Broad-Scale Weather Patterns Encountered during Flight Influence Landbird Stopover Distributions

Abstract

The dynamic weather conditions that migrating birds experience during flight likely influence where they stop to rest and refuel, particularly after navigating inhospitable terrain or large water bodies, but effects of weather on stopover patterns remain poorly studied. We examined the influence of broad-scale weather conditions encountered by nocturnally migrating Nearctic-Neotropical birds during northward flight over the Gulf of Mexico (GOM) on subsequent coastal stopover distributions. We categorized nightly weather patterns using historic maps and quantified region-wide densities of birds in stopover habitat with data collected by 10 weather surveillance radars from 2008 to 2015. We found spring weather patterns over the GOM were most often favorable for migrating birds, with winds assisting northward flight, and document regional stopover patterns in response to specific unfavorable weather conditions. For example, Midwest Continental High is characterized by strong northerly winds over the western GOM, resulting in high-density concentrations of migrants along the immediate coastlines of Texas and Louisiana. We show, for the first time, that broad-scale weather experienced during flight influences when and where birds stop to rest and refuel. Linking synoptic weather patterns encountered during flight with stopover distributions contributes to the emerging macro-ecological understanding of bird migration, which is critical to consider in systems undergoing rapid human-induced changes.

A place to land: spatiotemporal drivers of stopover habitat use by migrating birds

“…Our comprehensive modelling of bird stopover distributions across this region supports important seasonal differences in the function of coastal sites. We found birds concentrated near the coastline during spring where they were less influenced by landscape composition, characteristic of fallout to rest (Gauthreaux et al 2006; Lafleur et al ., 2016; Clipp et al ., 2020), while birds stopped further inland during autumn in landscapes with increased forest cover, characteristic of habitat selection for refuelling (Buler and Moore, 2011). When spring weather conditions are favourable, coastal habitats may offer refuge for birds that are young, physiologically stressed, dehydrated or in poor energetic condition incapable of sustained flight (Spengler et al ., 1995; Leberg et al ., 1996).…”

“…While bats cannot be distinguished from birds during movement through the airspace, ‘roost rings’ of bats are visible when they depart en mass for nocturnal flight (Stepanian and Wainwright, 2018). Therefore, we excluded data from areas around the Houston (KHGX) and Corpus Christi (KCRP) radars in Texas with persistent free‐tailed bat ( Tadarida brasiliensis ) roosts (Horn and Kunz, 2008; Clipp et al ., 2020). Mapping the relative density of birds departing stopover habitat among radars requires an instantaneous measure of the density of migrating birds aloft during the onset of nocturnal flight for each sampled night.…”

“…The patterns of where and when migrating birds stop during spring and autumn reflect variability in the drivers of the filtering process and how migrants use stopover habitats. For example, poor energetic condition and weather may concentrate migrants in the first available habitats along coastlines, irrespective of food resources, where they can rest and recover after making non‐stop over‐water flights (Gauthreaux et al 2006; Lafleur et al ., 2016; Clipp et al ., 2020). Alternatively, migrants may concentrate in food‐rich habitats to forage and build energetic reserves before making a non‐stop flight (Buler et al ., 2007).…”

Inclement weather forces stopovers and prevents migratory progress for obligate soaring migrants

“…We found that Turkey Vultures frequently stopped actively migrating to avoid flying during poor weather conditions, adding to the growing literature recognizing weather avoidance as an important driver of stopover use by migrating birds [ 37 – 39 ]. Turkey Vultures ceased directed flight quickly in response to high rates of change in several weather variables and resumed migration when conditions improved.…”

Stopover behavior of Red-eyed Vireos (Vireo olivaceus) during fall migration on the coast of the Yucatan Peninsula

“…Prior to crossing water bodies, for example, migrants-especially lean individuals-may rest briefly along the coast before relocating inland, presumably to seek more food-rich habitats (Deutschlander and Muheim 2009;Smolinsky et al 2013). In contrast, after crossing, birds may be fat depleted (Moore and Kerlinger 1987), which leads to a high concentration of individuals in coastal habitats (Lafleur et al 2016;Clipp et al 2020;Cohen et al 2021) where they may refuel (Moore et al 2017), take long stopovers (Dossman et al 2016), and move among coastal habitats (Deutschlander and Muheim 2009). Importantly, lean birds are known to gain more mass and take longer stopovers than heavier birds (Moore and Kerlinger 1987;Loria and Moore 1990;Dossman et al 2018) because there may be a minimum fuel load to attain before a bird can move on, depending on the distance that needs to be covered.…”