Do predator energy demands or previous exposure influence protection by aposematic coloration of prey?

Abstract

Growing evidence exists that aposematic and toxic prey may be included in a predator's diet whenthe predator experiences physiological stress. The tree sparrow Passer montanus is known to havea significant portion of aposematic and toxic ladybirds in its natural diet. Here, we present experimentstesting the attack and eating rate of the tree sparrow toward the invasive aposematic harlequinladybird Harmonia axyridis. We wondered whether the sparrow's ability to prey on nativeladybirds predisposes them to also prey on harlequin ladybirds. We compared the attack and eatingrates of tree sparrows of particular age and/or experience classes to test for any changes duringontogeny (hand-reared × young wild-caught ×adult wild-caught) and with differing perceived levelsof physiological stress (summer adult × winter adult). Winter adult tree sparrows commonly attackedand ate the offered ladybirds with no evidence of disgust or metabolic difficulties after ingestion.Naïve and wild immature tree sparrows attacked the ladybirds but hesitated to eat them.Adult tree sparrows caught in the summer avoided attacking the ladybirds. These results suggestthat tree sparrows are able to cope with chemicals ingested along with the ladybirds. This preadaptationenables them to include ladybirds in their diet; though they commonly do this only intimes of shortage in insect availability (winter). Young sparrows showed avoidance toward thechemical protection of the ladybirds.