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The brown-headed cowbird, a brood parasite

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    An immature male cowbird rides the back of a pony waiting to gobble up disturbed insects. BARRY REED/SPECIAL TO THE TIMES NEWS

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    A cowbird egg sits among chipping sparrow eggs. Ten days later only one sparrow and cowbird nestling remained.

Published September 29. 2017 11:28AM

I’ve discussed previously how some birds tirelessly and painstakingly build their nests. (cliff swallows, Baltimore orioles, etc.) One bird species. the cowbird, does not even build a nest.

The cowbird, a blackbird species, places its eggs in other bird nests and never raises its own young. Why is this bird’s reproductive process so different?

Cowbirds, grouse, warblers, vireos, robins, thrushes, etc. were all present in North America before the European settlers arrived. Some eventually suffered from this human intrusion into their habitats. The cowbird did not and in fact, its range probably more than doubled.

Cowbirds adapted to life with the American Bison on North America’s Great Plains. Untold numbers of bison grazed Northward from spring to summer, and grazed southward again in the fall. The great herds were constantly on the move. Cowbirds would feed among the bison relying on them to “stir up” insects as they grazed. They even sat on the backs of the bison which offered a great vantage point to drop down and feed. The key here is: the bison were constantly on the move.

If cowbirds started building a nest, laid an egg a day for 4 or 5 days, then incubated and fed the nestlings, that process could take 45 days or more. Meanwhile, the bison herd could now be a hundred miles away. Their food providers were now gone. They needed to adapt.

Cowbirds located grassland’s birds which did not follow the bison and placed their eggs into the “foster parent’s nests. But now, moving westward were the “human” settlers. They cleared large forest areas and brought with them their livestock. With the bison all but gone, the cowbirds now had livestock to rely on and the pastures and homestead lawns made for easy times to find insects.

Eventually, the cowbirds moved both eastward and westward from the Great Plains and now live throughout the U.S. With all the human intrusion the cowbirds flourished.

Most forest bird species were sheltered from the cowbird intrusion. However, with our ever growing population, housing developments, utility line rights of way, oil and gas pipe lines, and cellphone towers we’ve opened up the deeper forests to the cowbird.

Cowbirds prefer the nests of yellow warblers, wood thrush, vireos, song sparrows, and about 115 other species. Now the frustrating news; cowbirds take advantage of the parental instinct of these other birds who now brood the intruder, feed it, and help it mature after fledging.

Cowbirds are great at locating the parasitized host’s nest, and usually early in the morning, slip in and lay an egg. Usually the cowbird young hatch first, they are larger than the host’s youngsters, and then get the lion’s share of the food. Often, the host bird only is successful raising the cowbird. The cowbird female sometimes even removes one or all the eggs from the host bird’s nests. Some of our native bird species are decreasing in numbers because of us opening up the forests. Cowbirds can lay 10 to 20 eggs a year in a variety of species’ nests.

A few bird species, such as the song sparrow and robin have become very good at recognizing the intruder’s egg and will remove it. Yellow warblers, have actually built a second nest on top of the original nest to stifle the cowbird. If you find a nest in your travels that has a cowbird egg, you may want to toss it aside.

Test your knowledge — Which of these is Pennsylvania’s smallest owl? A. Barn Owl B. Screech Owl C. Saw Whet Owl D. Barred Owl


Last week’s trivia answer: Barn owls may be the most nocturnal of all Pennsylvania’s owls. The chipmunk is very common; however it is a diurnal (daytime) animal thus barn owls don’t encounter them at night and are not one of their normal prey items.

Write to Barry Reed at

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