In many species of snakes, particularly viperids from temperate regions, production of offspring by individuals occurs on a less-than-annual schedule. Accordingly, acquiring sufficient energy and nutrient reserves for reproduction in females often requires more than a single active season. This is termed capital mode. Yet, in some instances, annual reproduction occurs under conditions where foraging success is high and environmental factors are compliant. This is termed income mode. Here, we addressed the hypothesis of annual versus less-than-annual reproduction from a long-term radio-telemetric study involving female western diamond-backed rattlesnakes Crotalus atrox from a population of the Sonoran Desert in Arizona. From 2001 to 2008, 16 of 20 radio-telemetered females produced 36 litters, which 32 were informative in addressing the hypothesis of reproductive frequency. In 14 females, litters were produced on a biennial or at-least-biennial (≥biennial) cycle. However, seven females demonstrated annual reproduction, of which several had previously reproduced on a biennial or greater cycle. Because our study was non-experimental, we were unable to unambiguously identify specific proximate factors that contributed to the shift in annual reproduction. Nonetheless, we established that greater annual rainfall was significantly correlated with shifts to annual reproduction. Based on other studies, we hypothesize that increased rainfall was causally linked with increases in rodent densities and the foraging success of female C. atrox, which in turn is linked to reproduction. We describe, moreover, several characteristics of female C. atrox that appear to facilitate the potential for annual reproduction. In long-lived species, such as C. atrox, our research underscores the necessity to follow individuals for extended periods to gain insights on reproductive cycles not captured by point sampling methods, such as short-term field studies or reliance on museum specimens.