Parasites

Molothrus aterWell, if I don’t recognize it, how will the other birds?

Spotted in Marine Park’s wild west side a week ago: the identity of this bird baffled me for while. And then it hit me. Young Brown-headed Cowbird (Molothrus ater). This bird was raised by another species, for Brown-headed Cowbirds are brood parasites: they lay their eggs in the nests of other bird species. Yellow Warbler, Song Sparrow, Chipping Sparrow, Red-eyed Vireo, Eastern Towhee, and Red-winged Blackbird are typical targets, but there are 220 species possibilities! Talk about adaptation.

Some birds will recognize the alien egg and push it out, or, if not strong enough for that, build a new nest on top to it. Other species, however, can’t recognize that the egg doesn’t belong to them (even though it may be larger). Hatchling cowbirds will then out-compete if not outright kill their step-siblings in the nest.

Brood parasitism is a remarkable adaptation by several bird species around the world. The BHC use other nests because, we think, they followed the bison around the grasslands of the American west. This left them no time to make a nest and brood a clutch of eggs themselves. They’ve expanded their range eastward as we’ve destroyed forests and otherwise paved paradise.

Some people get very moralistic about BHCs–the birds can negatively affect rare bird populations–but as usual the problem was created and/or exacerbated by us humans, definitely the world champion nest-wreckers.
Pipilo erythrophthalmusAlso, close by was a singing Eastern Towhee male (Pipilo erythrophthalmus) atop a cherry tree. Coincidence?

6 Responses to “Parasites”


  1. 1 belindagroverphotography August 7, 2016 at 8:31 am

    Interesting post, terrific shots!

  2. 3 Kass August 7, 2016 at 8:56 am

    I think you have solved a mystery for me. There was a dead bird on our side porch, looking very much like this one, that I could not identify. We have had phoebes nesting in our eves sporadically for many years. Was this guy pushed out, the last hatchling left, and subsequently abandoned, I don’t know. Even if I am wrong, it’s all fascinating.
    Thanks!

    • 4 mthew August 7, 2016 at 10:07 am

      Isn’t it? There’s so much going on out there! I remember hearing that a single female Brown-headed Cowbird can lay egg after egg in a nest if the first (etc.) is disposed of. The determination to continue those genes is awe-inspiring.

  3. 5 elwnyc August 7, 2016 at 10:08 am

    I find it interesting that young cowbirds don’t imprint on their care providers but manage to seek out their own species to breed. Do you know if there are any studies investigating whether cowbirds tend to lay their eggs in the nests of their own foster parent species?

    • 6 mthew August 7, 2016 at 10:28 am

      I don’t know the answer to that. I understand that some BHCs will specialize in specific species for nesting, but don’t know if this is transmitted to the next generation.

      How also do they learn to sing like other cow birds? Not that they’re not noted singers…


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