52 
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 1, March 2011 
■ Saltcedar EDMesquite DCottonwood □ All habitats 
Bell's Vireo Yellow-breasted Chat Abert's Towhee 
FIG. 2. Finite rate of population increase (X) ±90% confidence intervals (Cl) for Bell’s Vireo, Yellow-breasted Chat, 
and Abert’s Towhee by vegetation type. Data to estimate X was drawn from field studies conducted on 23 sites along the 
San Pedro River 1999-2001 and from the literature. 
survivorship and average number of nesting 
attempts —would be required to attain X = 1 at 
observed levels of population-specific vital rates 
from the San Pedro—nest survival and number of 
young per successful nest—within each vegeta¬ 
tion type. 
RESULTS 
We monitored 86, 147, and 154 Bell’s Vireo, 
Abert’s Towhee, and Yellow-breasted Chat nests, 
respectively, during a 72-day interval during the 
avian breeding season across 3 years. Clutch sizes 
were similar among vegetation types for all three 
species (Table 1). Estimated young per nest, young 
per successful nest, and seasonal fecundity were 
similar among vegetation types for Abert’s Towhee 
and Yellow-breasted Chat although seasonal fe¬ 
cundity estimates for Yellow-breasted Chat were 
quite low (Table 1). Bell’s Vireo had extremely 
low seasonal fecundity, particularly in saltcedar, 
and had significantly fewer fledglings per nest in 
saltcedar compared with cottonwood based on non¬ 
overlapping confidence intervals (Table 1). 
Point estimates of X were substantially < 1 for 
Bell’s Vireo, Yellow-breasted Chat, and Abert’s 
Towhee in all vegetation types (Fig. 2). Bell’s 
Vireo had the greatest variation in X across 
vegetation types of the three species with 27 and 
17% higher estimates in cottonwood compared 
with saltcedar and mesquite, respectively. The 
upper 90% confidence limit for X was <1 for 
Bell s Vireo and Yellow-breasted Chat in all 
habitat types with the exception of Bell’s Vireo in 
cottonwood where the upper confidence limit 
barely exceeded 1. Cls (90%) for Abert’s Towhee 
surrounding X were large and included 1 for all 
vegetation types. 
Annual adult survivorship required to maintain 
X = 1, at observed levels of seasonal fecundity, 
was 0.60 across vegetation types for Abert’s 
Towhee with little variation between vegetation 
types; estimates were within 90% Cl of assumed 
survivorship (Fig. 3). Annual adult survivorship 
required to maintain X = 1, at observed levels of 
seasonal fecundity, were 0.77, 0.72, 0.71, and 0.73 
for Yellow-breasted Chat and 0.95, 0.87, 0.75, and 
0.87 for Bell’s Vireo in saltcedar, mesquite. 
cottonwood, and across vegetation types, respec¬ 
tively; these estimates were higher than the upper 
90% Cl of assumed survivorship estimates. 
Seasonal fecundity estimates needed to main¬ 
tain a stable population (^=1), given assumed 
annual survivorship, were higher than observed 
for all species in all vegetation types and not 
within the 90% Cls of our estimates (Fig. 3). 
Seasonal fecundity needed for Bell’s Vireo to 
maintain a stable population was —15, five, two. 
and live times higher than observed in saltcedar. 
mesquite, cottonwood, and across vegetation 
types, respectively. Seasonal fecundity to main¬ 
tain a stable population needed to be about two 
and three times higher than observed across 
vegetation types for Abert’s Towhee and Yel¬ 
low-breasted Chat (Fig. 3). 
There was a strong trade-off between adult 
survivorship and average number of nesting 
