Longer-lived tropical songbirds reduce breeding activity as they buffer impacts of drought

Tropical songbirds in the New and Old World reduced reproduction during droughts. Longer-lived species reduced reproduction and experienced increased adult survival, whereas shorter-lived species maintained reproduction and experienced reduced survival during drought than non-drought years.

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Climate change is expected to alter patterns of rainfall across the globe, increasing the frequency and severity of droughts in many places. Drought can have major implications for population dynamics, making population responses to drought critical to understand for predicting long-term consequences under future conditions.

 

Determining the relative impact that droughts have on reproduction and survival is critical because these fitness components influence population growth. Yet, species are thought to differ in how sensitive their populations are to changes in these two components. Population models suggest that longer-lived species are more sensitive to changes in survival than reproduction. In contrast, shorter-lived species are predicted to be more sensitive to changes in reproduction than survival.

 

Droughts may reduce either survival or reproduction and, thus, may have serious consequences for populations of both short- and long-lived species. However, behavioral responses to drought may determine the relative impact on survival and reproduction. For example, long-lived species may reduce reproduction during droughts to minimize the impact of drought on survival. We tested this idea by studying 38 species of tropical songbirds during severe droughts that occurred during long-term studies of tropical bird communities in Malaysia (2016) and Venezuela (2008). At each site, we located and monitored nests of all species over many years to examine reproductive activity prior to and during the droughts. We also banded birds with colored bands and used intensive re-sighting of these birds to obtain rigorous estimates of survival.

 

Longer-lived species showed strong decreases in breeding activity while shorter-lived species tended to breed just as much as normal in both tropical communities. This supported the idea that longer-lived species may obtain a benefit to survival impacts of the drought. Did these behavioral responses to drought actually help buffer populations by limiting reductions in survival?

 

Negative impacts of drought on survival are well documented. We therefore also expected the droughts to reduce survival, but thought that the reduced breeding activity might limit the decrease in survival. We were extremely surprised to find that not only did reductions in breeding activity mitigate costs to survival, many long-lived species actually experienced higher survival rates during the drought year than during non-drought years. In contrast, shorter-lived species that kept breeding during the drought faced strong reductions in survival.

 

To examine how these behavioral responses might affect population growth over time, we simulated populations across future drought scenarios. We found that the population impacts of droughts were largely mitigated by the behavioral shifts in longer-lived species, but not in shorter-lived species.

 

Overall, our results have several major implications. First, we show that understanding behavioral responses to drought are critical for predicting population responses. Behavioral responses to environmental conditions can help buffer the most sensitive vital rates for a given species and mitigate the overall effect on fitness. Second, our results provide unique support to the idea that reproduction can yield costs for survival (i.e. the cost of reproduction). This idea of a “cost of reproduction” is central to life history theory, but only rarely documented in wild populations. Finally, long-lived species are argued to be most sensitive to climate change, but our results suggest that many longer-lived species may actually be more resilient to drought impacts of climate change than previously expected. Ultimately, we hope our study can help motivate future studies into behavioral and demographic responses to shifting patterns of rainfall in more species so we can better anticipate and soften the impact of climate change.

Thomas E Martin

Research Biologist, USGS, University of Montana

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