Brand and Noon • SHRUB-NESTING BIRDS IN A RIPARIAN CORRIDOR 
55 
were also quite high for Yellow-breasted Chat 
(37-46%) and Abert’s Towhee (43-50%) on the 
San Pedro across vegetation types (Brand et al. 
2010 ). 
Nest parasitism by Brown-headed Cowbirds 
can reduce both nest survival and young fledged 
per successful nest. Nest survival rates observed 
for Bell’s Vireos on the San Pedro were low, 
particularly in saltcedar and mesquite, and similar 
to those recorded along the Bill Williams River 
(Averill-Murray et al. 1999). Parasitism rate 
decreased on the Bill Williams River and Bell’s 
Vireo nest survival increased by over 400% in the 
breeding season following initiation of cowbird 
control and cessation of adjacent ranching oper¬ 
ations (Averill-Murray et al. 1999). Cowbird 
parasitism rates for Arizona Bell’s Vireos on the 
San Pedro were substantially higher in saltcedar 
(73%) and mesquite (60%) compared with 
cottonwood (33%; Brand et al. 2010). These rates 
in saltcedar and mesquite were lower than those 
observed on the Bill Williams River, but higher 
than most of those observed for federally listed 
Least Bell’s Vireo (Vireo bellii pusillus) and 
Southwestern Willow Flycatcher (Empidonax 
traillii extimus) across the region (Averill-Murray 
et al. 1999, Finch and Stoleson 2000, Kus and 
Whitfield 2005, Kus et al. 2010). In contrast, 
Abert’s Towhee parasitism rates were low on the 
San Pedro across vegetation types (11-17%) 
compared with those on the Lower Colorado 
River (Finch 1983, Brand et al. 2010). There were 
also substantially fewer fledglings for parasitized 
versus unparasitized nests on the Lower Colorado 
(Finch 1983). Parasitism rates were intermediate 
for Yellow-breasted Chat on the San Pedro (42, 
41, and 23% in saltcedar, mesquite, and cotton¬ 
wood, respectively; Brand et al. 2010). In 
comparison, no chat nests were parasitized by 
Brown-headed Cowbirds in a study in central 
Kentucky (Ricketts and Ritchison 2000). 
The average number of nesting attempts per 
breeding season has a strong effect on annual 
fecundity (Schmidt and Whelan 1999, Grzybow- 
ski and Pease 2005). This parameter is difficult to 
measure because it requires birds to be color 
banded and followed throughout a breeding 
season. Renesting can be influenced by cowbird 
parasitism and we were able to incorporate 
different average nesting attempts by vegetation 
type for Bell’s Vireo (Budnik et al. 2001). Our 
estimate of the average number of nesting 
attempts for Abert’s Towhee came from a 
population with about twice the rate of parasitism 
compared with the San Pedro (Finch 1984, Brand 
et al. 2010); thus, the estimate may have been 
high for the San Pedro population. Of the three 
species, average number of nesting attempts was 
most likely underestimated for Yellow-breasted 
Chat, since estimated nesting attempts came from 
a population with apparently lower parasitism 
rates (Thompson and Nolan 1973). 
The average number of nesting attempts per 
season required to maintain a stable population 
based on our sensitivity analysis was 6.0 for 
Abert’s Towhee and 3.5 for Yellow-breasted Chat 
with little variation between vegetation types. 
Abert’s Towhee is a resident species with a long 
nesting season, but an average of six attempts per 
season is beyond the 90% Cl of what was 
observed for 10 pairs; only two (20%) females 
attempted to nest > five times in a season (Finch 
1984). The Yellow-breasted Chat is a neotropical 
migrant with a much shorter nesting season, and 
3.5 nests per season is beyond the 90% Cl of what 
was observed for 24 pairs; only two (8%) females 
attempted to nest > two times in a season 
(Thompson and Nolan 1973). We estimated that 
10 average nesting attempts per season would be 
required for a stable Bell’s Vireo population on 
the San Pedro across all vegetation types (33, 11, 
and 4 in saltcedar, mesquite, and cottonwood, 
respectively) when holding other vital rates at 
assumed or observed levels. Kus et al. (2010) 
reported a maximum of seven nesting attempts for 
individual Bell’s Vireos across all studies with 
population averages similar to those used in this 
study; >10 average nesting attempts per season to 
maintain a stable population in all vegetation 
types except cottonwood would almost certainly 
be beyond the physiological capabilities of Bell’s 
Vireos. 
Year-to-year variation in fecundity rates can be 
substantial for some species (Eckerle and Thomp¬ 
son 2001) and it is important to evaluate if our 
fecundity estimates are representative. Average 
annual precipitation can strongly influence bird 
populations in semi-arid regions. For example, 
Abert’s Towhee density and productivity de¬ 
creased in years with lower precipitation (Mar¬ 
shall 1960, Meents et al. 1981). The years of data 
collection in this study occurred over one of three 
major statewide droughts during the 20 th century 
(Jacobs et al. 2005) and the subsequent 7 years of 
combined precipitation (2003-2010) in southeast¬ 
ern Arizona averaged 85% of normal (1971- 
