Heteroptera are often preyed upon by birds in spite of their noxious scent-gland secretions. Consequently, the question is raised as to whether or not birds possess physiological immunity to the secretions, simply tolerate any discomfort, or use special prey-handling techniques.
This study reports the responses of 12 European blackbirds, ranging in age from 45 days to 8 years, to 583 adult Heteroptera from 8 species (Coreidae, Pyrrhocoridae, Pentatomidae). The main results are:
(1) The non-aposematic bugs were eaten (63.1%) far more often than the aposematic ones (16.3%). Palomena prasina and Coreus marginatus had the highest edibility rank; Eurydema ornatum, the lowest. The highest rate of predation took place in the youngest birds.
(2) The metathoracic secretion of 4 of the pentatomid species functioned as a topical contactirritant of the buccal cavity, head and eyes; the other test species provoked little or no discomfort. Some birds showed little sensitivity to the secretions. Among the behaviors induced were head-shaking, dirt-eating, respiratory difficulties, head-scratching, and eye-blinking. One bird engaged in “anting” with Graphosoma lineatum. No intoxication resulted from ingestion of the bugs.
(3) Several prey-handling techniques were used: seize, mandibulate, peck, shake, and wipe. The last three are probably correlated with the distinct physical properties of the Heteroptera. In general, the bugs were handled more during the first trials than during the last. The blackbirds learned to modify their handling techniques in such ways as to avoid or minimize exposure to the insects' secretions.
(4) The caustic action of the scent-gland secretion did not constitute an effective deterrent. Rejection of the Heteroptera was primarily based on gustatory cues.