Musings of a roaming nature nerd

Those Dastardly Cowbirds



The Brown-headed Cowbird has a bad reputation. Being, what is called an “obligate brood parasite,” cowbirds exclusively lay their eggs in the nests of other birds and then leave, letting the “adoptive host parents” do all the hard work of raising their chick! The nest hosts often don’t understand what has happened and end up incubating the new egg along with their own. For several days the cowbird mother revisits the nest to ensure that her egg is still there. If she sees that it has been removed or destroyed, she might lay another one or even destroy the host eggs, forcing the parents to re-nest, thereby giving her another chance to lay. Females will do this up to 90 times with different nests during a breeding season! The successful cowbird egg typically hatches a day or so before the host eggs and the baby demands copious amounts of food with its loud calls. The host parents feed it constantly, to the detriment of their own eggs and young. The cowbird baby grows quickly and grows large- often developing to twice the size of much smaller frantic parents. In the end, these parents will exhaust their entire breeding season feeding a baby that isn’t even their own! For an endangered sub-species like the Least Bell’s Vireo, the loss of a nesting cycle to brood parasites is devastating.

Despite their negative portrayal as a destroyer of happy homes, the Brown-headed Cowbird has a fascinating natural history, which also may explain why they evolved to parasitise nests in the first place. True to their name, cowbirds are found around cattle. Before cattle appeared on this continent, they mixed with bison herds, and prior to that may have evolved alongside large mammals in Africa. They are often seen perched atop the backs of plodding bovine, waiting to pluck tasty insects from the churned up dust or from the backs of their mammalian hosts. At least in part because of this great food source, cowbirds tended to follow the herds as they roamed. Well, if you’re following migrating bison chances are you don’t have time to build a nest, incubate eggs, and feed your babies for weeks on end. By that time the herds would have moved on and your food supply would have vanished with them. It is much easier to let someone else care for your eggs and feed your babies.

Cowbirds are gregarious and flock together in large groups everyday. Newly fledged juveniles eventually make their way into these social groups and learn the family trade. The cowbird is adaptable and now lives in most habitats across North America. They don’t mind the disturbances that humans cause to the landscape and take advantage of the habitats and nest of birds that they might not have historically encountered. As one biologist put it ‘Brown-headed Cowbirds behave in such a dastardly way!’  But are the cowbirds to blame for the endangered status of a bird like the vireo? About 97% of their attempts at parasitism actually fail. Unfortunately, the poor state of things for vireos gets chalked up to good old human degradation of habitat. Really, the cowbird’s unique natural history makes a lot sense for the world in which they evolved. We are the ones who changed the game and now point fingers at the cowbirds. Cowbirds are not helping the matter, but are they really the dastardly ones?!

Biologists have different methods for handling nest parasitism. One way, of course, is to just let nature run its course. This sounds realistic but perhaps a little heartless. And that strategy works best when humans haven’t already interfered with the process from the beginning. When endangered species are involved some sort of action is taken. Some biologists remove cowbird eggs from host nests and replace them with clay ones. This is to trick the cowbird mother who might be monitoring the situation, into thinking that her egg is still in the nest. Also, the nest hosts occasionally abandon the nest if they see that one of their eggs is gone. The clay egg is a good decoy. It will never hatch, but provokes the right stimuli from both the cowbird and host parents.

Other biologists remove the actual cowbirds from the equation. A few birds are captured and put in a large cage. Because cowbirds flock together, other birds see the captured ones and sneak into the cage to engage in their daily chat! Once they get inside, they can’t get out. They are provided with shade, perches, food and water, but are successfully removed from endangered species habitat during the critical breeding season. Once the breeding season winds down, they are released unharmed. These temporary measures provide reasonable solutions to tricky problems.

Managing the impact of cowbirds is necessary, but admittedly while I keep a non-biased perspective in the field, I do find the clever ways and unique story of the cowbird fascinating. Dastardly or not it has somehow parasitised its way right into my heart!


These aren't the best shots, but at the top of the page you see a male and female cowbird, and in this photo you see one of the holding areas for the cowbirds during breeding season.


3 Responses

  1. Dad

    Can’t beat Mom’s comment(!!) so I’ll simply say, “Cool!” Well, not that deadbeat dads are cool! How about, “I enjoyed the behind the scenes look at nature.” :-)

    June 30, 2012 at 3:57 pm

  2. Momchester

    Cowbirds sound like the “dead beat dads” of the bird world! Perhaps we could run parenting classes for them? Anyway, always interesting to read your blogs – I learn so much! (Even if my comments don’t reflect this!)

    June 30, 2012 at 4:40 am

    • annie

      Hee Hee! Glad you’re enjoying these, Mom.

      June 30, 2012 at 6:50 am