Stick supply to nests by cliff-nesting raptors as an evolutionary load of past tree-nesting

The supply of sticks to cliff nests by many European raptors has been explained only as a functional means of decreasing ectoparasite loads in nests and for signalling nest occupancy. We provide here a historical explanation of this behaviour as we consider it represents an evolutionary load of formerly tree-nesting species. Basically, from this perspective, facultative tree/cliff-nesting species reproduce the nests they used to build originally on top of trees, but on cliffs. Facultative species (likely evolved in forested areas) that supply their cliff nests with sticks include Pandion haliaetus, Haliaetus albicilla, Milvus migrans, Circaetus gallicus, Buteo buteo, Aquila fasciata, A. pennata, A. chrysaetos, A. heliaca, Gypaetus barbatus Gyps fulvus and Neophron percnopterus. On the contrary, the only Falco species that solely nests in cliffs (F. eleonorae)) and does not supply its nests with sticks and should be considered a true cliff-nester, likely evolved in non-forested areas. All other Falco species that do not supply their cliff nests with sticks but can make use of tree nests made by other non-raptorial species, should also be considered as true cliff-nesters, likely evolved in more forested areas or times. Milvus milvus, Elanus caeruleus, Accipiter nisus, A. gentilis, Pernis apivorus, Aquila adalberti, A. clanga, A. pomarina and Aegypius monachus are true tree nesters, likely evolved in forested areas, which did not evolve the plasticity to nest directly on cliffs.