This investigation delineates the phenology of the annual cycle (gonadal activity, nesting, moult, behavioural events) in the Rufous-collared sparrow and attempts to identify the environmental factors involved in the temporal coordination of the annual cycle with environmental seasonality. Populations of sparrows were studied in arid chaco plains, subtropical foothill transition zone, and mountain semi-desert along a 100-km east-west transect at 26.9d̀S in the eastern piedmont of the Andes. This is a region of summer rainfall and winter drought characterized by a temporally unpredictable onset of vernal rains and vegetative rejuvenation.

All three populations have a strong gonadal cycle including a period of histological collapse and reorganization. Measureable gonadal growth began in spring concurrently in all three populations when the daily photoperiod surpassed about 12 hours. The rates of gonadal growth in the log-linear phase were alike in the chaco and mountain populations, but were only 35–45% of the rate in the intervening foothill population. The earliest nestings in all populations were closely correlated with the first growth of vegetative ground cover, but less closely correlated with other environmental variables. The growth of ground cover to an arbitrary index stage differed by several weeks in the three biotopes, and the onset of the nesting season differed accordingly. Rufous-collared sparrows in this region nest almost exclusively on the ground in dense grass, and are evidently dependent on its concealment. Gonadal regression began at different times in the three populations and was not uniformly correlated with any obvious changes in the condition of the habitat. In all cases, regression began after the summer solstice, and thus coincided with decreasing daylength.

Postnuptial moult in adults began almost simultaneously in all three populations regardless of the phase of the nesting cycle. Moult (suspended temporarily in some individuals) thus overlapped the end of the nesting season in the chaco and foothill populations, but not in the mountain population. Postjuvenal moult in immatures also began almost simultaneously in the three populations, about two weeks later than the onset of postnuptial moult.

The temporal coordination of the annual cycle of Rufous-collared sparrows in northwestern Argentina appears to be controlled basically by the annual photocycle, with habitat conditions (probably vegetative growth in spring) acting as essential modifiers in the final adjustment of nesting to local conditions. This system allows the sparrows to exploit suitable nesting conditions as soon as they occur each year in a temporally unpredictable environment in which an exclusively photoperiod control of the gonadal cycle would be deleterious in a large proportion of years.