Dennis P. wrote -

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>>>>
Brood parasitism is undoubtedly one of the major sources of mortality on
passerine birds in this area. When you're entertaining cowbirds at your
feeders during the summer, that's worth a thought. The feeder is probably
more important for them than it is for the other birds coming to it, most
of which have a higher proportion of insects in their diets, so by
discontinuing feeding in summer you may actually be helping the songbirds
in your neighborhood.
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It has been easy to quantify the effects of cowbird brood parasitism on
relatively few host species of restricted range and small population such as the
Kirtland's Warbler. Seems to me that the only adequate objective survey method
to gauge the cowbird's effect on a more widespread and abundant species such as
the junco would be to census a cowbird affected area first, then remove the
cowbirds and re-census. Although one would assume that the plight of Kirtland's
Warblers can be extrapolated to other species, there may be some surprises
awaiting the researcher re. the relience of other species to cowbird brood
parasitism, or even self-limiting effects of parasitism itself as mentioned by
Dennis. Some may argue that the Breeding Bird Survey is an adequate objective
method but having run these censuses for quite a while, it is obvious that one
has to consider too many variables other than cowbirds to try to explain
year-to-year variability in population sizes.

- Jack

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Jack Bowling
Prince George, BC
jbowling at direct.ca