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The avisphere

2022; Elsevier BV; Volume: 32; Issue: 20 Linguagem: Inglês

10.1016/j.cub.2022.09.046

1879-0445

Florian Maderspacher,

This July, while pruning a pear tree, I came across an empty bird’s nest. Not long after, three juvenile blackbirds appeared out of nowhere, hopping about irritatedly, as though trying to find their way among the newly shaped branches; no doubt, this had been their nest and their native tree. Though long departed, they did not seem to appreciate the upheaval. What to me was superfluous, undesirable growth, to them was the hallmark of home. I had crossed, destructively, into their sphere. By some measure, birds really do live in their own sphere — the avisphere, as it were — which is more than just the air through which they flutter, glide and soar. Their ability to move about swiftly in three dimensions has enabled a lifestyle that is entirely different from that of other terrestrial animals — more dynamic, more wide-ranging, but also more conspicuous and exuberant. The aerial, volant lifestyle has shaped their bodies in ways that contribute to the eerie otherness of their appearance: their often richly ornamented aerodynamic plumage, their lightweight and efficient bills and beaks. It has also shaped their behavior, their reproduction and their brains. This special issue aims to provide a showcase of bird biology, a slice of the avisphere. Birds’ breakout into three dimensions was ultimately enabled by that most crucial evolutionary innovation — the feather. Sometime during the Jurassic, complex feathers with interlocking barbules evolved in a few lineages of theropod dinosaur. In some of them, feathers grew big, creating an airfoil for gliding and ultimately powered flight. Tiled feathers form the outer hull of the bird, supporting aerodynamic shape. But feathers also double as formidable billboards in all sorts of colors and shapes, advertising the evolutionary prowess of their bearer. The swift-moving, elevated life off-the-ground allowed birds to become more daring in their visual displays. Some of them use their feathers in protean ways, assuming entirely new forms, like some birds of paradise do. The stunning visual beauty of birds may have tuned our own sense of aesthetics, if only in simple appropriation whereby humans from Bavaria to Papua New Guinea adorn themselves with (stolen) feathers. The same liberated exuberance evolved in the acoustic domain in the myriad songs and sounds of birds that dominate Earth’s soundscapes. Most birds are highly mobile, often commuting between multiple habitats and ecosystems. About four in ten bird species are migratory, seasonally traversing climatic zones. Birds have colonized remote and hostile regions, such as inland Antarctica or otherwise lifeless deserts. The avisphere is a truly global sphere, transcending countries, cultural spaces and continents. Perhaps the connectedness of birds can teach our own species a much-needed planetary outlook. The mobility and conspicuousness of birds has made the avisphere at once tangible and out of reach to humans. This has led to a human fascination with birds that has both dark and bright sides. Birds are trapped or killed for meat or eggs, for feathers or their song, as pests or for sport. But because birds are so conspicuous, changes to the avisphere are felt poignantly. Birds have become sentinels of environmental change and ambassadors of conservation. Birds were the focus of some of the first conservation efforts. The decline of the Bald Eagle, along with many other species, led to a ban on pesticides, and the prospect of a Silent Spring invigorated the environmental movement. Birds like the California Condor or the Kākāpō have become icons of vulnerability and recovery. The avisphere is like the colored plumage of the living world, a display of its health and fragility. Because it is so accessible and so alluring, millions of people enjoy birdwatching as a pastime, fuelling tourism and, more importantly, collecting invaluable data on location and abundance of birds, providing a real-time view of the avisphere. Some bright spots notwithstanding, that plumage of the living world looks sparse and plucked. With all their skill to evade predators, birds are not able to evade that most rapacious and systemic of predators and the damages it inflicts on the biosphere. Like all of nature, they are paying the toll for our expansiveness. About one in eight of the 11,000 bird species are threatened by extinction. Historically, bird extinctions mainly played out on islands, especially through introduced predators, but now they increasingly affect continental species. Among the latest possible losses are — call them by their names! — Eskimo Curlew, Alagoas Foliage-gleaner, Pernambuco Pygmy-owl, Glaucous Macaw or Purple-winged Ground Dove. Many of these species come from Brazilian coastal forests, biodiversity hotspots that have been all but destroyed. The loss of each species is a tragedy, the severing of a unique thread in the fabric of life on Earth. But apart from the extinction of entire species, the overall avisphere is also shrinking globally at an alarming rate. Nearly half of all bird species are declining in number. How many fewer birds this means is only known for the best-researched parts of the world, Europe and North America. Today, these regions harbor nearly four billion fewer birds than in 1970 — almost one third. In North America, birds in certain habitats, such as grasslands, or birds with certain lifestyles, such as migratory birds, are particularly vulnerable. A similar trend is seen in Europe, where common farmland birds have declined by nearly 60% since 1980. Tens of millions of birds are actively killed in Mediterranean countries, for food or sport, mainly by men probably trying to re-enact a long-outdated image of masculinity. Nearly half of all bird species are traded for body parts or as pets. In Indonesia in particular, millions of birds are kept in captivity — a sad attempt to pin down their fleeting nature, to encapsulate their beauty. In North America, hundreds of millions of birds die in collisions with tall buildings, temples of greed that breach the sky in concrete and glass. The most existential threats are those that affect birds indirectly, through changes in their habitat. And it is these threats that birds render much more readily apparent than other, less well-characterized and less visible animals. Human activity and encroachment and the ensuing changes in land use, as well as deforestation or the degradation of farmlands, are the biggest threats to ecosystems worldwide and often manifest themselves first as bird declines. Therefore, it is likewise through acting on habitats — their restoration and preservation and the adoption of bird-friendly land-use practices — that birds and the ecosystems they inhabit can be safeguarded. Other measures, such as bird-safe buildings and infrastructure and minimizing light pollution, complement these fundamental actions. The most common charitable act people commonly do for birds is feeding them, which, while helping some species at the feeder, may have negative repercussions in the wider ecological network. The most bizarre and perhaps the most revealing relationship we have is with one particular bird, the Red Junglefowl. A native of Southern and Southeastern Asia, this bird was domesticated about 8,000 years ago and — as domestic chickens — has since conquered Earth in the wake of human expansion. The consequences have been momentous. Today about 33 billion chickens live on Earth and about twice that are slaughtered every year. By comparison, the most abundant wild bird, the Red-billed Quelea of Sub-Saharan Africa is thought to number about 1.5 billion birds. In terms of biomass, chickens may outnumber wild birds by a factor of two. In the same timeframe during which wild birds have decreased by a third, the amount of chicken meat production has quintupled. We have created a separate bubble of the avisphere around this single bird species. The chicken is a mirror of the transformative dominance of our species on Earth. As much as we fancy ourselves living in the information or the space age, we really live in the chicken age. For most humans, chickens are the prime — or only — source of animal protein. In many agricultural societies, chickens are an essential part of the farming ecosystem, for instance by controlling pests or by providing fertilizer. But in the poultry industry of so-called developed societies, the chicken itself has transmuted into an altogether different bird: a miracle and a nightmare of biology. A modern broiler chicken reaches two kilograms in about a month. It often does not live to see the end of its second month. Its entire life cycle is optimized and controlled — from artificial egg incubation to rearing under crowded conditions, with over 20 birds per square meter. Skeletal problems and other health issues ensue, as well as genomic impoverishment and gait anomalies; and of course, it cannot fly. This industrial bird is the distorted mirror image of the intricate complexity and foreign beauty of the avisphere. It suggests that we can only co-exist with animals if we bereave them of their essence. It cannot survive on its own, and neither can we, its creators, if we shape the entire biosphere according to our own worst image. Instead, we should let the birds lead the way.