While on the subject of black headed grosbeaks:
Monarch butterflies feed on milkweeds containing cardenolide derivatives, steroids that are poisonous and usually affect the heart. Cardenolides accumulate in the monarch butterflies that ingest them and act as a defense mechanism. Most vertebrate predators of monarch butterflies avoid the monarchs because of the butterflies’ bitter taste and toxicity. Many predators recognize the monarch’s bright color pattern and avoid them. Others, such as jays will vomit after eating poisonous monarch butterflies.
Two birds, the black backed oriole and the black headed grosbeak (Pheucticus melancephalus) have a high tolerance to cardenolides and can eat these beautiful, poisonous butterflies. The black backed oriole is endemic to Mexico and I probably will never see one. So I will skip discussing its tolerance to monarch butterflies.
Black headed grosbeaks, as I mentioned in an earlier post, winter in Central Mexico – the same area the monarch butterflies overwinter. According to some references, the black backed oriole and black headed grosbeak account for over 60% of all monarch butterfly mortality on their wintering grounds.
Black headed grosbeaks remove the two wings of monarch butterflies before eating their abdomen. The monarch butterfly abdomen contains high concentrations of cardenolides with the cardenolide concentration in females being as much as 30% higher than the concentration in males. Black headed grosbeaks selectively eat more male butterflies than female butterflies, thus reducing the concentration ingested. The grosbeaks also eat monarchs in eight day cycles. The time between feedings is thought to allow elimination of excess toxins from their bodies. Additionally, black headed grosbeaks appear to have a natural moderate tolerance to cardenolides.
The monarch butterfly’s defense mechanism, although highly effective, is not entirely successful. Our summer resident, the black headed grosbeak is capable of thwarting the butterfly’s toxic effects.
This male black headed grosbeak was photographed last spring on Lower Hat Creek (Shasta County CA).