that the oystercatchers have to go through. Unfortunately, it was getting darker and there were still lots of birds under the net, so part of the group continued removing those birds and the rest started the main procedure of ringing and measuring body condition inside a large garage that a friendly fisher man let us use. With the precious help of some volunteers we did a big part of the work; however after they left we continued working until around 1:00am. In total we ringed and measured 131 oystercatchers. At the accommodation Magali and Henk-Jan immediately started centrifuging capillaries with blood in order to measure the hematocrit, this has to be done within 6h after taking the blood from the bird, and hence they stayed up until quite late. We came back to the location in the next morning to remove the net and the cannons from the field, pack all our stuff and clean the garage.

Oystercatchers caught with a cannon net. - Scholeksters gevangen met kanonnet. - Picture by Henk-Jan van der Kolk

The weather forecast for the night of 21th January was quite optimistic on Ameland, Vogelpolle, so the plan was to catch with mist nets. We left Wageningen at 10:00am and while we were waiting in the harbor for the others I realized that I was going to be for the first time on an island, so this catch became even more exciting for me than it already was. On Ameland we went straight to the location and around 15:00 started setting the poles, pegs and nets. We built two rows of mist nets, each had 11 poles, and after that we went to our new home. Jan de Jong offered us a place to sleep and ring the birds. There we prepared everything for the ringing, taking biometry, blood, feathers and pictures, and not less important the dinner for ourselves. After having a tasty spaghetti alla bolognese, we went to the catch location. There were already some oystercatchers in the nets, so we removed them, placed in crates and Martijn and Laurens took them to the tents near the house by van. The rest of us waited for half an hour and then went to the nets for another check, and then repeated this one more time before leaving. In total we caught 102 oystercatchers. On our way home we saw lots of hares and a gorgeous owl that flew for some time in front of the van. We started the measurements around 22:00 and finished almost at 3:00am, this would be way harder to do without coffee and sugary snacks. Even though, Henk-Jan and Martijn still had to process the blood during the rest of the night. The birds were released right after the sunrise, since it allow us to leave them smoothly and it’s easier for them to orientate. During the morning we cleaned up everything, removed the nets and washed the tents. At the afternoon we packed all our stuff and went to the mainland.

Mistnets and processing birds on Ameland. - Mistnetten en verwerken van scholeksters op Ameland. - Pictures by Iryna Litovska

‘t Kuitje
On 26th January we did a catch with cannon net in Balgzand, 't Kuitje, Den Helder. Part of the team arrived at 8:00 and started placing the cannons and the net on the field. Soon enough we realized that we left the tents in NIOO. After some stressful messages and calls we managed to talk with Magali and Foteini, that haven’t departed yet from NIOO, so thankfully they brought the tents. At 11:15am we captured 77 oystercatchers and afterwards we removed the cannons from the field and the net that was somewhat muddy. After a quick break we started ringing, measuring, and immediately releasing the birds. We finished at 16:30, cleaned the room that we were working in and left. Nonetheless there was still work to do related to the hematocrit, so 2 people stayed a little bit longer. With a big thank to the Jaap Booij and Henk Arts that we were allowed to use the Natuurinformatiecentrum Balgzand, ‘t Kuitje, to ring the oystercatchers.

We did the next catch with cannon net as well on 30th January in Schor van Wilhelmina, Delta. In order to facilitate the logistics of the catch we arrived in Zeeland a day before. We cooked spaghetti alla bolognese and had it for the dinner. This time I wasn’t supposed to come in a first place so there was no bed available for me, but there were 2 comfortable couches, therefore I slept in the living room (approved!). In the next morning we saw an amazing sunrise on our way to the catch location. Around 9:00am we placed the cannons and the net on the field, and prepared the rest to work outside right over the dike (it would be perfect if we could finish before the sunset). During the day we spent a couple of hours waiting for the right moment to shoot, but luckily the weather was very pleasant and we observed a common kestrel hunting nearby. In the afternoon some volunteers (Sander Lillipaly, Jos Tramper, Koos Minnaar, Harmke de Hoogh) started to appear to help. At 15:00 we heard the shot and everyone ran with the crates to the field. We captured around 150 oystercatchers, however they were too many because we were working outside, so we couldn’t physically manage to work with such an amount of birds. Subsequently, we had to release straightway some of them (not ringed ones), thus we measured 124 oystercatchers this time. Another successful catch! While transporting the crates with oystercatchers we noticed that someone was documenting this catch. Kees Polderdijk made a movie from this catching session. We started ringing and measuring at 16:30 and at 17:15 we witnessed a spectacular sunset. After this moment it was getting darker and colder, hence tougher to work (even with a beautiful sky full of stars and a bright moon). As we were working outside I was using an electricity generator to have the light turned on in the picture-box, and when we plugged in the boiler the generator stopped working (we need to have hot water to make the plastic from the rings more flexible). We managed to turn on the generator again but just with the light for the picture-box on. At one point the water was too cold, consequently Kees and Symon couldn’t ring anymore and everyone had to stop. Therefore some locals went home, boiled some water and kindly brought it to us in thermos, so we could continue with the process. We finished at 20:30, placed everything in the vans and went accommodation. There were not enough boxes to put the capillaries with blood, so I had the responsibility of not breaking 3 rows of capillaries during a 30min road trip (I succeed). At home Magali and Henk-Jan started with the hematocrit while Laurens, Foteini and I prepared not surprisingly spaghetti alla bolognese for the dinner. As Kees and Symon left earlier I moved to their room with not less comfortable beds than the couch.

Processing oystercatchers outside at Wilhelminaschor. - Ringen en meten van scholeksters buiten bij Wilhelminaschor. - Picture by Iryna Litovska

Terschelling - Part 1
In February the first catch happened right after the Oystercatcher weekend (Schiermonnikoog). The plan was quite clear: catch the boat on Schier at 7:30, pick up NIOO’s cars in the mainland, go to Harlingen and catch another boat at 9:45 to Terschelling, set the mist nets during the afternoon and catch oystercatchers at night. Makkelijk! In the morning of 5th February beside some stress we managed to reach the harbor on time, but due to the tide the boat got delayed for an hour, which meant that we wouldn’t get on time to catch the other boat to Terschelling (and consequently we wouldn’t catch?). We entered the boat from Schier at 8:30, and right after Henk-Jan and Magali started having lots of calls. At this point the plan has changed: Kees, Symon, Laurens, Foteini, Katrin and I would go on a boat that doesn’t bring cars at 12:30 with the poles, pegs, and nets (the most important), so we could set the mist nets still during the afternoon, and later on Magali and Martijn would come on another boat at 15:00 with NIOO’s cars and the rest of our stuff. And so it happened, when we reached Terschelling Jacob Jan de Vries was already waiting for us, so we packed everything in his car and had a warm lunch at his place. Then we went to the location and set the mist nets on the mud flat. Around 17:00 we arrived at the accommodation, and right after Magali and Martijn arrived as well. Some of us went back to Jacob’s place to arrange all the other stuff (rings, biometry, blood, feather, pictures, and tents) and others started cooking the dinner. This time we had a delicious stamppot, after which 5 people went outside to check the nets (nothing). At 21:00 I joined for the second check (nothing). Then at 23:00 everyone went to the nets location and stayed there. The night was very bright because of the moon, the tide increased very fast in a short period of time and it was fascinating to see how fast the sea water frizzes. We caught some geese, curlews, knots and 3 oystercatchers. In the next day we went to the location and it was quite amusing and frustrating to see a group of oystercatchers feeding almost under one of the rows of mist nets! We adjusted one row of nets, and removed the other row, setting it in another (muddier) location where we saw a group of oystercatchers as well. We were all quite tired so after coming back to the accommodation we slept 2h, and then some of us went to check the nets for the first time, while the others cooked bifinhos com cogumelos for the dinner (Portuguese dish with rice, pork and mushrooms with cream). At 23:00 everyone went to the location, and after checking a couple of times we realized that there were no oystercatchers in the nets at all. The water was freezing and Kees decided that it would be better to remove the poles while they weren’t frozen to the ground. Not everyone went into the water because the tide was too high, so the rest of us transported the crates, nets and poles from the seaside to the car. Around 1:00am we finished and went to the accommodation. In the next day we went to one of the locations to remove the pegs that we left the night before (they were somehow frozen to the ground), and to another location to remove all the other mist nets. Interestingly, while we were working a single oystercatcher appeared, flew over us and stayed in the same field looking at us until we were gone (I’m pretty sure that he was laughing at us). Later we packed, drove to the harbor, and had a wonderful lunch on the boat.

Terschelling - Part 2
Nevertheless, we still had hope that it’s possible to catch oystercatchers on Terschelling! Hence on 15th February we caught a boat to Terschelling at 15:00, did some birdwatching on our way, went to the accommodation, checked some locations, prepared the tents, and cooked spaghetti alla Bolognese for dinner. In the next day we departed from the accommodation at 5:00am, set the cannons and the net, and waited for a couple of hours. The oystercatchers started to appear on the seaside but they weren’t exactly where the net would reach, so after a few attempts to disturb the group, in order to make them move in the right direction, they flew away. It was already midday and the tide was getting lower again, consequently we removed the net without shooting. This meant that we had to do mist netting in that night. We had a quick lunch and at 15:00 we went to a new location to set the mist nets (Striep). Then we had a dinner in a restaurant and went straight to the location. We caught some curlews, knots, ringed plovers and 18 oystercatchers. We did the rest of the work outside in the surroundings of the accommodation, and released the birds on the nearest dike. In the next day we changed the location of both rows of mist nets (Sehael). While removing the pegs I injured myself and discovered that there are better techniques of handling the rope to take the pegs from the ground (always learning!). At 20:00 we started checking the nets and luckily we caught 51 oystercatchers (and some curlews). At 22:30 we started ringing and measuring the oystercatchers, and releasing them afterwards in the same place. In the next day we removed the nets, packed everything and got to the mainland. From Harlingen we drove straight to Holwerd to catch a boat at 18:00 to Schiermonnikoog.

Mistnetting on Terschelling. - Mistnetten op Terschelling. - Picture by Iryna Litovska

At this moment we were just 5 people (Kees, Magali, Henk-Jan, Katrin and I). It is important to emphasize the fact that cars are not allowed to enter this island. Therefore we had to carry everything (ringing, biometrie, blood related stuff, picture’s stuff, poles, pegs, nets and our luggage) with hand cars by ourselves, first to the boat, then to the taxi, which drove to the Herdershut on Schier. But the tougher part was yet to come. In the field station we prepared couscous with vegetables for the dinner, and had some time to rest. In the next day we prepared all the ringing and measuring material, and after Symen and Martijn arrived we took the hand cars and started the journey. I soon realized that pushing a hand car full of heavy stuff was a great opportunity to grow some muscles. At 15:00 we started setting 3 rows of mist nets on the mudflat, which was beautifully reflecting the sun and the clouds. Then we came back to the Herdershut and had a delightful meal of stamppot. At 19:00 part of the group went to the nets location, and the others waited until the tide got a bit higher. At 21:00 we joined the rest of the group on the mudflat. Most of the crates were already full with oystercatchers. We did one more check in each row of mist nets and afterwards Magali and I carried one hand car full of oystercatchers, and Martijn carried another one to the Herdershut, Koos van Ee helped guiding us (I still find it hard to understand how they knew the path through the darkness). When we reached the station we were quite exhausted, but after a while the warmness started to disappear and it didn’t take much more to realize that my waders were not waterproof at all (at least now we know for sure!). The remaining people checked the nets again and brought a next set of oystercatchers with them to the Herdershut. At 1:00am we started ringing and measuring the birds. We were done around 3:00 and we released all of them right after that. The hematocrit took until 5:00am to finish. In total during this night we captured 86 oystercatchers. In the next day, unexpectedly, we didn’t change the mist nets location. At that moment we didn’t know for sure if we would catch in the next day in Ternaard (at the Frisian coast), so some of us had to go there and check whether it is a good place to catch or not, and then come back to the island. Everyone else stayed in Schier to catch during the night as we did the night before. We arrived at Ternaard’s location at 17:30, but the tide was too low at that time, hence we had dinner and came back around 21:00. We heard some groups of oystercatchers but still weren’t sure if we should catch there. Then we drove to Holwerd’s harbor and there the water taxi was already waiting for us. I have to say that it was an incredible trip that took less than 20min. When we reached the station no one was there, accordingly Herdershut was closed. At 1:15am the rest of the group arrived at the station with 36 oystercatchers. This was our last night of winter catches; we finished at same time as the night before. In the next morning, we started early removing the mist nets, carrying them back to the station and cleaning everything because it was the end of the season.

Getting on the boat with lots of equipment... - Vervoer van spullen op de boot... - Picture by Katrin Schifferle

... and then to the edge of the mudflats. - ... en vervolgens naar de kwelderrand. - Picture by Iryna Litovska

To conclude, this season of winter catches was quite successful if we look at the total amount of oystercatchers that we caught despite some less fortunate catches. We worked in more locations than last year covering a greater number of populations. It is not an easy work, but it easy to understand why so many of us are so passionate about this kind of work. I am more grateful for having this opportunity to be part of this project!

Author: Iryna Litovska

Formulier haalt reacties op...