Naturespeak A naturalist's view of the world

July 4, 2010

Little Momma

Filed under: Uncategorized — wykes @ 9:16 pm

One thing you will never see is a cowbird young being fed by a cowbird momma. It just doesn’t happen because cowbirds never raise their own young. As nest parasites, cowbirds lay their eggs in other bird’s nests and let the “host birds” do the task for them. So, the sight of a tiny Chipping Sparrow diligently feeding a hulking cowbird chick (like shown above) is only slightly unusual.

Cowbirds are known to use over 40 host species in Michigan, but Chipping Sparrows are generally at the bottom of their victim list. Field Sparrows are preferred over Chippers by a ratio of nearly 9:1 (approximately, of course). One of the reasons has to do with the color of their eggs. When female cowbirds perform sneak egg-laying, they carefully remove one of the host bird eggs first and then later replace it with one of their own. They tend to look for eggs similar to their own brown-speckled white eggs. It is a matter of fact that Field Sparrow eggs, which tend to the creamy white and speckled side, look more “cowbirdy” than the blueish spotted eggs of the Chipping Sparrow. But, apparently this is not a set rule in cowbird land as evidenced by the mom/chick pair that occupied my front lawn for several days. They were living proof of at least one successful act of Cowbird/Chipping Sparrow sabotage.

The diminutive sparrow was stuffing insects into the open maw of her monster chick at a record pace – at least three times a minute. She was as diligent and prideful as if it were her own flesh and feather. In typical fashion, the nearly full grown cowbird would stoop forward, open her mouth, and wiggle her wings whenever her surrogate mom would approach with some recently acquired prize.

As a human, it is hard for us to understand how this mom could fail to recognize that the out-sized specimen before her was not one of her own offspring. You are tempted to yell out something, but there are no words to convey the proper thought without sounding callous (i.e. “this chick killed your babies – let it starve”). Human nature is not nature nature, however.

In a natural setting this chick was one of her very own from the beginning. She was there when the egg hatched and bound by instinct to care for her charges. The blindness of instinctive motherhood required her to feed the biggest mouth in the nest and thus starve out her own young in favor of the rapidly growing cowbird. In larger birds, the cowbird is able to become one of the brood but in small birds, like this one, the even smaller nest mates usually die – leaving the cowbird as the sole chick.

There is no need to get angry at the bumbling chick any more than it is to get frustrated at the zealous mom. In fact, this is cause to reflect on the ingenious system created by cowbirds long ago. It is believed cowbirds evolved alongside wandering herds of western buffalo. They ate the insects kicked up by the bison. Since bison herds are nomadic, the cowbirds had little time to nest and raise their own young before their food source lumbered on so they developed the ability to put their eggs out for adoption. The tactic was so successful that the bird continued to employ it long after the bison disappeared. Sure is has been damaging to some local bird populations, but that is another concern.

We are left then to view this mismatched mother/daughter pair (see here) in a strictly historical and behavioral sense and leave the hysterical sense for another time.

2 Comments »

  1. I learn the most fascinating things here.

    Comment by Monica the Garden Faerie — July 5, 2010 @ 2:05 am

  2. Check out P.D. Eastman’s “Flap you Wings!” for an imaginative take on nest predation.

    As usual, we are the species to blame for the destruction of the bison herds and the expansion of the cowbird range.

    Comment by barefootheart — July 10, 2010 @ 11:05 am

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