A long term study on initial orientation in sand martins (Riparia riparia)

Over a period of 18 years sand martins (Riparia riparia) were displaced fro m their breeding colonies and released to investigate the relationship betw een distance of displacement and initial orientation. The results show sign ificant homeward orientation at the shortest distances of displacement (7.1 -15.2 km, series A), bur not over longer distances (49-180 km, series B and C). Birds displaced over 49 lull from their breeding colonies only seldom show homeward directedness, while pseudo-pooling and second order analysis of their directional choices seem to indicate the tendency to fly in a give n compass direction. Possible causal factors that can induce this behaviour in sand martins are discussed, taking into account not only local biasing factors but also the existence of a so called preferred compass direction. These data show that sand martins Ne able to determine the correct homing d irection inside a probably familiar area. The response over longer distance s indicates difficulties in determining the appropriate homing course at ta keoff.