Should I stay or should I go now? New study in PNAS explains how birds decide to jump the Gulf of Mexico

[Fern Davies]

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

1. Jill L. Deppea,1, 
2. Michael P. Wardb, 
3. Rachel T. Bolusb,c, 
4. Robert H. Diehlc, 
5. Antonio Celis-Murillob,
6. Theodore J. Zenzal, Jr.d, 
7. Frank R. Moored, 
8. Thomas J. Bensone, 
9. Jaclyn A. Smolinskyf, 
10. Lynn N. Schofielda,
11. David A. Enstrome, 
12. Eben H. Paxtong, 
13. Gil Bohrerh, 
14. Tara A. Beverothe, 
15. Arlo Raime,2, 
16. Renee L. Obringerh,
17. David Delaneyi, and 
18. William W. Cochrane

 

Significance

 

Bird migration has captivated the attention of scientists and lay people for centuries, but many unanswered questions remain about how birds negotiate large geographic features during migration. We tracked songbirds across the Gulf of Mexico to investigate the factors associated with birds’ departure decisions, arrival at the Yucatan Peninsula (YP), and crossing times. Our findings suggest that a bird’s fat reserves and low humidity, indicative of favorable synoptic weather patterns, shape departure decisions. Fat, date, and wind conditions predict birds’ detection in the YP. This study highlights the complex decision-making process involved in crossing the Gulf and its effects on migratory routes and speeds. A better understanding of the factors influencing migration across these features will inform conservation of migratory animals.

1. 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.

 

Published 2 Nov 2015 by the Proceedings of the National Academy of Sciences.