Effects of Drought and Fire on Bird Communities of the Kofa National Wildlife Refuge, Arizona

Abstract

In southeastern California, from 2004 to 2009, we found that, among all 13 bird species for which we have sufficient sample size, the nest initiation date was later when there was less rainfall during the four-month winter rainy season preceding nesting. Nesting was delayed more than three weeks for some species during extreme droughts in 2006 and 2007. During the same period, we found nesting success to be positively associated with winter rainfall for the Black-tailed Gnatcatcher (Polioptila melanura). Because climate models are nearly unanimous in their predictions for increased drought frequency in the southwestern U.S., drought-delayed nesting may play an important role in nesting success and productivity for many Sonoran Desert bird species in the future. To investigate the effect of breeding phenology on nesting success and productivity, we conducted a manipulative experiment with Black-tailed Gnatcatchers and Verdins (Auriparus flaviceps). Following a wet winter in 2009–2010, we delayed clutch initiation dates for treatment pairs in spring 2010 to match the late first- egg dates that we had observed during previous droughts in 2006 and 2007. Undelayed first clutches had 1.8 times (Verdin) to 2.8 times (Black-tailed Gnatcatcher) higher odds of success than delayed first clutches. Diminishing daily nest survival rates over the remainder of the breeding season prohibited delayed pairs from overcoming the lost period of high productivity in March and early April, and delayed treatment pairs had lower productivity over the course of the entire breeding season than undelayed control pairs. As nest depredation and parasitism were the most common causes of nest failure, we conclude that drought impacts on annual reproductive output in the Sonoran Desert may be mediated by parasitism and depredation among Black-tailed Gnatcatchers and by depredation among Verdins.