Biologists go the distance for the safe release of migratory birds

Featured Image Caption: The orange-bellied parrot is a critically endangered bird that migrates from Tasmania to mainland Australia each year (Image Source: “File:Neophema chrysogaster female 2 – Melaleuca.jpg” by JJ Harrison ( is licensed under CC BY-SA 3.0.)

Reference: Stojanovic, D. 2023. Altered wing phenotypes of captive-bred migratory birds lower post-release fitness. Ecology Letters, 26, 789-796.

Conservation Programs Don’t Wing It

As winter approaches, temperatures cool, days shorten, and resources become depleted. This ignites an instinctual fire in many living things, prompting them to take action. For example, some birds take to the skies in search of areas with more resources, like food or nesting sites to raise their young. Massive bird migrations are suspected to have occurred for hundreds of thousands of years. However, human activity and climate change have upped the danger factor of an already treacherous feat. As many migratory birds now face extinction, there are efforts to help conserve and recover failing wild populations. These recovery programs often involve breeding birds in captivity and releasing them into the wild.

Rounder wing tips (top) occur when the longest feather is not much longer than the adjacent feather, while sharper wings result from a larger difference (Image Source: Figure 3 in the featured paper; Stojanovic, 2023).

While captive-breeding programs are attractive attempts to boost natural populations, conservation biologists ask the tricky question: Are they working?

The primary objective of any release program is to optimize the chances of surviving in the wild. Birds’ wings are specially designed to improve aerodynamic function, enabling them to make such distant flights. Sharper wing tips are more aerodynamic, while rounder tips increase drag, slowing the bird’s flight. Research has recently shown that being bred into captivity can alter the shape of the wing. It is vital to understand whether that change impacts their survival upon release. As such, conservation researchers must identify traits that differ between captive and wild populations and learn how those differences affect survival in the wild.

Trait Determines Fate

Captive-breeding programs typically select only the birds with the highest probability of surviving for release, including birds with good body mass and feather condition. Dr. Dejan Stojanovic at the Australian National University adds wing shape to the list of considerations for bird selection. He used decades-old specimens from Australian museums to detect captivity-related changes in wing shape among 16 species of birds. Four of the 16 bird species exhibited changes in the shape of their wings when born in captivity versus the wild. Still, some closely related species showed opposite effects, with some wings becoming increasingly rounded in captivity while others became sharper.

Tasmania (darker shade) is below the southeast shore of mainland Australia (Image Source: “Tigris-Australia location Tasmania” by theEmirr is licensed under CC BY 3.0).

To test whether changes to wing shape significantly impact migration ability, Dr. Stojanovic studied a threatened Australian bird: the orange-bellied parrot. This critically endangered parrot species breeds during the fruitful summers of southwestern Tasmania, feeding on seeds and flowers. As resources deplete, they migrate north along the western coast of Tasmania to the southeastern shores of mainland Australia. During this trip, juveniles are experiencing increasingly high death rates compared to adults. While the reason for this stage-specific decline remains unclear, researchers hypothesize that habitat loss and drought are challenging the inexperienced juveniles at the wintering sites. To address this gap in knowledge, recovery plans explain the state of a vulnerable species and provide a research and management guide to recover the wild population to a sustainable size. The 2016 recovery plan for orange-bellied parrots reported only 50 birds in the wild and 320 in captivity.

Dr. Stojanovic has contributed significantly to the research effort for this species. He measured the wing shape of 78 captive-bred juveniles before their release and observed which birds returned the following year. Wing shape played a significant role in survival – minimally sharper wing tips exhibited almost three times higher chance of survival. Only about a fourth of the released birds returned the following year.

A Fight for the Flight

Wing shape varies between wild- and captive-born birds of several species tested, but not all species experienced similar changes. While some birds displayed sharper wing tips in captivity – thought to be a beneficial trait in the wild – others had rounder, less-efficient wings. Even slight changes in wing shape were sufficient to alter the probability of returning from the first migration of the critically endangered orange-bellied parrot. This research has vast implications for conservation practices. Before the study, birds were selected for release based on details like body and feather condition and wing length. Recovery programs now have an additional metric to identify the best-fit birds for release to the wild.

Wing shape is important for these flying orange-bellied parrots spotted getting a refreshing sip of water (Image Source: “orange-bellied parakeet, orange-bellied parrot” by outsidefourwalls is licensed under CC BY 4.0.).

While this information will surely help conservation efforts, many questions remain open. For example, why does wing shape change when some birds are born in captivity? Researchers are also uncertain whether captive traits can be reverted to wild traits and what cues are needed for that to occur. An additional question is why minor wing shape differences have such effects on survival. One explanation is that drag on rounded wings might slow migrating birds down and cause them to expend more energy to make the trip. The wing shape also affects take-off, which could slow escape from predators. As the research pushes forward, each study brings new knowledge of ways to further protect declining migrating bird populations.

Wing shape is a promising new avenue for ensuring the best survival chances during long-distance bird migrations. With the help of conservation biologists and research like this, we hope that all threatened migrating bird species will soar once more.

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Brandi Pessman

I am a fifth-year Ph.D. student at the University of Nebraska-Lincoln in the School of Biological Sciences. Growing up on a farm in a small town in Illinois, I developed an early love for animals and a fascination with their behaviors. When I was younger, however, it never crossed my mind that I would be using spiders to investigate how human presence affects animal behavior, but I am loving every second of it. Studying the behaviors of animals can tell us a lot about the role that we play in their survival (or death), which is becoming increasingly important as human populations continue to grow. When I am not studying spiders, I enjoy playing with my cat or being outdoors!

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