Swainson’s Thrushes do not show strong wind selectivity prior to crossing the Gulf of Mexico

Abstract

During long-distance fall migrations, nocturnally migrating Swainson’s Thrushes often stop on the northern Gulf of Mexico coast before flying across the Gulf. To minimize energetic costs, trans-Gulf migrants should stop over when they encounter crosswinds or headwinds, and depart with supportive tailwinds. However, time constrained migrants should be less selective, balancing costs of headwinds with benefits of continuing their migrations. To test the hypotheses that birds select supportive winds and that selectivity is mediated by seasonal time constraints, we examined whether local winds affected Swainson’s Thrushes’ arrival and departure at Ft. Morgan, Alabama, USA at annual, seasonal, and nightly time scales. Additionally, migrants could benefit from forecasting future wind conditions, crossing on nights when winds are consistently supportive across the Gulf, thereby avoiding the potentially lethal consequences of depleting their energetic reserves over water. To test whether birds forecast, we developed a movement model, calculated to what extent departure winds were predictive of future Gulf winds, and tested whether birds responded to predictability. Swainson’s Thrushes were only slightly selective and did not appear to forecast. By following the simple rule of avoiding only the strongest headwinds at departure, Swainson’s Thrushes could survive the 1500 km flight between Alabama and Veracruz, Mexico.

Nocturnal departure timing in songbirds facing distinct migratory challenges

“…The effects of environmental cues predictive of the upcoming synoptic weather conditions on birds’ nocturnal departure decisions are, however, only marginally investigated (e.g. Bolus et al., ; Bulyuk, ; Deppe et al., ) and thus require further consideration in future studies.…”

“…Y-axis is given on a log 10 -scale nocturnal departure decisions are, however, only marginally investigated (e.g. Bolus et al, 2017;Bulyuk, 2012;Deppe et al, 2015) and thus require further consideration in future studies.…”

“…Yet, several studies revealed considerable temporal variation in nocturnal departure timing, reviewed in Müller et al (2016). Part of this variation seems to be explained by intrinsic factors (fuel stores) and extrinsic factors (wind; Müller et al, 2016), which are known to act as cues for day-to-day departure decisions in migrants (Bolus et al, 2017;Deppe et al, 2015;Schmaljohann & Eikenaar, 2017;Shamoun-Baranes, Liechti, & Vansteelant, 2017;Smolinsky, Diehl, Radzio, Delaney, & Moore, 2013). Several studies revealed that birds carrying large fuel stores and experiencing favourable winds depart early at night, whereas birds migrating with little fuel and under unfavourable winds show considerable variation in their nocturnal departure timing (Eikenaar, Müller, Kämpfer, & Schmaljohann, 2016;Schmaljohann & Naef-Daenzer, 2011;Sjöberg, Alerstam, Åkesson, & Muheim, 2017;Sjöberg et al, 2015;Smolinsky et al, 2013), but this pattern is not ubiquitous (Bolshakov et al, 2007;Bulyuk, 2012;Bulyuk & Tsvey, 2006).…”

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

“…The defining role of weather patterns in bird migration has a long history [1,2], including observations of unusually early migrant arrivals and occurrences of regionally rare birds in response to intense storms [3]. More recent research supports the influence of weather on many aspects of avian migration, including energetic costs of flight [4], timing [5], flight duration [4], route choice [6], and risk of mortality [7,8]. Relationships of wind and precipitation with the flight behavior of nocturnally migrating landbirds (i.e., passerines and related species with terrestrial life histories) are particularly well studied.…”

“…Migrants "fall out" after crossing water when weather conditions along a coastline are unfavorable, but they can fly farther inland when weather is favorable [25,26]. Broad-scale weather conditions encountered during flight over ecological barriers (e.g., large water bodies or terrain inhospitable for landing) presumably influence subsequent stopover distributions [7]. However, support for this has been difficult to measure because neither precise locations of small birds in airspace over such regions nor origins of their flights are known.…”