Fat, weather, and date affect migratory songbirds’ departure decisions, routes, and time it takes to cross the Gulf of Mexico

Abstract

Approximately two thirds of migratory songbirds in eastern North America negotiate the Gulf of Mexico (GOM), where inclement weather coupled with no refueling or resting opportunities can be lethal. However, decisions made when navigating such features and their consequences remain largely unknown due to technological limitations of tracking small animals over large areas. We used automated radio telemetry to track three songbird species (Red-eyed Vireo, Swainson's Thrush, Wood Thrush) from coastal Alabama to the northern Yucatan Peninsula (YP) during fall migration. Detecting songbirds after crossing ∼1,000 km of open water allowed us to examine intrinsic (age, wing length, fat) and extrinsic (weather, date) variables shaping departure decisions, arrival at the YP, and crossing times. Large fat reserves and low humidity, indicative of beneficial synoptic weather patterns, favored southward departure across the Gulf. Individuals detected in the YP departed with large fat reserves and later in the fall with profitable winds, and flight durations (mean = 22.4 h) were positively related to wind profit. Age was not related to departure behavior, arrival, or travel time. However, vireos negotiated the GOM differently than thrushes, including different departure decisions, lower probability of detection in the YP, and longer crossing times. Defense of winter territories by thrushes but not vireos and species-specific foraging habits may explain the divergent migratory behaviors. Fat reserves appear extremely important to departure decisions and arrival in the YP. As habitat along the GOM is degraded, birds may be limited in their ability to acquire fat to cross the Gulf. migration | ecological barrier | Gulf of Mexico | songbirds | weather D uring migration, animals encounter ecological barriers, inhospitable environmental features that prevent or impede movement due to increased risk of mortality from starvation, predation, collision, and severe environmental conditions (e.g., weather for aerial migrants, aquatic temperature or chemical gradients for aquatic migrants) (1-5). Because barriers can have important consequences on survival and future reproductive success (6), animals have evolved behavioral, morphological, and/or physiological means to safely negotiate them (7-9). Barriers can include large geographic features (e.g., large water bodies, deserts, mountains), inhospitable land cover types (e.g., agricultural "deserts"), anthropogenic structures (e.g., tall buildings, towers, dams, weirs), and unfavorable weather and aquatic conditions (e.g., droughts, storms, strong temperature gradients), although the extent to which any of these functions as a barrier to migration varies (4,5,(10)(11)(12).Approximately two thirds of all songbird species and millions of individuals breeding in eastern Canada and the United States encounter the Gulf of Mexico (GOM) while migrating to tropical or subtropical wintering grounds in the Caribbean, Mexico, and Central and South America (13). Unfavorable weather conditions combine...

The Motus Wildlife Tracking System: a collaborative research network to enhance the understanding of wildlife movement

“…This wealth of data has led to comprehensive and diverse applications in the study of breeding and postbreeding dispersal, stopover and migration behavior, and overwintering ecology. For example, detection data from Motus collaborators have been used to estimate stopover duration (Dossman et al 2016, Mann et al 2017, Neima 2017, activity level, e.g., onset of diel activity, stopover versus active migration (Crysler et al 2016;Morbey et al, unpublished data), regional and site fidelity during migratory stopover (Mann et al 2017, Neima 2017, precise departure and arrival times , Dossman et al 2016, Müller et al 2016), departure and flight orientation , Crysler et al 2016, Kishkinev et al 2016, Neima 2017, flight distance, time, and therefore flight speed (Woodworth et al 2014, 2015, Brown and Taylor 2015, Crysler et al 2016, Falconer et al 2016, colony attendance patterns (Loring 2016), and types of movements, e.g., migratory, relocation (Woodworth et al 2014, 2015, Crysler et al 2016). …”

Prolonged stopover and consequences of migratory strategy on local-scale movements within a regional songbird staging area

“…Although geolocators have revealed prolonged stops for multiple songbird species, understanding of stopover and movement behavior in these staging areas is still limited, since the coarse latitudinal precision of geolocators (~300 km) precludes study of fine-scale behavior (McKinnon et al 2014, Deppe et al 2015. Even relatively large landscape-scale stopover movements recorded with automated telemetry (e.g., 250 km; Brown and Taylor 2017) would not be registered with geolocators.…”

“…During fall migration, vireos are regularly sited along most of the eastern U.S. coastline (Sullivan et al 2009). They reach the breeding grounds by either crossing or circumnavigating the Gulf of Mexico (Deppe et al 2015) and thus represent a short-jump migrant. By contrast, most blackpolls exhibit an extreme jump strategy that involves a 2,500 km transatlantic flight from the eastern coast of the U.S. (DeLuca et al 2015).…”

Ghrelin affects stopover decisions and food intake in a long-distance migrant

“…However, the final decision to resume migration depends on the interplay between environmental conditions, the endogenous program, and the physiological condition of an individual. Theory predicts that stopover decisions depend on amount and rate of deposition of energy stores (1,2), and empirical findings have confirmed this hypothesis (3)(4)(5)(6)(7)(8)(9)(10)(11)(12). The big and yet unanswered question is, How do birds know that they are ready to depart?…”

“…Previous work has shown that fat stores are the best predictor for migratory disposition and stopover duration: Birds with larger subcutaneous (s.c.) fat stores show more migratory restlessness in captivity (8,10,13,14), stay shorter times at stopover sites (9,11), and migrate faster (12). Because birds fuel their migratory flights primarily through fatty acid metabolism (15)(16)(17), we expect that factors reflecting fuel load or the energetic condition must be involved in the behavioral switch between resting at stopover sites and resuming migration.…”