Captive Birth Reduces Survival In The Wild For Migratory Birds

Captivity can alter the wing shape of birds, reducing their chances of surviving after they are released into the wild

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Breeding animals in captivity and releasing these individuals into the wild is an important conservation tool. It is typically used to increase numbers and, sometimes, the genetic diversity, of rare or endangered species in relative safety from their predators, diseases and other issues. For example, captive breeding followed by release back into the wild is credited with assisting the conservation of the California condor (more here), which almost went extinct but is now breeding again in the wild.

But captive breeding is not a panacea: it can alter a species’ natural behaviors that are necessary to survive in the wild. Migration is one such behavior that is negatively affected by captive breeding, which can inspire conservation biologists to ‘think outside of the box’ as they strive to re-establish or guide its expression in released animals and birds, as seen in whooping cranes (more here).

A new study (ref) suggests that being born in captivity can affect more than behavior: it can also change morphology, such as the shape or length of a bird’s wings. Bird wing shape is critical for migration success and suboptimal phenotypes are strongly selected against. If captive bred animals show strong fitness costs in the wild, conservation breeding could be less effective than hoped for (more here).

“Previously we’ve shown that captivity can change the shape of orange-bellied parrot wings, which we suspected might make their migration flights more difficult”, said the sole author of the new study, conservation biologist Dejan Stojanovic, a postdoctoral fellow at the Australian National University’s College of Science. Dr Stojanovic has a particular interest in parrots, and his doctoral research led to the discovery of severe predation by sugar gliders on endangered swift parrots. He also studies the conservation and breeding biology of orange bellied parrots and Carnaby’s cockatoos, both of which are endangered species.

“But this new study reveals the first direct evidence that altered wing shapes in captivity lower migration success after release to the wild”, Dr Stojanovic said in a statement.

Orange-bellied parrots are subject to one of the largest, longest running breeding programs of any Australian species. Teetering on the edge of extinction, their wild population is supplemented annually with releases of juveniles bred in captivity.

In this study, Dr Stojanovic found that captive-born juvenile orange-bellied parrots with a ‘wild wing phenotype’ (a 1mm longer distal primary flight feather) survived post-release at 2.7 times the rate of those with a ‘captive wing phenotype’ (i.e. a shorter distal feather) (Figure 1).

Surprisingly, even such subtle phenotypic changes and their fitness impacts are more common than widely realised because they are easily overlooked, as Dr Stojanovich discovered when he examined the wing phenotypes for 16 other species of birds commonly bred in captivity. These findings suggest that such easily overlooked changes may be more common in captivity than previously thought (Figure 2).

This is abslutely astonishing: just a 1mm reduction in wing length from the optimal wild type is associated with a much greater juvenile mortality.

“This is likely only the tip of an iceberg of subtle physical changes to the bodies of captive bred animals that, although easily overlooked, have a big impact after release”, Dr Stojanovic explained.

Of course, this raises the obvious questions: why are these birds’ wing lengths altered by simply being born in captivity? What is it about the captive environment that has this damaging effect? Can captive-born individuals recover their optimal wing length after extensive flight training? Or are they stuck with short wings forever? Further, how can such a gross waste of extremely limited conservation resources be justified? Equally worth asking is how ethical is it to release captive-born individuals, knowing almost all of them will die almost immediately?

“These questions need to be answered so that we can work out how to breed animals well suited for life in the wild”, Dr Stojanovic agreed.

As a breeder of a wide variety of parrot species and other birds myself, I cannot even imagine the intense heartbreak and despair this reality must cause those who work with these critically endangered birds, hoping that some of these precious youngsters will survive and live and contribute to building a future in the wild for their critically endangered species.

“We should be aware of this and find ways to mitigate the effects of captivity if we want to give our breeding programs the best chance of supporting wild populations”, Dr Stojanovic said.

“This could become especially important as the global extinction crisis forces more species into captive breeding programs.”

Source:

Dejan Stojanovic (2023). Altered wing phenotypes of captive-bred migratory birds lower post-release fitness, Ecology Letters | doi:10.1111/ele.14200

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