A Tale of Two Birds



Photo by Dendroica cerulea

    This morning I saw this bird on the deck outside my office at Lutherlyn's nature center.  It is a male brown-headed cowbird (Molothrus ater).  I have a love/hate relationship with these birds.  Actually, its more of a respect/derision relationship.  If you are familiar with the brown-headed cowbird, then I imagine that you understand why I have such a range of emotions associated with a robin-sized blackbird.
  I find the appearance of this bird striking because its black plumage has an iridescence that catches the eye, yet it has a brown head. (To me, the bird appears to be a strange hybrid of a blackbird and a female rufous-sided towhee.)
  The respect part of the equation comes from knowledge of its adaptations.  Brown-headed cowbird lived for centuries on the Great Plains following the bison herds.  These birds lived off of the insects that the bison kicked up as they grazed.  Since bison herds didn't stay in one location for very long, the cowbirds didn't have time to raise their young before the herd moved on.  A unique, if lazy, adaptation enabled them to survive - the cowbird females laid their eggs in the nests of other birds.  So all cowbirds have been raised by surrogate moms, often to the detriment of the host's young, as young cowbirds are very aggressive and will often out-compete their foster siblings.
   As the bison herds disappeared, the cowbirds had to adapt once again.  This time they moved east and started occupying the newly cleared farmlands of the eastern U.S.  They no longer had to follow bison herds and had the time to raise their own young - but now they encountered other bird species who were not familiar with their parasitic ways and did a fine job of raising their children for them - so why change?


male Scarlet Tanager

    I imagine that you can now see where the feeling of derision comes into play.  Let me introduce you to the brightest bird in our forest:  the male scarlet tanager (Piranga olivacea).  This bird travels from South America to reproduce in the mature oak forest of Pennsylvania.  In fact, 19% of all the scarlet tanagers in the world nest in PA and spend their summer eating lots of our insects.
  However, tanager numbers are declining and have been for some time.  One of the culprits is the brown-headed cowbird.  Its parasitic ways are causing more scarlet tanagers to spend their summers raising cowbird young - and the tanager's don't even seem to realize it.
   In the end, the blame can not be placed on the cowbirds, though.  Scarlet tanagers like to nest in deep forest habitats and cowbirds are usually only on the edge of forests.  However, as people continue to carve up our mature forests for homes, logging, and shale gas development, they are exposing the tanager's habitat to cowbirds.
  So, my derision is misplaced, as the cowbirds would not be able to reach the tanagers, if our mature forests were kept intact.  Indeed this is yet another example of how everything in creation is intricately connected - and how easy it is for people to unwittingly throw off the delicate balance that was meant to be.