Brand and Noon • SHRUB-NESTING BIRDS IN A RIPARIAN CORRIDOR 
53 
■ Saltcedar 
I Mesquite 
I Cottonwood □ All habitats ^ Survivorship if A=1 
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Bell's Vireo Yellow-breasted Chat Abert's Towhee 
■ Saltcedar ■ Mesquite ■ Cottonwood □ All habitats Fecundity if A. = 1 
FIG. 3. Observed estimates of (A) adult survivorship and (B) seasonal fecundity ±90% confidence intervals (Cl) for 
Bell’s Vireo, Yellow-breasted Chat, and Abert’s Towhee by vegetation type drawn from 23 sites along the San Pedro River 
1999-2001 or the literature in solid colors. Fecundity or survivorship required to obtain A = 1 with other vital rates held 
constant in pattern color. 
attempts needed to maintain a stable population in 
the absence of immigration at observed levels of 
nest productivity on the San Pedro (Fig. 4). Adult 
survivorship and/or the average number of nesting 
attempts required to maintain a stable population 
was high for Bell’s Vireo, and higher in mesquite 
and saltcedar than cottonwood, given observed 
nest survivorship and young per successful nest in 
the different vegetation types (Fig. 4). Levels of 
adult survivorship and average nesting attempts 
required to maintain a stable population were 
lower for Abert’s Towhee and Yellow-brested 
Chat than Bell’s Vireo and varied little by 
vegetation type (Fig. 4). 
DISCUSSION 
Possible population sources (based on 90% Cl 
of A that included 1) only occurred for Abert’s 
Towhee across vegetation types and Bell’s Vireo 
in cottonwood vegetation using nest survival and 
productivity estimates from local populations and 
survivorship estimates from the literature. Our 
point estimates of A were all substantially <1 
indicating the entire study area may be operating 
as a population sink. These results are surprising 
given the San Pedro is considered to be one of the 
best remaining occurrences of lowland native 
riparian vegetation in North America (Noss et al. 
1995). 
We calculated annual survivorship required to 
maintain a stable population given observed 
fecundity levels to compare with what could 
conceivably occur. Annual survivorship required 
to maintain a stable population across vegetation 
types was within the 90% Cl of our estimate for 
Abert’s Towhee (0.60), although we know of no 
