Research Objectives:
- Quantify parasitism
frequency on the black-capped
vireo and white-eyed
vireo on private lands
in
central Texas.
- Investigate the relationship
between frequency of
parasitism on black-capped
and white-eyed vireos
and characteristics of
the neighboring avian
assemblage, including:
(1) cumulative bird abundance,
(2) cowbird host abundance
and cowbird egg acceptor
abundance, (3) abundance
of each species detected,
(4) proportional abundance
of each species detected,
and (5) abundance of
brown-headed cowbirds
detected.
Management Implications:
- The parasitism frequencies
we observed for the black-capped
and white-eyed vireos
were high enough to substantially
limit productivity (Porneluzi
and Faaborg 1999, Powell
et al 1999), suggesting
that cowbird management
may be an important tool
for successful black-capped
vireo recovery efforts.
- Our findings showed
a significant relationship
between cowbird number
and parasitism frequency,
suggesting that the number
of cowbirds detected
at a site may be a useful
predictor of expected
parasitism frequency.
The number of cowbirds
may be useful, practical
means to inform management
decisions on where to
implement cowbird control
for efficiency and efficacy.
- The relationship between
decreased parasitism
frequency and increased
black-and-white warbler
abundance may be due
to vegetation characteristics;
black-and-white warblers
are associated with mature
trees or woodlands.
There may be an interaction
between surrounding vegetation
characteristics and observed
parasitism frequency
which warrant future
study.
Project Summary:
Management
for the endangered black-capped vireo (Vireo atricapilla)
in Texas has focused on habitat restoration through
vegetation modification. However, loss of productivity
due to parasitism by brown-headed cowbirds (Molothrus
ater) may limit the success of habitat restoration
efforts in restoring black-capped vireo populations.
We investigated the relationships between frequency
of parasitism on black-capped and white-eyed (V. griseus)
vireos and characteristics of the neighboring avian
assemblage to identify assemblage characteristics useful
for predicting frequency of parasitism. We monitored
vireo nests from March to July 2006 to determine frequency
of cowbird parasitism on the 2 vireo species on 6 study
sites on private land in east-central Texas. We conducted
100-m-radius circular-plot point count surveys to identify
neighboring avian assemblages. All black-capped vireo
nests (n = 20) and 81% of white-eyed vireo nests (n
= 27) were parasitized. Parasitism frequency on the
2 vireos was positively correlated with the number
of cowbirds detected (r2 = 0.673, P = 0.045). Parasitism
frequency was negatively correlated with abundance
(r2 = 0.687, P = 0.042) of black-and-white warblers
(Mniotilta varia), but was not correlated with abundance
of other cowbird hosts. The relationship between black-and-white
warbler abundance and parasitism frequency may reflect
an interaction between parasitism and the vegetation
characteristics associated with black-and-white warblers,
suggesting the need for future study of the relationships
between parasitism frequency and vegetation characteristics.
The frequency of parasitism we observed suggests that
cowbird control may be an important tool if management
objectives include recovery of black-capped vireos.
Number of cowbirds may be a reasonable predictor of
expected parasitism frequency in an area, which may
be useful to land managers in selecting areas for implementation
of cowbird control. |
