Nomadic birds depart with moderate—but not large—fuel stores

Joely G. DeSimone, Beverly S. Domschot, Megan A. Fylling, William M. Blake, Creagh W. Breuner

This is a plain language summary of a Functional Ecology research article which can be found here.

              Migration is a common animal behavior, allowing animals to track food sources that vary across time and space. Many birds migrate seasonally—breeding in the north and spending the non-breeding season to the south—because the insects and fruits they rely on are only abundant during warm months. Because these bird species have predictable migrations, they can anticipate and prepare for long-distance flights. They prepare by eating more and storing fat which they use to fuel their flights.

Birds that rely on seeds from conifer trees have very different migration patterns. Conifer trees produce seeds irregularly, so the birds that eat these seeds do not develop predictable, seasonal movements. Instead, scientists think their movements are often in direct response to declining food availability. However, it is unknown if these birds can still prepare for flight by packing on fat stores, similar to seasonal migrants, if they flee an area when they’re out of food, or something in between. Our study is the first time scientists have related physiology (body mass, fat stores, and a hormone called corticosterone) to the movements of a nomadic bird in the wild.

A siskin’s-eye-view of the Bitterroot River in western Montana (credit: Joely DeSimone)

We studied pine siskins in western Montana and tracked their local and long-distance movements with radiotransmitters. We found that siskins need moderate—but not large—fuel stores before leaving an area. Surprisingly, the hormone corticosterone, which usually relates to birds’ seasonal migrations and escape movements, did not underlie siskins’ behavior. Overall, the patterns we observed differ from studies in captivity, highlighting the importance of fieldwork, where food and behavior vary in more realistic ways. We conclude by describing ways that physiology interacts with food availability to drive different movements and behaviors.

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