Tuesday, November 22, 2016

Bison and Magpie

Look closely at this Bison from Yellowstone National Park. A Black-billed Magpie is perched on its head. Magpies form symbiotic relationships with large ungulates. The magpie feeds on ticks and other ectoparasties. The advantage to the bison is obvious, but the situation may be more complicated. Magpies cache the ticks for later consumption, but often don’t kill the ticks first. If the ticks survive, they reproduce, thus increasing tick numbers. For the bison, tick borne illnesses and outright blood loss from tick wounds can cause deaths. Magpies feast on the bison carcasses, and both carcasses and ticks are probably critical food sources for the birds (Trost 1999).

In yet another twist, I have been told that ranchers often treat cattle with poisons to kill ticks. If their cattle die from other causes, the poisons sometimes collaterally kill scavengers like magpies and Golden Eagles.

Magpies are omnivorous, consuming grains, vegetable matter, and both living and dead animals. Magpies cache food for only a couple of days. Often their caches are stolen by other magpies, that find the caches by smell. Female magpies watch males cache food and later steal the males’ hoard. Males will often move caches away from carcasses on which they feed and take the food closer to their nests. Magpies cache whatever food is most abundant. Both wild and pet magpies also cache shiny trash.

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