Water availability affects clutch size in a desert sparrow

Birds that breed in hot, arid environments experience unique physiological demands resulting from efforts to maintain positive water balance when ambient temperatures are high, but humidity and water availability are low. We provided water to Black-throated Sparrows (Amphispiza bilineata) on breeding territories in the Mojave Desert of southern California to examine the effects of water availability on reproduction in desert birds, independent of the increase in food availability commonly associated with increased precipitation in and ecosystems. In both years of the study (2000 and 2001), water-supplemented (treatment) pairs laid significantly larger clutches than did control pairs. In 2000, the drier of the two years, the mean-clutch size of treatment pairs was almost 0.5 egg larger, or about 16% larger, than control pairs. Overall, the mean egg volume per nest in treatment and control pairs was not significantly different, and the mean proportion of eggs hatched was not significantly different at alpha = 0.05, but was nearly so. Daily nest survival probabilities were not greater in treatment nests than in control nests in 2000 or in 2001. Treatment nests did not produce a greater mean number of young surviving to day 8 than did control nests, although the trend is of greater means in treatment nests. The results of our study indicate that for species like the Black-throated Sparrow that are abundant in deserts, are successful in breeding there, and can survive without drinking water, water availability in arid environments can nevertheless impose limitations on reproduction.