At 01:09 AM 08/21/98 -0400, Chris Maack wrote:
[Snip]

I read
>somewhere that female cowbirds tend to parasitize
>the nest of the same species that raised them, which
>would tend to cut down on cases where the foster
>parents aren't going to be bringing proper nutrition.
>

This is not true for cowbirds. Cowbirds do not seem to prefer to
parasitize the same species that raised them. There are several lines of
evidence suggesting cowbirds are not very selective.
1.) Cowbirds often lay their eggs in nests of birds that are
clearly unsuitable. Cowbird eggs have been reported in nests of Mourning
Doves, terns, and gulls. Clearly, these females were not raised by these
species.
2.) Cowbirds often lay eggs in nests of known rejector species.
Several species of birds avidly reject cowbird eggs including Ameircan
Robins, Grey Catbirds, Scissor-tailed Flycatchers, Baltimores and Bullock's
Orioles, and Great-tailed Grackles. Cowbird eggs have been found in nests
of these species but they disappear as soon as the female returns.
3.) High rates of parasitism of unsuitable hosts. Studies of
Northern Cardianls and House Finches have shown that they are unsuitable
parents for cowbirds. In one study, a large percent of cardinal nests
contained cowbird eggs but no cowbirds fledged from these nests; many
cardinals fledged. A similar study was done with House Finches with the
same results. Clearly the females that laid eggs in these nests were not
raised by cardinals or finches.
4.) Captive females will lay in any nest-like structure found in
their cage regardless of whether it resembled a natural nest.
5.) There is no genetic evidence for host preference. If a female
lays an egg in the same species she was raised by, her daugher should follow
suit. Over evolutionary time, you would start to see genetic differences in
female-inhereted DNA (such as the mitochondrial DNA) between eggs found in
nests of different host species. No such evidence has been found.
There are other lines of evidence as well but the general consensus
on cowbirds is that they do not differentiate between host species. The
cowbird's strategy is to lay as many eggs as possible as fast as it can.
This explains why cowbirds can readily invade new areas and parasitize novel
hosts. It also explains why cowbirds show no egg or nestling mimicry.
Which host are you going to mimic?
The birds Chris was thinking of were cuckoos. The Cuckoo (the only
one in Europe) is an obligate brood parasite, just like the cowbirds.
However, they have a different strategy than the cowbirds because they lay
fewer eggs. (A cuckoo can make an egg every other day while a cowbird can
make one egg a day.) Cuckoos are much more selective and females do tend to
lay eggs in the nests of the same species they were raised by. Over
evolutionary time, this has result in different "strains" of cuckoos. These
"strains" parasitize different species, their eggs tend to mimic these
species, and there is significant genetic differences between the different
"strains". (I use the term "strains" because these are not really
subspecies; the genetic differences occur only in the mtDNA, which is
inherited only from the mother, and not in the nuclear DNA, which is a
combination of the mother's and the father's DNA.)
The Viduan finches of Africa have taken selectivity one step
farther. There are about eight different species; all of the females and
fledgelings look identical. The males of the differnt species look and
sound very different. The male viduan finches mimic the songs and calls of
the host species. Each of the eight species parasitizes only one host and
each has a different host. The eggs and nestlings closely mimic the host
species. It is hypothesized that there was only one species which
parasitized several species of hosts. Over time, the females imprinted on
the hosts (just like the cuckoos); their eggs and nestlings began to mimic
the host. However, the mimcry wasn't very good (like the cuckoo) because
the females were choosing males that were raised by different host species
and the selective pressure was diluted. So, over evolutionary time, the
males began to imprint on the songs and calls of the host species and sing
the song of the host. The females could then tell which males had been
raised by the same species. Over time, the mimicry of the eggs and
nestlings have become so tight that humans have difficulty telling the host
nestlings from the parasite's nestlings. The result is eight different
species where there used to be only one. Males of these eight species look
very different from one another and from the hosts; this might be to help
females chose the right male.
In cowbirds, it appears that being a generalist parasite has evolved
from a specialist parasite. There are five species which differ in their
degree of specialization. One parasitizes only one host, two have several
hosts and two (Brown-headed and Bronzed Cowbirds) have a large variety of
hosts. The ones with the wide variety of hosts evolved more recently than
the specialists, suggesting that there is an advantage for cowbirds to not
be picky (rapid range expansion?). The pattern is less clear in cuckoos.
There are over 40 species of parasitic cuckoos; specialists and generalists
are sprinkled all over their family tree. The trend seems to favor
specialists. North American Cuckoos (Black-billed and Yellow-billed) are
not parasitic (except to each other); apparently, this is a reversal and
they evolved from an obligate brood parasite ancestor. There are three
other groups of obligate brood parasites but they don't help us out much.
The Vidua finches (mentioned above) are all specialists so no trends there.
The Honeyguides (also of Africa) parasitize cavitiy nesters (primarily
woodpeckers) and nothing is known about them. There is a lone duck in South
America that is an obligate brood parasite; it is a bit of a generalist,
parasiting ducks, terns, coots, and gulls. So we don't know if it is better
to be a generalist or a specialist in brood parasitism.
If you would like to know the specific references for this
information, please email me. I had to learn this stuff for my prelims last
year; I can remember the results and why it is important in the story of
avian brood parasitism but I can't remember the exact references; I have a
reference database for that but it is at home and I am at school.

More than you every wanted to know about brood parasites.

Cheers
Deb
Deborah K. Beutler
Department of Zoology
Washington State University
Pullman, WA

dbeutler at wsunix.wsu.edu