54 
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 1, March 2011 
—cw 
-S-MQ 
—♦—ALL 
• CW 
—IS— MQ 
—♦—ALL 
Average number of nesting attempts 
FIG. 4. Sensitivity analysis of different levels of adult 
survivorship by average number of nesting attempts at a 
stable population (X = 1) when holding nest survivorship 
and number of female young per successful nest at observed 
levels from 23 sites along the San Pedro River 1999-2001 
for Bell’s Vireo, Yellow-breasted Chat, and Abert’s 
Towhee in saltcedar (SC), cotton wood-willow (CW), 
mesquite (MQ), and across all vegetation types. 
other estimates from the literature. Estimated 
annual survivorship for Yellow-breasted Chat 
needed to maintain X = 1 across vegetation types 
on the San Pedro (0.73) was higher than estimates 
from the Southeast (0.35 ± 0.04; mean ± SE) and 
Midwest (0.610 ± 0.067) (DeSante et al. 2001, 
DeSante and Kaschube 2006, Michel et al. 2006). 
Estimated annual survivorship for Bell’s Vireo 
needed to maintain X = 1 across vegetation types 
(0.87) was well above estimates from the south- 
central region (0.56 ± 0.04; mean ± SE) and 
Midwest (0.61 ± 0.04) (Budnik et al. 2000, 
DeSante and Kaschube 2006, Michel et al. 2006); 
it was more similar to that of a long-lived raptor 
species such as California Condor (Gytnnogyps 
californianus) (Meretsky et al. 1999). Estimates 
of annual survivorship required to maintain a 
stable population for Yellow-breasted Chat and 
Bell’s Vireo were higher than generally found for 
small passerine species (0.40-0.62; Martin 1995, 
DeSante and Kaschube 2006). Thus, at observed 
levels of fecundity and within the range of 
possible survivorship, it appears unlikely that 
Bell’s Vireo or Yellow-breasted Chat populations 
could maintain stable populations in the absence 
of immigration on the San Pedro over the study 
period. 
Local estimates of adult and juvenile survivor¬ 
ship are needed to draw strong conclusions about 
population status. Population-specific estimates of 
adult and juvenile survivorship are lacking for the 
species we examined, and our results must be 
interpreted with caution. The estimates we used 
were drawn from the southwestern region (De¬ 
Sante and Kaschube 2006, Michel et al. 2006), 
although we can only assume these estimates 
represent the specific location and vegetation 
types of interest. We used the common assump¬ 
tion of juvenile survivorship equal to half of adult 
survivorship. Data are lacking to assess the quality 
of this assumption, but the few studies that have 
directly estimated juvenile survivorship suggest 
this estimate may be slightly high and thus 
conservative (Gardali et al. 2003, Yackel Adams 
et al. 2007). The survival estimates we used were 
based on mark-recapture models that did not 
distinguish between mortality and emigration; 
they may underestimate true survivorship (McCoy 
et al. 1999). One approach that has been used is to 
add 0.1 to published estimates of adult survival to 
account for birds that dispersed (McCoy et al. 
1999, Yackel Adams et al. 2007). When adult 
survivorship estimates are raised by 0.1, point 
estimates of X across vegetation types become 
0.98 for Abert’s Towhee but still well below 1 
(0.85 and 0.78) for Yellow-breasted Chat and 
Bell’s Vireo, respectively. 
Observed fecundity was low on the San Pedro, 
particularly for Yellow-breasted Chat and Bell s 
Vireo. High nest-predation rates can profoundly 
decrease nest survival. Nest-predation rates for 
Bell s Vireo on the San Pedro varied by 
vegetation type with 55, 51, and 33% of nests 
preyed upon in saltcedar, mesquite, and cotton¬ 
wood, respectively (Brand et al. 2010). These 
nest-predation rates were generally higher than 
those on the Bill Williams River 06-31%; 
Averill-Murray et al. 1999). Nest-predation rates 
