At 00:38 4/9/97 PST, you wrote:

(snip)

>Which gets us back to cowbirds once again: both these species are well-known
>cowbird hosts. If one were to assume, rightly or wrongly, that cowbirds are
>one of the main checks on these species' populations, then one could posit
>that Yellow Warblers are being more negatively affected than Red-eyed Vireos
>these days by cowbird hosting. If so - why?
>
>Jack Bowling
>Prince George, BC
>jcbowling at mindlink.bc.ca

Temporarily, I will assume that cowbirds can affect the population
dynamics of both Red-eyed Vireos and Yellow Warblers. The effects of
cowbirds on these two birds may differ for several reasons. The most
obvious that I can think of is cowbird preferences. Perhaps, when given a
choice between Red-eyed Vireo nest and Yellow Warbler nests, the cowbird
parasitizes the warbler. Cowbirds are known to parasitize species at
different levels in the same place. Cowbirds may even switch host
preference. In the tallgrass prairie of Kansas, where I studied the
species, cowbirds would heavily parasitize Field Sparrows, Red-winged
Blackbirds, and Bell's Vireos early in the season before the Dickcissels
began nesting. Once the Dickcissels began nesting, parasitism rates on
other species dropped and almost all of the Dickcissel nests were
parasitized. For whatever reason, cowbirds seem to prefer some species over
other species.
Back to the original question. Even though they nest in the same
habitat (riparian shrubs), the warbler and the vireo choose different nest
sites. Theses sites might affect whether the cowbird locates the nest.
The two species could differ in their nest defense strategies.
Perhaps Red-eyed Vireos are better than Yellow Warblers at driving cowbirds
away from their nest site.
I could go on for several more reasons but I won't. None of these
have possibilities have been tested to any great extent and it is all just
speculation on my part.
I am more interested in the assumption that it is the cowbirds that
are regulating the population of Yellow Warblers and Red-eyed Vireos. Many
studies have been published documenting the negative effects of cowbird
parasitism on a given individual nester. However, very few studies have
actually extended the analysis to the population level. If cowbird
parasitism reduces the number of young fledged by the host, what effect does
that have on the population as a whole. The answer may seem straight
forward but it isn't. There are many other factors at work. What about
predation? For most nesting population, predation by natural enemies is
higher than parasitism. In addition, the survival rate of fledglings is
low. Does the reduction in fledgling rate as a result of cowbird parasitism
have any effect or is it swamped out by nest predation and juvenile
survival? We don't know. The main problem with researching this question
is determining juvenile survival. We are relatively sure it is low but it
is difficult to determine.
Usually, people point to cases where cowbirds were eliminated from
the nesting area around an endangered species, such as Kirtland's Warbler,
and the population increases. But again, there are multiple factors working
here. In the case of the Kirtland's Warbler, the nesting success greatly
increased after cowbird removal but the population didn't really increase
very much until they were provided with more nesting habitat.
While it is obvious that cowbirds have detrimental effects on
individual host parents, it is difficult to determine whether that is enough
to affect the population or the species. We don't know (yet). So much more
work to be done :-)
Deb Beutler
Department of Zoology
Washington State Univerisity
Pullman, Whitman Co., WA
dbeutler at wsunix.wsu.edu