Water availability and reproductive status in desert birds: the effects of water supplementation in the black-throated sparrow (Amphispiza bilineata deserticola)

Coe, Sharon

The author tested the effects of water availability on clutch size of birds. 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. I examined the effects of water availability on reproduction in desert birds. During two consecutive breeding seasons (years 2000 and 2001) I provided water to Black-throated Sparrows (Amphispiza bilineata deserticola) on breeding territories in the Mojave Desert of southern California. In both years, Treatment pairs laid significantly larger clutches than did Control pairs. In 2000, the average clutch size of Treatment pairs was almost 0.5 an egg larger than Control pairs. Overall, the average egg volume per nest in Treatment and Control pairs was not significantly different, and the average proportion of eggs hatched was not significantly different at α = 0.05 but was nearly so. Daily nest survival probabilities were not greater in Treatment nests than in Control nests in 2000 nor in 2001. Treatment nests did not produce a greater average number of young surviving to Day 8 than did Control nests, although the trend is of greater averages in Treatment nests. The results of this study indicate that for species like the Black-throated Sparrow that are abundant in the desert, are successful in breeding there, and survive without drinking water, arid environments can nevertheless impose limitations on reproduction. The results should be considered in estimating the proximate effects on bird reproduction from global climate change, and for assessing the effects from artificial water sources placed for wildlife in arid regions of the United States. This is the first known controlled field experiment to examine the effects of water availability on avian reproduction.