RESPONSES TO DISTRESS CALLS BY BLACK-HEADED GULLS, LARUS-RIDIBUNDUS - THE ROLE OF NONDEGRADED FEATURES

Distress calls of black-headed gulls have a complex structure. They are efficient over long distances, despite the alterations that occur during propagation. This degradation was analysed by broadcasting distress calls in the natural environment where gulls spent most of the day. Degradation was greatest for amplitude modulation (AM) and also for fast frequency modulation (FM). It was least for slow FM and for the lowest harmonics of the complex carrier frequency. Synthesized distress calls that contained only these non-degradable structures (some without AM, others with a simplified FM) elicited normal responses from the gulls. This suggests that the gulls could use only those parameters that were not degraded, which may be an adaptation to the constraints of the transmission channel. © 1990 The Association for the Study of Animal Behaviour.