Similarly, stimuli that communicate a "warning signal" to the target bird that a predator is nearby (e.g., distress/alarm calls), or has been in the vicinity (e.g., model of dead prey bird), work well. Even with the best models, unless the presentation is occasionally supplemented (with killing, for example), birds eventually learn that there is no real threat. A real owl tethered to a post works even better. They will habituate more slowly to a stuffed owl with a crow in its talons that moves and calls. Birds will quickly habituate to a "Canadian Tire" owl. The period of effectiveness is related directly to the realism of the model – in appearance, behaviour, and sound. The biological basis behind bird control products/techniques that mimic known threats to birds, such as scarecrows and hawk kites, tends to be stronger and longer-lived. Although there is a biological basis to these products, any deterrent/dispersal effects are short-lived. Similarly, "startle" devices (e.g., gas cannons) lose their effectiveness once they become an expected part of the birds' environment and no longer startle. ![]() ![]() (Some birds may initially investigate, rather than avoid, a novel stimulus.) Once the stimulus is no longer novel, however (and birds quickly learn what is a threat and what is not), the stimulus has lost its effectiveness on those birds. Birds tend to avoid any novel stimulus, such as the synthetic sounds produced electronically by the Phoenix Wailer, because birds do not know whether this is a threat or not. Many of the least effective products/techniques are based on the presentation of novel stimuli and/or stimuli that startle birds by the suddenness or loudness of their presentation. Bird control products can be categorized by the manner in which they deter or disperse birds - novelty avoidance, startle reaction, predator mimics, warning signals, and killing (Rochard 1996).
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