A new SELVA study highlights the importance of stopover sites in determining the outcome of migration
How birds migrate is one of the most intriguing and difficult to answer questions in ornithology. Recent technological advances are beginning to shed light on this question, as illustrated by a study by SELVA researchers published in Scientific Reports this week. Gray-cheeked Thrushes fitted with miniature radio-transmitters, known as nano-tags, were tracked from Colombia to North America through an array of automated receivers, revealing remarkable 3000 km non-stop flights but also showing that the time taken to migrate is directly related to the conditions birds experienced in Colombia.
“The behavior of migrating birds has always been really difficult to study and so stopover sites do not figure highly in conservation strategies, but we wanted to change that” says lead author, PhD student and SELVA researcher Camila Gómez, “new research techniques now make it possible to understand how the conditions encountered by birds along the way affect the duration and length of subsequent migratory flights”.
The Motus Wildlife Tracking System consists of more than 300 automated receiving stations that record signals from tiny radio-transmitters, making it possible to track individual birds across continents with great precision. In the past researchers had to chase birds with a handheld receiver, such that birds could only be tracked over distances rarely exceeding 100 km, or recapture birds to retrieve information from data loggers.
A multi-institution team from three countries including Camila Gómez and Daniel Cadena (Universidad de los Andes), Ryan Norris (Guelph University), Keith Hobson (Environment and Climate Change Canada), Phil Taylor (Acadia University), Stuart Mackenzie (Bird Studies Canada), Nick Bayly (SELVA), and Ken Rosenberg (Cornell Lab of Ornithology) used the Motus system to track 97 Gray-cheeked Thrushes after leaving an intensively studied stopover site in the Sierra Nevada de Santa Marta, an isolated mountain on Colombia’s Caribbean coast.
Nick Bayly, co-author and manager of SELVA’s migratory species research area, could not believe it when he got the first detections of birds tagged in Colombia from North America, “we already knew that Gray-cheeked Thrush left Colombia with enough fuel to make it to North America (link) but that doesn’t mean they would do it in one direct flight. When we received news of a bird that flew 3,255 km from Colombia to Indiana in just 3.3 days, we thought there must be an error in the data. We kept thinking, why wouldn’t the bird stop further south but it seems that if birds have enough fuel and flight conditions are good, they just keep going, flying day and night”.
Stu Mackenzie, director of Motus, responding to the news of the first long-distance non-stop flights in 2015 wrote, “if these figures are right, they are freaking AMAZEBALLS”, which goes someway to emphasizing just how remarkable and game-changing these data were.
Ken Rosenberg from the Cornell Lab of Ornithology, who has supported SELVA’s work on migratory birds since 2010, highlights what this means for the conservation of migratory birds, “we used to think that landbirds used lots of stopover sites along the length of their migration route to complete their migration but this does not seem to be the case and for species like the Gray-cheeked Thrush, the Sierra Nevada de Santa Marta in Colombia is the equivalent of what Delaware Bay is to Red Knots”. For Cornell, this kind of information is vital to determining conservation priorities for Neotropical migratory birds at all stages of their complex life cycles. “This study not only highlights how important the Sierra Nevada de Santa Marta is for the migration of Gray-checked Thrush”, says Ken Rosenberg, “but also the urgent need to determine where other species are stopping over”.
“We really wanted to know how habitat loss or the use of different habitats might affect the success of migration”, says Camila Gómez, who is completing her PhD under the supervision of Daniel Cadena at the Universidad de los Andes in Colombia, “and showing that birds attaining smaller fuel reserves took up to 30 days longer to migrate to Ontario than birds leaving Colombia with large reserves, was a major breakthrough”. This means that if birds lose key habitats or food availability goes down because of habitat degradation, they will take longer to migrate, which could set in motion a chain reaction of events such as reduced breeding success. It could also mean more mortality, if birds attempt to “catch up”, taking desperate measures such as crossing large bodies of water, like the Gulf of Mexico, without emergency energy reserves. “These types of carry-over effects have been very challenging to study in the wild but they can have large impacts on both individual success as well as the abundance of populations” says co-author Ryan Norris.
Aside from revealing details about the biology of a little-known species which are important for conservation, “this study exemplifies that understanding the life cycles of migratory birds requires extensive cross-disciplinary collaboration among scientists” says Daniel Cadena, “our work involved researchers from seven institutions in three different countries”. He added that, “there are still many open questions about the connectivity between populations of Gray-cheeked Thrush and about the influence of events occurring in different phases of the annual cycle, affecting migratory strategies and population dynamics. We hope to solve some of the outstanding questions through ongoing analyses integrating the tracking data presented in this study with data generated using other tools including molecular genetic data and stable-isotope analyses”.