Grosbeaks and Monarchs

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Grosbeaks and Monarchs -
Black-headed Grosbeaks Dine on Poisonous Butterflies

Written by Bob Sundstrom

This is BirdNote!
    [Black-headed Grosbeak song]
    The Black-headed Grosbeak’s rollicking refrain may be the most joyous bird song of the western summer. Beginning in early May and continuing right through July, the Black-headed Grosbeak sings and sings: from the treetops, from deep in the leaves, and even while sitting on its nest. [Black-headed Grosbeak song]
    Black-headed Grosbeaks are one of very few birds that regularly eat Monarch butterflies, which most birds and other animals find unpalatable, if not downright toxic.
    The caterpillars of Monarch butterflies consume milkweeds containing toxic substances known as cardenolides (pronounced kar-DEN-oh-lydz). The poison is stored in the adult butterfly’s abdomen, giving it a powerful form of defense.
    Many Black-headed Grosbeaks winter in the same areas of central Mexico as millions of Monarchs. The birds consume a great number of Monarchs through the winter, including their toxin-laden abdomens. One other bird in the area, the Black-backed Oriole, also preys on the butterflies, but studiously avoids the tainted abdomen.
    So perhaps what other birds find toxic is just the tonic the grosbeak needs to put that jazzy feel in its song.
[Black-headed Grosbeak song]
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Song of the Black-headed Grosbeak 126546 provided by The Macaulay Library at the Cornell Lab of Ornithology, Ithaca, New York. Recorded by T.G. Sander.
Producer: John Kessler
Executive Producer: Chris Peterson
© 2012 Tune In to Nature.org   June 2012   Narrator: Mary McCann

ID#062507BHGR2KPLU      BHGR-02b

Key source: Journey North website, 2004 article contributed by Bill Calvert.