| from the March 31, 2002 Newsletter, issued from near
Natchez, Mississippi: BROWN-HEADED COWBIRD MORALITY Thursday morning as I prepared my campfire breakfast, four blackish, dumpy-looking birds landed in the top of a nearby Pecan tree. They made squeaky gurgling calls and fluttered in a curious way so even without binoculars I knew I had four Brown-headed Cowbirds. I watched as three males orbited around a female displaying. During the "Bill-Tilt Display" a male would lift his head and point his bill skyward. This would often be followed by the "Topple-Over Display," during which the bird would fluff his body feathers, arch his neck, spread his tail and wings, and sort of lurch forward, sometimes issuing the gurgling song. Apparently these displays excite females, and probably females choose to mate with males doing the best job. The above Web address goes into more detail about these and other cowbird displays. Female cowbirds do not lay eggs in their own nests. Being careful to go unobserved, sneaking quietly through undergrowth or among dense leaves, they look for the nests of birds of other species. Often they locate nests still under construction. Then the female cowbird watches the nest until egg laying begins, and one dawn she sneaks in, removes and sometimes eats the nest-owner's egg, and lays her own. If only one "host" egg is present, she does not remove it, apparently because doing so might clue the nest owner that something is amiss, and the nest might be abandoned. Not only do cowbird eggs usually hatch one day ahead of the host's eggs, but also cowbird nestlings typically are larger, are more aggressive in begging for food, and grow faster than the host's own young. Even when the cowbird fledgling grows much larger than the host mother herself, the mother just doesn't catch on that there's a problem. Of course this is hard on "host" families. Before humans began cutting up the landscape, cowbird "nest parasitism" wasn't as important as it is now because cowbirds in most places tend to focus their activities in open areas and forest edges. However now humans have broken vast forests into tiny plots and there are so many access roads that many remaining forests consist of nothing but "ecological edges." Cowbird nest parasitism is a very serious problem contributing to the ongoing collapse of many bird populations. Species hurt particularly hard include the Song Sparrow, Chipping Sparrow, Eastern Phoebe, and Northern Cardinal. How can Mother Nature tolerate such a free-loading species? Maybe we just have to recognize that Nature rejoices over diversity to the same extent that She doesn't really care much whether individuals like you and me get exactly what we want. Nature exults in the robust feeling embodied in the music, not in the destinies of us individual notes comprising the score. from the June 27, 2009 Newsletter, issued from the
Siskiyou Mountains west of Grants Pass, Oregon: However, once the great forests were broken into today's tiny woodlots, vast acreages became "forest edge" habitat very inviting to cowbirds. Now, cowbird nest parasitism affects a much greater percentage of songbird nests than in the past. My picture of a male displaying while issuing his upward swinging bubbling call before a female almost has a slightly degenerate, villainous feeling to it, as you can see below:
However, the effects of cowbird nest parasitism vary locally. A study in canyon woodlands around Fort Hood, Texas found that 90% of all Black-capped Vireo nests were parasitized, and that clearly was bad news for Black-capped Vireos. Yet in California's Sacramento River watershed where 83% of Lazuli Bunting nests were parasitized, the buntings' nests were no less successful than nonparasitized ones. Often parasitized nests are abandoned and renesting takes place. A well documented Audubon page discussing many sides of the issue is at http://www.audubon.org/bird/research/. |
