In the arid southwestern United States, many birds initiate breeding in the driest months of the year, March to May, long before the monsoon rains arrive in July and August. Although breeding success in these species is thought to be sensitive to precipitation, the relationships have not been rigorously described based on long-term study of a single species. We studied the relationships among reproductive success, precipitation, and temperature in Mexican Jays (Aphelocoma ultramarina) in the Chiricahua Mountains, Arizona. We identified three orthogonal factors that accounted for 85.8% of the variance in the 11 reproductive-success variables. Factor 1, the most important, was associated with production of young and with the fraction of females that were breeding. Because the Mexican Jay is a plural breeder (more than one breeding female per group), reproduction in the population can be influenced not only by the success of individual females, but also by the proportion of females in each flock that breed in a given year. This factor was positively related to the amount of precipitation both at the onset of breeding in March and April and during the previous eight months. Brood size at banding (14 to 15 days), which was strongly associated with Factor 2, was negatively related to the number of adult females per flock and relatively insensitive to yearly variation in climate. Success of the youngest females was associated with Factor 3 and depended on a different set of variables than that of older females. Although production of young was predictably depressed in drought years, the significant relationships between reproductive success and climate did not otherwise enable precise predictions based on climate alone. Because predation appears to be highly correlated with the number of nestlings per unit, the lack of strong predictability of reproductive success using climate variables alone may be caused by the independence of predation from climate variables.
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