KEY FINDINGS – 2021 and 2022 tagging seasons
05.09.22 – All our GPS-GSM deployed in Malta and Tunisia on Yelkouan shearwaters have now stopped transmitting – roughly around the same period as for our tracking of Croatian yelkouans last year. Fixed to the bird feathers with water-resistant tape, the transmitters are expected to come off after a few weeks, in order to make the study minimally intrusive for a species classified as Vulnerable by IUCN (2018). It is now time to draw some preliminary conclusions at this stage, based on our two seasons of tracking.
The strength of this project is the deployment for the first time of highly accurate devices (GPS-GSM) to explore the poorly known post-breeding movements of Yelkouan shearwaters. This fine-tuned tracking has been set so as to record location every 45 min (on average), adjusted to a 10 min frequency on critical passages (e.g. the Bosphorus crossing), with a live tracking day after day. In total, 48 GPS-GSM were deployed between 2021 and 2022 in nesting sites located three countries: Croatia, Malta and Tunisia (see Table 1).
Table 1. Summary data of the tracking of adults and juveniles Yelkouan shearwaters in 2021 and 2022.
- Migration to the Black Sea
Last year, we discovered that one third of the Yelkouans we tracked from Croatia traveled 2500 km to reach the Black Sea, with asynchronous but similar trajectories. While some birds’ signals stopped in vicinity to the Sporades Islands or near the Gulf of Patras, the others were flying in the northern/central Adriatic when we lost, one by one, their signal. Are we facing distinct migration strategies? Or was it just a delay for some individuals in reaching the Black Sea? This year, from our sample, two male adults from Malta also reached the eastern Black Sea (Fig. 1), suggesting a possible common pattern driving the species all the way there during the post-breeding period in order to feed on the vast resources of juvenile anchovies.
Figure 1. Trajectories of yelkouans shearwaters that made the journey to the Black Sea.
- Exploration of the southern coasts
This second year of tracking also revealed some journeys from Malta towards the southern coasts (Tunisia, Libya and Egypt), whereas it was not the case last year for birds leaving Croatia. This information is precious since the distribution of the species is almost unknown in these areas. It is interesting here that only adults visited the Gulf of Gabes, an area rich in nutrients, before heading in a quite different direction – whereas individuals that reached Libya and Egypt were only juveniles. The latter may have strayed (out of ignorance?) to regions that are much poorer in food, like the one found dead on a beach east of Alexandria.
- Unaccompanied minor: a strong independence from their parents
Among the individuals that reached the Black Sea, some juveniles caught our attention. Is it common to undertake such a long migration at this early age? Are the juveniles guided by their parents? For this second year of tracking, we focused on these questions and deployed efforts to track ‘full families’ consisting of both parents and their juvenile. We discovered that juveniles left their nest alone without the help of their parents. Even more striking, each member (mother / father / juvenile) followed significantly different routes. If we cannot omit a possible accompaniment of juveniles by other conspecifics, we clearly see here a strong independence from the parents.
