Global, anthropogenic climate change is projected to trigger intense but variable droughts, especially in arid lands, with unknown but potentially severe implications for animal populations. Juvenile animal survival is an important parameter for population growth. Annual variation in juvenile survival can cause stark demographic fluctuations, highlighting the importance of studying this life stage to identify limiting demographic factors. Because of their inexperience and incomplete plumage development, juvenile birds may be more sensitive to adverse climatic conditions, such as drought, than adults. During 2017 and 2018 we monitored nest survival and juvenile survival of a migratory songbird, the Gray Vireo (Vireo vicinior), at Sevilleta National Wildlife Refuge, New Mexico, in the Desert Southwest, USA. We used radio telemetry to track juveniles daily during the post-fledging period (i.e., after nest departure). We then used logistic exposure models to estimate nest and juvenile survival rates in 2017 and 2018 while including the effects of multiple covariates.
Results/Conclusions
Juvenile survival significantly differed between 2017 and 2018 (p=0.04), while nest survival did not (p=1.00). During extreme drought conditions in 2018, the model-averaged juvenile survival rate (0.38; n=19) was 53% lower than in 2017 (0.80; n=30) when the drought index was near the long-term average. The primary difference in juvenile survival between years was the rate of mortality associated with exposure compared to predation. Nest survival did not differ between years (0.25, n=51 in 2017; 0.24, n=34 in 2018). We suggest that juveniles that died of exposure in 2018 may have had high rates of evaporative water loss and therefore may have been dehydrated. Our study highlights that the predicted increases in drought severity and duration due to climate change will likely have negative impacts on population growth of Gray Vireos and perhaps other songbirds in arid regions. We also show that the life stage during which this effect presents is the early juvenile stage and not the more commonly studied nesting stage.