Ongoing changes in migration phenology and winter residency at Bracken Bat Cave

Abstract

Bats play an important role in agroecology and are effective bioindicators of environmental conditions, but little is known about their fundamental migration ecology, much less how these systems are responding to global change. Some of the world's largest bat populations occur during the summer in the south-central United States, when millions of pregnant females migrate from lower latitudes to give birth in communal maternity colonies. Despite a relatively large volume of research into these colonies, many fundamental questions regarding their abundance-including their intra- and interseasonal variability-remain unanswered, and even estimating the size of individual populations has been a long-running challenge. Overall, monitoring these bat populations at high temporal resolution (e.g., nightly) and across long time spans (e.g., decades) has been impossible. Here, we show 22 continuous years of nightly population counts at Bracken Cave, a large bat colony in south-central Texas, enabling the first climate-scale phenological analysis. Using quantitative radar monitoring, we found that spring migration and the summer reproductive cycle have advanced by approximately 2 weeks over the study period. Furthermore, we quantify the ongoing growth of a newly-established overwintering population that indicates a system-wide response to changing environmental conditions. Our observations reveal behavioral plasticity in bats' ability to adapt to changing resource availability, and provide the first long-term quantification of their response to a changing climate. As aerial insectivores, these changes in bat phenology and propensity for overwintering indicate probable shifts in prey availability, with clear implications for pest management across wider regional agrisystems.

bioRad: biological analysis and visualization of weather radar data

“…Following a proliferation of advances in information technologies, data infrastructure, and open data policies, access to low‐level weather radar data has greatly improved over the last decade (Huuskonen et al , Ansari et al ). These low‐level data consist of scans (sweeps) in polar coordinates of each of the observed quantities by the radar, collected at multiple beam elevations (in the European OPERA network called single‐site polar volumes, in the US NEXRAD network called level II data).…”

“…These profile quantities can be combined into multiple measures summarizing the number and passage of animals aloft. In the literature a large variety of measures can be found to report the amount of biological targets detected in the airspace by radar, like reflectivity factor z (Buler and Diehl 2009), reflectivity η (Dokter et al 2011, Chilson et al 2012b), vertically integrated reflectivity VIR (Gasteren et al 2008, Shamoun‐Baranes et al 2011, McLaren et al ), vertically integrated density VID (Buler and Diehl 2009, Dokter et al 2011, Horton et al ), migration traffic rate MTR (Nilsson et al 2019), or migration traffic MT (Dokter et al ). This section and Fig.…”

“…This value may be refined using more detailed knowledge about which species are migrating, e.g. from information on phenology or other independent measurements (Horton et al ).…”

“…1). Traffic rate measures have the important additional advantage of suppressing stationary (non‐migratory) signal components in weather radar data: reflectivity signal components with zero velocity will bias velocity estimates down by the same amount as their contribution to the total reflectivity, hence, measures that are based on the product of speed and reflectivity, like MTR and RTR, are effectively insensitive to these zero‐velocity signal components (Dokter et al ).…”

“…We recommend using the traffic measures dependent on transect angle α when estimating the actual passage across a geographic transect line of interest. Examples are the estimation of influx or efflux from a geographic region (Dokter et al a), or when comparing weather radar data to other sensors surveying along a stationary geographic line or plane, like a fixed vertically rotating ship radar (Fijn et al 2015). The measures independent of transect angle are most appropriate when quantifying traffic irrespective of the direction of movement, e.g.…”