In January, the Dutch embassy organized a valuable meeting in l’Albufera to explore how typically Dutch expertise can contribute to the integrated management of wetlands in Spain – naturally under the banner of the Dutch national bird! Wouter Vansteelant was invited to speak about the tracking research of BirdEyes, and Rene Faber (Fjildzaken) provided context based on years of field experience. Wouter wrote a blog post about the interplay between satellite technology and hands-on fieldwork.
A blog post by Wouter Vansteelant
Imagine this. You are a Black-tailed Godwit researcher in Spain, and your study area covers more than 200 km² of tancats (polders) with rice fields. Within that vast area, you want to relocate groups of Black-tailed Godwits as quickly as possible after their arrival from Africa, in order to estimate their timing and ultimately also the duration of their stay as accurately as possible. Birds carrying color rings are individually recognizable, and over the course of days you see individuals come and go. But how can you be sure that a bird wasn’t already present in your study area long before you first saw it? Or that it isn’t simply a few kilometers away when you think it has left?
That is the great challenge facing Toni Alcocer, who for more than five springs has headed out every single day into the rice complex of l’Albufera, Valencia. He does so with support from Dutch godwit expert Rene Faber , who is trying to unravel the ins and outs of godwit behavior in this area. Local birdwatchers closely follow the godwit news and tip off Toni when they spot groups. Each spring they naturally encounter familiar individuals that were ringed or tagged in the Frisian polders by researchers from BirdEyes. Sometimes such a bird grows into a real crowd favorite. That was also the case with the tagged godwit Warnserpolder – or TANCAT DE WARNS, as they elegantly call it in refined Valencian.
WARNSERPOLDER in l’Albufera on February 6, 2026, photographed by Toni Alcocer
Toni versus satellite
Warnserpolder is a bird that was fitted with a transmitter in 2019 by researchers from BirdEyes in southwest Friesland, and that has been observed every spring by Toni since 2020. In four of the seven years, Warnserpolder was first seen between 6–9 February. In 2021, 2024, and again this spring of 2026, it was a week later. But are those observations truly representative of the actual arrival time?
Warnserpolder’s satellite transmitter stopped working in 2023, so for the last three years we cannot be certain. However, during the springs of 2020–2023, Toni managed to detect Warnserpolder within 48 hours of the first GPS locations in l’Albufera. Sometimes they already knew the bird had arrived thanks to the “live” positions on Global Flyway Network, the site where all BirdEyes-tagged birds can be followed. But in 2020 and 2023, Toni had already spotted Warnserpolder before the satellite transmitter had recorded and transmitted its first location in l’Albufera. Human and satellite clearly match each other stride for stride. That gives the passionate godwit researcher confidence!
Tracks of WARNSERPOLDER between 2019–2023, after which the transmitter stopped working.
Networking in the wake of the godwit
The story of Warnserpolder and l’Albufera does not stand alone. In Extremadura and along the banks of the Tagus in Portugal, the passage of anonymous and color-ringed godwits is also closely monitored by local researchers and observers from BirdEyes.org. There too we know that many birds are discovered within a day or even just hours after arrival. The observations provide insight not only into timing but also into the birds’ spatial use. Here again we see a striking match between the data from tagged birds and ground-based observations. Tagged birds turn up where we find large groups, and when observers lose track, tagged birds point the way to the flocks. A beautiful interplay of high-tech and old-school, of Frisians and Valencians, of Dutch and Portuguese, of birds and people.
