This blog post is provided by Ana M. González and tells the #StoryBehindThePaper for the article “Earlier and slower or later and faster: Spring migration pace linked to departure time in a Neotropical migrant songbird”.

Several species of migratory birds, many populations of which are in steep decline, spend the winter exclusively in montane forest in the Andes of northern South America between 1000-2500 m above sea level. Unprecedented rates of deforestation in the Andes have forced some migrants into agro-ecosystems that maintain native trees and provide “forest-like” habitats. Shade-grown coffee plantations are the best-known example. Agro-ecosystems are the predominant land cover in much of the region occupied by Neotropical migrants during the winter. While a growing body of research has shown that shade-grown coffee plantations provide suitable habitats for Neotropical migrants, we lack information about the effect of using shade coffee vs. native forest on migratory strategies.

Previous work has demonstrated that the onset of migration is a critical decision in the life of migratory birds because it can affect their survival and reproductive success. We have also learned to accept that early departure from wintering grounds in Central America and the Caribbean is related to the use of high-quality habitats where food is abundant. This is key, because early departures lead to early arrival on the breeding grounds and higher reproductive success, but is early departure the only strategy for achieving this? Shedding light on this question has been challenging until recently. Now technology has made it possible to reliably track small birds with incredibly high precision, over incredibly long distances. 

For the past five years, we have been researching Swainson’s Thrushes in shade-grown coffee plantations and forests in the eastern Andes of Colombia.  Early in the morning of March 19, 2015, I captured a thrush in a mid-elevation forest, 200 km southwest of Bogotá. It was a bird born in the summer of 2014 and was spending its first winter in South America. From here on let’s call the bird “Pecas” (meaning ‘freckles’ in Spanish). He will carry a tiny backpack with a radio-transmitter from Colombia to North America. Upon release, Birds Canada’s vast Motus Wildlife Tracking System, an international research network that operates hundreds of automated radio telemetry stations across the Americas, tracked Pecas’ local movements 24/7 and his journey northwards in Spring.

Pecas set off on his first spring migration from the Colombian Andes to North America the night of April 12. A month after departing from the Andes, Pecas was detected by the Motus array nearly 6000 km away, in Canada. If this was not remarkable enough, the Motus station responsible for the detection was less than 200 km south from the campus of the University of Saskatchewan, where I was doing my PhD at the time! Pecas was one of several Swainson’s Thrushes tracked from Colombia and each bird’s journey raised a series of interesting questions. For example, did Pecas depart earlier or later than other individuals in the forest, or relative to individuals overwintering in adjacent habitats in the Andes? Did he spend more or less time migrating? Did he arrive early or late to the breeding grounds? Were migratory strategies related to the quality of the habitat where the birds were captured? The answers to these questions are explored in our recent publication in the Journal of Animal Ecology.

To understand the relative quality of shade coffee vs.  forest, we measured stable carbon isotopes in the thrushes’ blood and assessed moisture patterns. This and other evidence pointed to forests being of higher quality, but despite this, Pecas and other thrushes wintering in forests departed later on spring migration than birds using adjacent shade coffee. These results contradict previous studies that described birds departing earlier from high quality habitats. The twist in the story, however, was that in the case of Swainson’s Thrushes, birds that departed later migrated faster and could essentially “catch up” to some degree, with early migrants, which challenges the generality that early departure leads to early arrival.

The differences we found in departure between habitats suggested that departure timing is a plastic response to differences in conditions experienced on the wintering grounds. Most thrushes in our sites departed in April and as late as the first week of May, way after the onset of the rainy season – it’s important to note that both fruit and insect abundance is positively associated with rainfall in the tropics. From a thrush’s perspective, leaving later to take advantage of abundant food allowing them to accumulate large fat reserves quickly, migrate faster, and ultimately catchup with earlier departing birds, might just be a safer strategy. So why leave earlier when you can stay longer and catch up? We showed that ‘early and slow’ and ‘late and fast’ strategies enabled birds to reach the breeding grounds within a narrow arrival window, so the main advantage of leaving early is likely associated with competition avoidance on the wintering grounds, and the possibility of finding higher quality stopover sites for birds that settled in low quality winter habitats.

For Pecas, and most thrushes, arrival time to the breeding region and the number of days spent migrating was linked to the quality of winter habitat and environmental conditions thousands of kilometers away in the Andean mountains of South America. This reinforces that the different stages of the life cycle of migratory birds are inextricably linked, and that climate and land cover changes on the wintering grounds likely impact survival, breeding success and ultimately population trends of long-distance migrants.

Our research also highlights the value of international collaborations for studying migratory animals at continental scales. Our Colombian field team was key to conducting ethical and sound field work that guaranteed the safety and wellbeing of the birds, allowing us to obtain quality data. We are grateful for their hard work and dedication. We also thank all members of the Motus collaborative network, our funders, and the generous landowners that provided unequivocal access to the farms over so many years.

Read the paper

Read the full paper: González, A.M., Bayly, N.J. and Hobson, K.A. (2020), Earlier and slower or later and faster: Spring migration pace linked to departure time in a Neotropical migrant songbird. Journal of Animal Ecology. Accepted Author Manuscript. doi:10.1111/1365-2656.13359