aerodynamic forces
ascending
beak
bird flight
birds
blue sky
common house martin
complex movements
concrete ceiling
dark blue head
delichon urbicum
eave
estremadura
fast aerobatics
fauna
flygin
head
mature
migratory
motion
movement
nature
olivenza
one animal
roof tiles
summer season
swallow
upward force
urban human habitation
vertical
western house martin
white rump
wings