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  • Survival of the Swiftest

    Monarchs face 3 predators during the winter months in Mexico. Mice visit the sanctuary and feast on the butterflies, as do 2 bird species. Here are 2 unlucky monarchs. What do you suppose happened to the monarch on the left? Notice the wings of the monarch on the right are shiny and dark. This is from the fat that was in the monarch's abdomen. When the bird slit the body, the fat was released.

    Monarch Predation by Birds
    Information for this report came from "Interspecific Comparisons of the Foraging Dynamics of Black-Backed Orioles and Black-Headed Grosbeaks on Overwintering Monarch Butterflies in Mexico" by Alfredo Arellano G.,John I. Glendinning, James B. Anderson, and Lincoln P. Brower

    Summarized by Adam Lipschultz and Anna Curtis, Blake School

    "While the monarchs overwinter in Mexico their two greatest predators are the black-headed grosbeaks and the black-backed orioles. The birds, depending on the size of the reserve, can eat anywhere between nine and forty four percent of the population. The percentage of monarchs eaten by these two birds is relatively high when compared to other insects. For example, the birds eat less than one percent of the spruce budworm in Canada.

    "Each bird has a different way of ingesting the monarchs because of the toxic cardenolides that are stored in the bodies of the monarchs (as a result of eating milkweed as larvae). The grosbeak has a higher immunity to the toxin so it is able to ingest more of the butterflies without vomiting. The orioles, on the other hand, are more sensitive to the toxin. They use their longer, sharper beaks to slit the butterflies abdominal cavity and suck out the less toxic parts of the monarchs.

    "The orioles have a shorter feeding cycle than the grosbeak that varies depending on the temperature. Both birds eat more monarchs when the temperature is colder. The orioles feeding cycle is shortened in the cold. Although when it is cold, they eat more during each cycle. The birds feed for a number of days until they become sick from the toxins, and resume eating when the toxin level in their bodies has gone down.

    "This data was gathered over several years in the reserves in Mexico (see footnote below). Nets were placed at the base of the trees to catch the dead bodies of the monarchs. They were examined to determine the sex of each butterfly and, depending on how the bird was eaten, which type of bird had eaten them. There was a weak correlation between the sex of the butterflies and the bird which had eaten them. This may mean that the birds are selective about preying on male or female butterflies. Why they would select one sex over the other is not certain."

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