Abstract
Climate change is drastically changing the timing of biological events across the globe. Changes in the phenology of seasonal migrations between the breeding and wintering grounds have been observed across biological taxa, including birds, mammals, and insects. For birds, strong links have been shown between changes in migration phenology and changes in weather conditions at the wintering, stopover, and breeding areas. For other animal taxa, the current understanding of, and evidence for, climate (change) influences on migration still remains rather limited, mainly due to the lack of long-term phenology datasets. Bracken Cave in Texas (USA) holds one of the largest bat colonies of the world. Using weather radar data, a unique 23-year (1995-2017) long time series was recently produced of the spring and autumn migration phenology of Brazilian free-tailed bats (Tadarida brasiliensis) at Bracken Cave. Here, we analyse these migration phenology time series in combination with gridded temperature, precipitation, and wind data across Mexico and southern USA, to identify the climatic drivers of (changes in) bat migration phenology. Perhaps surprisingly, our extensive spatiotemporal search did not find temperature to influence either spring or autumn migration. Instead, spring migration phenology seems to be predominantly driven by wind conditions at likely wintering or spring stopover areas during the migration period. Autumn migration phenology, on the other hand, seems to be dominated by precipitation to the east and north-east of Bracken Cave. Long-term changes towards more frequent migration and favourable wind conditions have, furthermore, allowed spring migration to occur 16days earlier. Our results illustrate how some of the remaining knowledge gaps on the influence of climate (change) on bat migration and abundance can be addressed using weather radar analyses.
Highlights
Every year, many billions of animals migrate across the globe in search of conditions that increase survival and reproductive success
A unique 23-year (1995-2017) long time series was recently produced of the spring and autumn migration phenology of Brazilian free-tailed bats (Tadarida brasiliensis) at Bracken Cave
These high temporal resolution population estimates revealed cyclic seasonal changes in the Bracken Cave colony size that are indicative of migration phenology, and were used to extract quantitative annual phenophases corresponding with spring and autumn migration timing (Stepanian & Wainwright, 2018)
Summary
Many billions of animals migrate across the globe in search of conditions that increase survival and reproductive success. For bat species living in highly seasonal (temperate) environments, migration is an essential part of their ecology (Fleming & Eby, 2003; Hutterer et al, 2005) Because of their cryptic nocturnal activity patterns and often secretive roosting, bat behaviour (including migration) has remained notoriously difficult to study (Fleming, 2019; Krauel & McCracken, 2013; Liechti & McGuire, 2017; Smith & McWilliams, 2016; Weller et al, 2016). We show to what extent these drivers may have caused the observed long-term temporal trends in migration phenology
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