It may be a sin to kill a mockingbird but that doesn’t keep a cowbird from trying. Or at the very least harassing the bird by laying eggs in their nest, a behavior biologists call brood parasitism.
A study published in December, however, makes a good case for mockingbirds that take care of themselves and offers up a new answer as to why some cowbirds get away with exploiting mockingbird parents.
In an experiment in Argentina, researchers examined the relationship between chalk-browed mockingbirds and the parasitic shiny cowbird. These cowbirds are particularly nasty neighbors. In addition to the usual nest parasitism they also periodically attack mockingbird nests, puncturing eggs present.
The presence of their own species’ eggs doesn’t deter the indiscriminate cowbird, but those extra eggs do dilute the danger of puncture to a mockingbird’s own young. In fact, the more cowbird eggs present, the better the chances that the mockingbird’s babies are safe, simply because the brutal intruder is more likely to hit a cowbird egg instead. The researchers found that the more parasites in the area (and therefore greater attack risk) the greater the advantage to being parasitized. Score one for the mockingbird!
To see a cowbird attack and mockingbird clean-up in action, check out the video below. For more on brood parasites, Michael Marshall’s Zoologger blog does a nice job rounding up some examples.
Mockingbird fans also shouldn't miss Katherine Tweed’s post on their impressive memories.