From my experience and research, three factors determine the impact
of cowbirds on hosts: 1) host size, 2) host incubation period, 3) host diet.
In general, bigger birds are less affected by cowbirds than smaller
birds. Cowbirds are fairly large birds (for a passerine) and large birds
have less problem raising cowbirds than smaller birds. Very large
passerines tend to eject eggs so it is difficult to tell how well cowbirds
would do in the nests of larger birds. But in general, the larger the
passerine, the less the impact of cowbird parasitism.
Host incubation period is very important. If the cowbird hatches
first, it generally takes over the nest and the host eggs do not hatch.
Dickcissels, meadowlarks, and Red-winged Blackbirds have incubation periods
that are similar to cowbirds (around 10 days). Flycatchers and vireos are
particularly vulnerable to cowbirds because they have a much longer
incubation time (as long as 20 days for flycatchers). Therefore, the
cowbird hatches first and the parents are so busy feeding the cowbird chick
they don't incubate the other eggs. If the other eggs do hatch, they are so
much smaller than the cowbird, they usually starve or are crushed by the
cowbird.
Host diet is important. Cowbird chicks that hatch in nests of birds
that feed their offspring insects (blackbirds, meadowlarks, vireos,
flycatchers, warblers, etc.) grow fast. However, if a cowbird hatches in
nests of birds that feed their young seeds, particularly American
Goldfinches, the cowbird chick starves to death. American Goldfinches are
not affected by cowbird parasitism eventhough they are much smaller than the
cowbird.
The birds that are most vulnerable to cowbirds are small
insectivores that have long incubation times (flycatchers, warblers and
vireo in particular).
I don't mean to suggest that evolutionary history isn't important.
Birds need time to develop defenses against cowbirds (nest defense by
parents, ejection of eggs, abandonment of parasitized nests, etc.) However,
I think size, diet and incubation period are important in determining which
birds develop defenses to cowbird parasitism and which ones just "live with it".
Generally, cowbirds do not eject young and eggs like cuckoos and the
do not kill the other nestling directly like the parasitic honey-guides of
Africa (they have a wicked hooked bill they use to kill other nestlings).
There is at one recorded instant of a cowbird pushing Indigo Bunting chicks
out of the nest (recorded on video) but such instances are apparently rare.
In fact, in one study, cowbird parasitism was found to be
beneficial. In South America, oropendolas (I'm not sure which species)
eject Giant Cowbird eggs when they were nesting near a hornet hive but left
the cowbird eggs in the nest when they weren't near hornets. Oropendola
chicks often die because of heavy botfly infestations; the botfly larvae
literally eat them alive. Oropendola can raise both cowbird and oropendola
chicks in the same nest. Cowbirds hatch earlier than the oropendolas and
they eat botflies. Oropendola chick do not get botflies in parasitized nests
but they are heavily infested in unparasitized nests. So what about the
hornets? Hornets also suffer from botflies so they run the botflies off.
The oropendola nesting near hornets don't need cowbird so they eject the
cowbird eggs! If you think I am making this up, I can look up the
references for this story. It was published in Nature several years ago.
So cowbird chicks aren't all bad!!!
You mention birds in the plains versus birds in the forest. The
Brown-headed Cowbird not only inhabited the Great Plains but was also
present in the Great Basin long before whites colonized the area. Birds of
rather open forests (such as Ponderosa Pine parklands) and riparian birds
have a long history of parasitism by cowbirds. Cowbirds appear to be
causing the greatest damage in areas that have recently been opened up by
humans such as eastern deciduous forests and western coastal forests.
Humans have helped the cowbird by opening new habitats for them and
increasing their winter survival by providing tons of waste grain for them
to eat. But cowbirds have been around for a long time and the birds that
are threatened by cowbird parasitism (Kirtland's Warbler, Least Bell's
Vireo, Southwestern Willow Flycatchers, Black-capped Vireo) generally were
in trouble from a lot of other causes (primarily habitat destruction).
More than you ever wanted to know about cowbirds and were afraid to
ask!!!

At 14:20 27/8/97 -0700, you wrote:
>Hi Tweets,
>
>Thanks for the enlightening corrections from Christine Maack and Deb
>Beutler. I fell into the 'I ain't seen it; it don't exist' hole,
>generalising from personal local observation. The positive outcome of my
>stupidity was learning for the first time that raising cowbirds isn't
>necessarily all-or-nothing for the other young. Must have been equating
>cowbird young behavior with that of European Cuckoo young, where it turfs
>the other eggs and young. So many assumptions, so little time. Sigh. '-)
>
>Would it be fair to assume then that plains species might have arrived at
>some evolutionary status quo with cowbirds that forest birds have yet to
evolve?
>
>Michael Price The Sleep of Reason Gives Birth to Monsters
>Vancouver BC Canada -Goya
>mprice at mindlink.net
>
>
>
Deb Beutler
Department of Zoology
Washington State Univerisity
Pullman, Whitman Co., WA
dbeutler at wsunix.wsu.edu