Foam Lake Birding No. 155

No. 155
                        The first signs of autumn are here.  The Golden Rod in our yard is blooming which has always reminded me that a new school year was about to begin.   Some grain fields are starting to turn and some of our summer birds, like the Yellow Warbler and Purple Martin, have already started heading south.  Finally, as I was writing this article, a Swainson's Thrush landed on the railing in our garden.  These thrushes nest in the boreal Forest and are found here only in the fall and spring during their migration. 
            However, summer is still officially here and so discussion about another summer bird is in order. 
            In the last article I mentioned that the nests of many small song birds were parasitized by the Brown Headed Cowbird, Canada's only completely parasitic bird.  In the southern US there are two more cowbirds along the Mexican border.  As a point of interest, Eurasia has the Cuckoo (the bird of the cuckoo clock).  We do have some other birds that are occasionally parasitic but, unlike the cowbirds, they do build their own nests and rear their own young. 
            Since the arrival of Europeans to the Americas the Brown Headed Cowbird has "morphed" from a unique and interesting bird to a bit of a pest.  Historically, the cowbird has had a symbiotic relationship with the vast roving herds of bison (buffalo) and antelope.  As the herds roamed about the continent the cowbirds moved with them making traditional nesting and rearing of young impossible.  As a result they became parasitic (lay their eggs in other birds' nests).  Whenever bison were present in a certain area in the spring the cowbirds would parasitize the nests in that particular area and then move on with the herd.  Next spring the herds would be in a different area and the nests in that area would be parasitized and so on.  This constant roaming minimized the harm caused by parasitism.  Since the disappearance of the bison the cowbirds have switched their attention to cattle which do not roam.  Since the cowbirds no longer roam they parasitize the same areas year after year causing considerable problems especially for species at risk.  The cowbird's egg (she lays only one) hatches out earlier than those of the host bird usually resulting in the cowbird being the only chick hatched.  It is not uncommon to see two 'tiny" Chipping Sparrows feeding a very much larger dark grey juvenile cowbird.  To reduce parasitism authorities have had to occasionally exterminate cowbirds in order to restore the reproductive capabilities of threatened species.  However, one recent turn of events has led to some unintended control of cowbird parasitism.  Many farmers have gotten out of the cattle business leaving large tracts of land without suitable habitat (no cattle) for the cowbirds with the resultant reduction of parasitism. 
            The sparrow sized Brown Headed Cowbird is quite easy to identify.  The males are pure black with a distinctive brown head; the females are an even dark brown.  To see cow birds, especially males, one has to drive out into the country and find a herd of grazing cattle.  The cowbirds are almost sure to be there.  In spring, the females can often be seen in town clambering in the shrubs and bushes looking for nests in which to lay their eggs, often being harassed by potential host birds. 
            Both of this week's photos were taken last year several miles east of Foam Lake near some pasturing bulls.  One picture shows two male cowbirds; the other shows a female cowbird and a Clay Coloured Sparrow.  I am not sure how well the distinctive features will show up in a black and white photo but a bird book will help.