Avian scavengers are declining throughout the world, and are affected by a large number of threats such as poisoning, electrocution, collision with man-made structures, direct persecution, changes in agricultural practices, landscape composition, and sanitary regulations that can reduce food availability. To formulate effective conservation strategies, it is important to quantify which of these factors has the greatest influence on demographic parameters such as territory occupancy and breeding success, and whether quantitative models can be transferred across geographic regions and political boundaries. We collated territory and nest monitoring data of the endangered Egyptian Vulture Neophron percnopterus in the Balkans to understand the relative influence of various factors on population declines. We monitored occupancy in 87 different territories and breeding performance of 405 territory-monitoring years between 2003 and 2015, with an overall territory occupancy rate of 69% and a mean productivity of 0.80 fledglings per occupied territory. We examined which of 48 different environmental variables were most influential in explaining variation in territory occupancy and breeding success in Bulgaria and Greece, and tested whether these models were transferrable to the Former Yugoslav Republic of Macedonia. Territory occupancy and breeding success were affected by a wide range of environmental variables, each of which had a small effect that may not be the same across political boundaries. Both models had reasonably good discriminative ability [area under the receiver-operated characteristic curve (AUC) for territory occupancy = 0.871, AUC for breeding success = 0.744], but were unsuccessful in predicting occupancy or breeding success in the external validation data set from a different country, possibly because the most influential factors vary geographically. Management focussing on a small number of environmental variables is unlikely to be effective in slowing the decline of Egyptian Vultures on the Balkan Peninsula. We recommend that in the short term the reduction of adult mortality through the enforcement of anti-poison laws, and in the long term the adoption of large-scale landscape conservation programs that retain or restore historical small-scale farming practices may benefit vultures and other biodiversity.