THE EFFECT OF BACKGROUND CUEING ON PREY DETECTION

Studies of prey detection have typically focused on how search image a ffects the capture of cryptic items. This study also considers how bac kground vegetation influences cryptic prey detection. Blue lays, Cyano citta cristata, searched digitized images for two Catocala moths: C. i lia, which is cryptic on oak, and C. relicta, which is cryptic on birc h. Some images contained moths while others did not. The ability of bl ue jays to detect prey during repeated presentations of one prey type within a session was compared with their performance during randomly a lternating presentations of both prey types within a session to examin e search-image formation under two background conditions (informative and ambiguous). In the informative background condition, both trees in the image were of the same species and therefore, the background was a reliable indicator of which prey type might be present. In the ambig uous background condition, there was one tree of each species in the i mage and either prey type could be present. The results indicate that: (1) a search-image effect was observed only for the more cryptic prey type and only when the background was informative; (2) as accuracy on prey images (those with moths) increased, response latency remained u nchanged; (3) performance on nonprey images (those without moths) was primarily determined by the difficulty of searching the background and not by the prey type in the accompanying prey images; and (4) search- image effects disappeared with extended practice. These results sugges t that the ability to detect prey is influenced by background and that the presence of either multiple backgrounds or multiple prey types in terferes with search-image formation. (C) 1998 The Association for the Study of Animal Behaviour.