NOTES 
reported apparent success rates ranging from 58.1 to 63.1% (Tweit 1996). Of the 
nests I studied, the one with four eggs was destroyed when strong winds displaced it 
(at 212 cm above ground it was the highest nest), and another clutch was destroyed 
when the three eggs were punctured. Five eggs from five nests were infertile, for a 
fertility rate of 94% in 89 eggs. Of the 84 young that hatched, two died when their 
nest was blown from a cholla, five disappeared from four nests, and 77 fledged. Thus 
of 96 eggs laid, 80% fledged young, a rate higher than the 44% reported for 54 eggs 
during an earlier study near Tucson (Edwards and Stacy 1968, in Tweit 1996). 
Predation has been identified as the most important factor in mortality of Curve- 
billed Thrasher eggs and nestlings (Tweit 1996). The vicious spines on the Chainfruit 
Cholla generally deter mammalian predators, but some local snakes, such as the 
Red Racer ( Masticophis flagellum), Common Kingsnake (Lampropeltis getulus), 
and Gopher Snake ( Pituophis catenifer), are capable of climbing into chollas and 
are known consumers of eggs and young birds. Predation by snakes, however, has 
not been reported early in the Curve-billed Thrasher’s nesting cycle (Tweit 1996). 
In the clutch whose three eggs were punctured the punctures were small, suggesting 
destruction by either a neighboring Cactus Wren ( Campylorhgnchus brunneicapillus ) 
or another Curve-billed Thrasher. 
Precipitation above normal from October 1978 through January 1979 was 
likely responsible for early nesting of the Curve-billed Thrasher in late winter 1979. 
Temperature did not seem as important, but Edwards and Stacy (1968) reported 
the species laying as early as 26 January after a mild winter. As reptiles have been 
implicated as important predators of Curve-billed Thrasher eggs and nestlings, the 
high success rates in 1979 were perhaps related to the first nesting cycle of the year 
being completed before snakes emerged. Smith (1971) and Edwards and Stacy (1968) 
reported rates of nesting and fledging success higher when pairs nested early. Early 
nesting can be risky, however; 41 mm of a cold rain near Tucson in March 1968 
resulted in death for several nestlings 3-6 days old (Anderson and Anderson 1973, 
Edwards and Stacy 1968). 
At the time of this study in 1979, global warming had received little attention, 
but global climate change and its effect on ecosystem survival are currently at the 
forefront of numerous biological investigations. If the desert Southwest becomes 
drier and warmer as predicted, years of early nesting and high nest success, such as 
I observed in 1979, will become ever less frequent. Much will depend on vegetation 
changes. Cholla is the principal component of the Curve-billed Thrasher’s nesting 
habitat at least in Arizona, with prickly pear, thorny shrubs, and other species used to 
a lesser extent. Corman (2005) reported that of 87 nests noted during research for 
the Arizona Breeding Bird Atlas, 90% were constructed in six species of cholla and 
that Curve-billed Thrashers avoided the Sonoran Desert where vegetation was sparse 
and cholla lacking. If a decrease in rainfall eliminates cholla, the thrasher’s populations 
would be affected severely, except perhaps in irrigated urban settings. 
My study’s original objective was to assess the Curve-billed Thrasher’s breeding 
success after a wet winter followed usually by a cooling trend in March; the winter 
was wet, but the March cooling did not occur. Stephen Russell suggested the study, 
and to him I am most grateful. I thank Robert Scholes for reviewing an early draft of 
the manuscript, and also thank reviewers Mathew Johnson and Robert Tweit, and 
editors Thomas Gardali and Philip Unitt who provided suggestions for improving a 
later draft. 
LITERATURE CITED 
Anderson, A. H., and Anderson, A. 1973. The Cactus Wren. Univ. of Arizona 
Press, Tucson. 
235 
