In April 2017, Polish environmental group EkoLogiczna placed a GPS tracker on the back of Kajtek, a white stork, to collect data about the bird's winter migration route as well as its feeding habits.
In the year that followed, Kajtek travelled some 6,000 kilometres but was last recorded on 26 April in the Blue Nile Valley in eastern Sudan.
In a Facebook post, the group explains that "for unknown reasons" Kajtek stayed in the area for over two months, "travelling 25 kms in different directions during the day".
Last month, EkoLogiczna was surprised with a phone bill linked to the SIM card installed in the Kajtek's GPS tracker for a total amount of 10,000 PLN (€2,278).
"Someone quite simply removed the card from the tracker, put it in a phone and used it for 20 hours of communication," the group said in the Facebook post.
According to the EuroNatur Foundation, Poland is a major centre of distribution for the white stork, accounting for 40% of the bird's world population.
Alison Tidey is a freelance journalist for NBCNews, Euronews. She has reported for Agence France-Presse while based in London and for CNBCi.