By Ron Efrat
They say it is the age of big data, but how do you get big data when your study species is globally endangered? You collaborate, big time! In a new study published in Journal of Animal Ecology, we studied the migratory population of the Egyptian vulture (Neophron percnopterus) using data from 12 countries, acquired over 12 years by 38 researchers. Our data span the species’ breeding grounds from western Europe through the Balkans and the Caucasus to the Middle East. Including the migratory movements, we covered about 70% of the species total range. Overall, we used GPS data from 220 vultures, to study when, where and why do Egyptian vultures die throughout the annual cycle.
Of the 220 vultures we classified 52 as confirmed mortalities (in which the dead vulture was located in the field) and 38 as assumed mortalities according to data from the transmitters. The probability for a vulture to survive was calculated according to its age, whether it was migrating or in a stationary period and whether it was north or south of the center of the Sahara Desert. We also tested the effects of the geographic origin by separating western Europe, which is the only population that isn’t decreasing and has a different and shorter migration route, from the rest of the populations. Finally, we tested whether there’s a difference between birds that were reared in captivity as part of a conservation project and those reared in the wild.
We found that western and older vultures survived better and that during migration survival was lower than during stationary periods. We found small differences in survival between captive and wild reared vultures, but these differences were not supported statistically and should be further studied. Finally, we found that vultures survived better when they were at the wintering, more southern grounds, than at the breeding, more northern grounds and that at least 49% of the confirmed mortalities were human induced.
Our findings first strengthen the existing theory and literature by showing that migration and young age are the most dangerous periods for migrating birds. However, our finding of higher mortality at northern latitudes and the very high effect of human activities on Egyptian vultures’ survival are both surprising and disturbing. The human induced mortality included mainly electrocution, collision with energy infrastructure and poisoning and was mainly based in the north-eastern part of our study area. This suggests a possible change in the migratory life-cycle tradeoff: if in the past the higher mortality during migration was rewarded by higher survival and higher fecundity at northern latitudes, human-induced mortality might mean that this tradeoff no longer exists. Further research is required to ascertain this hypothesis by studying the fitness of migratory vs. stationary Egyptian vultures, i.e. not only their survival but also the number of fledglings their migratory or stationary strategies allow them to raise throughout their lives.
This study was the second, but not last, that came out of this large collaboration. We started with 60 vultures when we wrote our first paper about the migration routes of Egyptian vultures, and got to 220 in the current paper. We are already thinking about future issues to study using this great dataset. Look for updates soon 😊
You can find the paper here: https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2656.13449