B5RY: Larus michahellis

12 August 2023

Last Thursday 10th August, shortly after 11 a.m., as part of our routine work at the Port d'Eivissa, where we provide wildlife control services, we were lucky enough to be able to read a PVC ring of a yellow-legged gull (Larus michahellis).

As we have indicated, this is a Yellow-legged Gull, and the exciting thing (so to speak) about the sighting is the rings on its legs: a metal one on its left leg, and an orange PVC one on its right leg. Both rings have similar, but different functions.

Are so many rings necessary?

As can be seen in the photos, in the metal ring, due to its size and orientation, it is much more complicated to read the code written on it; while in the PVC ring, despite its damage, it is much more feasible to read the code. Therefore, both rings have a code, and this code will give us the same information, i.e. the biometric data collected at the time of tagging and the date, location and person responsible for the tagging. However, without the PVC ring it would be almost impossible to have this remote reading, i.e. without capturing it. Therefore, the sense of putting 2 rings, a small, resistant metal one, which is the official one (the one you have to put on), and a PVC one, more visible, without mandatory character and more susceptible to damage, lies in being able to access the unique information of that individual in an easier way (at a distance), but at the same time making sure that if something happens to the PVC ring, the metal one is still there.

On the other hand, and probably equally or more important than the previous aspects (after all, materials are constantly evolving), a ring with large codes will inevitably take up more space for a smaller series of numbers and letters than one with smaller ones. That is to say: to mark birds routinely, official metal rings with small codes are used, while for specific projects, where the aim is to be able to make remote readings for some justified reason related to the marking project itself, after requesting specific authorisation to ensure that codes are not duplicated (this is very important in both metal and PVC rings), PVC rings with specific colour and alphanumeric codes are used. In this case, the colour code is orange engraved in black, with the letter-number-letter-letter-letter combination.

What we can know about B5RY

And what does this PVC ring tell us about the bird in question? Well, that it is a male yellow-legged gull ringed as a chick in the nest on the neighbouring island of Sa Dragonera, specifically on 30 April 2016. The project to which it belongs is part of the IMEDEA-CSIC's study of the yellow-legged gull population, and it seems to be the first record of this specimen since it was ringed.

This marking project in particular, aims to better understand the degree of regional dispersion of the species, and above all, its degree of dispersion during migration, since outside the breeding season, much of the population of yellow-legged gulls leave the Balearic Islands, to go to other parts of the Mediterranean coast, but also reach the Cantabrian Sea and even the Atlantic, To better unravel these routes, apart from PVC rings, some specimens have also been marked with GPS transmitters. This has made it possible, for example, to know the exact route taken by a specimen in 2017 to the Cantabrian Sea, following the course of the Ebro and finally crossing the Basque mountains. Below, in the "To know more" section, for those who want to find out more about this subject, there are links to the original sources. With these markings, we have also studied how the closures of some landfills in the Balearic Islands have affected local populations.

On 5 May, also in Port d'Eivissa, we detected a specimen of the same project with a GPS transmitter, but in this case, without being able to read the ring. For those who are not familiar with how GPS transmitters work in wildlife tracking, the transmitters do not work constantly, they emit signals only when requested and are very limited by the battery, which is why they have solar panels. Therefore, despite having a transmitter, the PVC ring still fulfils its function of tracking the bird.

Is B5RY OK?

To the attentive eye or with some experience with wildlife, it will not have gone unnoticed neither the strange posture of the bird (it does not rest its leg well), nor the certain degree of slovenliness in the state and order of its feathers, and above all, the wound on its right leg, which looks a little infected. Indeed, B5RY has some kind of recent injury, which, although it does not prevent him from flying, clearly affects him. Injuries to wildlife are often fatal, because of the negative impact they can have on their ability to find food or stay safe from predators.

Fortunately for him, B5RY is an adult Larus michahellis, that is, a bird with character, opportunistic, in good health and not subject to particularly high predation pressure (although it can be, for example, prey for a peregrine falcon (Falco peregrinus) or Bonelli's eagle (Aquila fasciata)). Undoubtedly, these are characteristics that give him an extra point in the long-distance race he is fighting to recover from this injury... we'll see if we are lucky enough to see him again in Port d`Eivissa!