FOAM LAKE BIRDING
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.