Subject: Cowbirds, was: Local fledging successes
Date: Jul 25 01:37:42 1995
From: Alvaro Jaramillo - alvaro at quake.net



>Important not to demonise here, Mark. In the cowbird parents' scheme of
>things, this summer was just as successful as for the other species you
>mentioned. Nature is not an entity that cares: nest or brood parasitism is
>just as valid a form of reproduction as the four-eggs-to-a-nest style of the
>Wilson's Warbler and other songbirds.

Tell is like it is! Thanks Michael, for once I didn't have to come to the
defence of the lowly cowbird. As I have said before, the real enemy is us!

>
>As I understand it, cowbirds evolved on the Great Plains as a species which
>followed the huge migrant herds of buffalo and other ungulates of pre-human
>North America. To keep up with the herds, they developed the technique of
>dropping their single eggs into other birds' nests, then moving on with the
>herds.
>
It is interesting that this theory has been largely accepted, or at least
makes sense to most people, as I have never seen it challenged. It's a great
story, but recent evidence suggests it is not all that likely. Scott Lanyon
recently (1993?) published a phylogeny of the cowbirds based on sequence
data of that infamous gene, Cytochrome b. He came up with a bunch of neat
questions/observations including that the whole group of parasitic cowbirds
is monophyletic (radiated from a common ancestor) and that the trend was for
more recently derived cowbirds to have a larger number of hosts (generalists
evolved from specialists). The actual phylogeny (evolutionary 'tree') showed
that of the five (six in my opinion, but that's another story) species of
parasitic cowbirds, the Shiny and Brown-headed were the most recently
derived. So, the Brown-headed Cowbird arose from ancestors that were already
brood parasites, if Lanyon's work is correct, not as a consequence of the
bison herds. It is more likely that the cowbird was pre-adapted to follow
the bison by being a brood parasite. However, it is not impossible that the
first cowbird evolved with the bison, due to the scenario you proposed, and
that this bird eventually spawned all of the other cowbirds. The problem is
that the two most basal (diverged from the ancestor earliest) types of
cowbirds are the Screaming Cowbird and the Giant Cowbird, both of which
would have the phones ringing for days if they were ever seen in bison country!
I have no idea how 'old' the cowbirds are, but I just wonder if they
were around when all of those, now extinct, grazing herds inhabited North
(and South?) America back in the Pleistocene, was it? In any case, the most
basal cowbird, the Screming Cowbird, lives in the Pampas of Argentina where
there was no large mammal like the bison. The closest contenders were the
Guanacos (South American camel, related to the Llama) and the Pampas Deer.
The next cowbird to have split off was the Giant Cowbird, which mainly
inhabits tropical forests and wetlands. The only large mammal that may have
served the role of the bison in its habitat is the Capybara, the world's
largest rodent. Giant Cowbirds do perch on Capybara and nibble away at
parasites, or flies stirred up by these large rodents, so don't laugh. In
any case, all cowbirds will take advantage of large mammals as potential
beaters or as attractants for a wide variety of insects. So the question
remains, what came first the cowbirds or the 'cows'. My opinion is that
brood parasitism evolved before this mammal-following strategy, but who
really knows.

BTW, all this talk of the yakathon has got me jealous. I share, Serge's
dissapointent in not being able to attend. I hope you all have a great deal
of fun, and do post some photos/sound bites/video on the web page for all us
folks who can't be there. Now, its my turn to make you jealous: Just 15
minutes from my place on Sunday- 3 Wandering Tattlers, 300 Elegant Terns,
700 Heermann's Gulls, 30 Willet, 2 Surfbird, 50 Black Turnstone,200 Brown
Pelican, 20 Marbled Murrelet, 3 California Towhee, 100 Common Murre, 5
Wrentit, 1 Sea Otter, 30 Harbour Seal.....if any of you are down this way,
let me know!

If you want a full reference of Lanyon's paper let me know. Lanyon also
recently published a paper, in Evolution, that showed the genus _Agelaius_
(Red-winged, Tricolored Blackbird etc.) to be polyphyletic. This means that
birds that have been classified in the genus _Agelaius_ are not all that
closely related to each other. South American _Agelaius_ are quite distant
from the North American and Caribbean ones.

Take care,




Alvaro Jaramillo
Half Moon Bay, CA

alvaro at quake.net