NOTES 
CURVE-BILLED THRASHER REPRODUCTIVE 
SUCCESS AFTER A WET WINTER IN THE 
SONORAN DESERT OF ARIZONA 
CARROLL D. LITTLEFIELD, The Bioresearch Ranch, P. O. Box 117, Rodeo, New 
Mexico 88056 
Studies of avian reproductive success provide important clues about the relationship 
between a species’ populations and its habitats. Although many species are hard to 
study because their nests are difficult to locate and assess, the Curve-billed Thrasher 
(' Toxostoma curuirostre) is an ideal subject. Its open-cup stick nests are easy to find 
and often at a height convenient for observing nest contents. For the subspecies in 
southeastern Arizona (T. c. palmeri), the breeding season can begin in late Janu- 
ary, but generally nesting does not increase sharply until late March, with a distinct 
peak from mid-April through mid-May (Corman 2005), The initiation of breeding, 
however, seems (in part) correlated with the amount of winter precipitation. Two 
or more wet months in succession lead to food resources sufficient to elicit early 
nesting (Smith 1971), but earlier clutches may be smaller than those laid later, as 
early in the season food supplies may be limited (Stahlecker 2003). As food supplies 
increase, clutch sizes increase (Smith 1971). On the other hand, the success rate of 
earlier nests may be greater, as important predators of eggs and nestlings, such as 
snakes, are less active early in the season (Tweit 1996). To assess the Curve-billed 
Thrasher’s nesting chronology and reproductive success after a period of abundant 
autumn-winter rainfall, I began a study of its nesting near Tucson, Pima County, 
Arizona, in late January 1979. 
The 130-ha study site, within the Sonoran Desert ecoregion (Ricketts et al. 1999), 
was on the east slope of the Tucson Mountains 3.2 km east-northeast of Gates 
Pass (111 0 06' N, 32° 13' W) at an elevation of 793 m. The dominant overstory 
vegetation consisted of Saguaro cacti ( Carnegiea gigantea), wolfberry ( Lycium sp.), 
mesquite ( Prosopis spp.), Chainfruit Cholla ( Cylindropuntia fulgida), and palo verde 
( Cercidium sp.), with an understory predominantly of bursage ( Ambrosia sp.) and 
annual forbs. All thrasher nests were easily located during nest building, egg laying, 
or early incubation, as all were in cholla and usually visible from a distance. I assessed 
nests’ contents weekly by using a bicycle mirror attached to a rib from a decomposed 
Saguaro. At the Tucson weather station, ~11 km east-southeast of the study site, 
mean precipitation for October through January 1971-2000 was 9.0 cm, but for 
October 1978-January 1979 total precipitation was 23.13 cm; October had 4.72 
cm, November 4.01 cm, December 6.93 cm, and January 7.47 cm, but February 
precipitation was below normal (1.07 versus 2.24 cm, respectively). Maximum tem- 
peratures from October 1978 through January 1979 averaged 16° C, lower than the 
30-year mean of 19° C, perhaps because of increased cloud cover. 
In 1979, laying of the first egg by 38 pairs of thrashers ranged from 14 February 
to 14 March; most laying was in mid-to late February. All 38 nests were in Chainfruit 
Cholla and averaged 154.9 cm (standard deviation ±27.32 cm) above ground (range 
108-212 cm). Mean clutch size was generally smaller (2.53 eggs, standard deviation 
±0.56) than reported in other studies of the Curve-billed Thrasher in southeastern 
Arizona, in which clutch sizes ranged from 2.5 to 3.2 eggs (Tweit 1996). For example, 
from 1963 to 1968 at nearby Saguaro National Monument, Anderson and Anderson 
(1973) found a mean clutch size in 86 Curve-billed Thrasher nests of 2.7 eggs. During 
my study, 19 of 38 clutches (50%) had two eggs, 18 (47%) had three eggs, and one 
(3%) had four eggs. Nesting success was high; among 38 clutches, at least one egg 
hatched in 36, for an apparent nest-success rate of 95%. Three other studies of the 
Curve-billed Thrasher in southeast Arizona during the late 1960s and early 1970s 
234 
Western Birds 40:234-236, 2009 
