2018 declared Year of the Bird!
In celebration of the centennial of the Migratory Bird Treaty Act (MBTA), 2018 has been declared the Year of the Bird.1,2 For those already enthralled by avian antics, this is welcome news: Birds are receiving the recognition they deserve – huzzah! Those who have only a passing familiarity with birds, on the other hand, may find the excitement surrounding the declaration puzzling: Yes, birds are cute, and there is that whole birds-are-little-feathered-dinosaurs thing, but, really, what’s all the fuss about the MBTA? Why do birds matter?
As it turns out, from an environmental standpoint – as well as from a societal/cultural perspective – birds matter quite a lot. This article will explore the importance of birds and highlight some of the many roles played by birds in nature, as well as avian contributions to human society and culture.
FYI: If you want to cut to the chase and find out about Citizen Science opportunities involving birds, scroll down to the end.
“If you could see every bird in the world, you’d see the whole world.”
Jonathan Franzen, novelist3
There are more than10,000 species of birds alive today, and class Aves exemplifies diversity in form and function. Birds occupy every continent, utilize all habitat types, and display incredible variety in behavior and appearance. They have adapted to some of Earth’s most extreme environments: grey gulls are masters of arid conditions, rearing their chicks in Chile’s Atacama Desert; emperor penguins take the prize for cold tolerance, breeding during the Antarctic winter. They are hunters and gatherers and scavengers, with diets as varied as their vocalizations. Nectar, fruit, seeds, insects, amphibians, fish, small reptiles and mammals, other birds, carcasses, and even blood (yes, there is a vampire ground finch!) can be found in avian diets. They range in size from the tiny bee hummingbird (~2 g) to the immense ostrich (~104 kg). Feathers reflect an impressionist’s palette of color – the result of a kaleidoscope of possibilities incorporating every hue, shade, and tone imaginable – producing plumage as wildly divergent as the subtle beauty of a sparrow’s humble browns and greys to the resplendent brilliance of birds of paradise. Nests run the gamut from the simple, barely functional scrapes made by killdeer to the complex, highly decorated designs of bowerbirds (boudoirs meant to woo and seduce). Birds are masters of flight . . . . or not. Some species migrate thousands of miles each year (Arctic terns, red knots), while others remain in the same locality year-round (blue jays, northern cardinals). Some species spend months at a time on the wing (European swifts), while other species can barely fly (turkeys), and still others are completely incapable of flight (kiwis, emus, penguins).
The value of birds
The tremendous diversity of birds contributes to their importance within ecosystems. Ecosystem services refer to the benefits that humans derive from the natural world, and birds are key players in providing many of these benefits. Ecosystem services provided by birds can be broadly grouped into four categories: provisioning services, regulating services, supporting services, and cultural services.4, 5
Provisioning services are easily understood: these are the “commodities” that can be extracted from natural environments. Wild birds are (or were) a source of game meat, down, and guano.5 Harvesting of seabird guano (a combination of urine and feces) for use as fertilizer was a thriving industry in the 1800s, but use of the material declined in the 1900s when synthetic mechanisms for manufacturing fertilizers were developed.6 In the developed world, wild birds are now largely obsolete as sources of raw materials, as these goods can now be obtained more easily through other means (farmed birds, synthetic alternatives). In less developed regions, however, wild birds are still important sources of raw materials.
Regulating services include activities such as pollinating plants, scavenging carcasses, controlling populations of pest species, and dispersing seeds.5
Birds reduce the destructive impacts of various pest species. Two examples illustrate this point: consumption of spruce budworms by songbirds and raptor impacts on rodents.
Spruce budworms are the caterpillars of moths in the genus Choristoneura. These caterpillars are destructive insects that feed on fir and spruce trees; in large numbers they cause extensive economic damage by defoliating – and sometimes killing – trees. In Washington state, the consumption of spruce budworms by birds yields an estimated benefit of $1473/km2/year.5
Rodents are favored prey of raptors (owls, hawks, etc.), and rodents themselves feed extensively on various crops. Over its lifetime, a barn owl eats approximately 11,000 mice – mice that would consume 13 tons of crops.5 With an average lifespan of 4 years, the owl’s annual take is 2750 mice/year – clearly, healthy populations of raptors can have a suppressing effect on rodent numbers.
Raptors do not actually have to kill the animals to reduce depredations by rodents. Research with owls trained to fly over (but not hunt in) areas occupied by wild gerbils showed that the presence of owls reduced gerbil activity and the amount of food consumed by the gerbils.7 This study was one of the pioneering works that lead to a new branch of ecological enquiry, a subdiscipline concentrating on how fear changes animal behavior and affects interactions in ecological communities.
Pollution and seed dispersal
Although the role of birds as pollinators is minimal – only 3-5% of some 1500 economically important crop and medicinal plants are bird-pollinated5 – birds are extremely important as seed dispersers. For many reasons, it is generally advantageous for seeds to be removed from the immediate vicinity of the parent plant prior to germination. The production of fleshy fruits, such as berries, is one tactic that plants have evolved to co-opt animals for this important task. Simply put, a fruit is a bribe. The sweet (or oily) pulp encourages consumption, and the seeds, provided they are not crushed during eating, travel unscathed through the animal’s gut and pass out with the feces (nutrients for the germinating seed! bonus!). Birds are exceptionally good seed dispersers: they lack teeth and tend to swallow food whole, and they often move long distances between the areas where they feed and the areas where they rest/defecate. Most woody tree species (92%) are dispersed by birds, a number that includes 85 timber species, 182 edible plants (including spices), 153 medicinal plants, 146 ornamental species, plus 84 genera with other economic/cultural significance.5 As an example of the value of the seed dispersal services of a single bird species, consider Clark’s nutcracker, a member of the corvid (crow) family: Clark’s nutcrackers disperse seeds of whitebark pine in western North America; the annual value of the birds’ service is estimated at ~$2200/ha (or ~ $12.6 billion over the entire range of whitebark pine).5
Carrion birds (vultures, buzzards, condors, etc.) play a vital role in carcass removal. When animals die intact (from disease, drowning, etc.), if the hide is thick or tough, carrion birds, with their powerful beaks, may be needed to “open” the carcass for scavenging. Work in Spain estimates the annual value of carcass removal at €1 million5 (~ $1.25 million). Vultures are particularly valuable as carrion birds because they feed on carcasses that are in a more advanced state of decay than other scavengers will handle. A vulture’s extremely acidic gut secretions destroy many of the bacteria, as well as bacterial spores and toxins, associated with rotting meat, and pathogenic bacteria that may have been the cause of death, such as anthrax, are also destroyed.8 Hence, consumption of carrion by vultures helps limit the spread of disease. Simple reduction of the number of carcasses available to other scavengers also has benefits. Unintentional poisoning of vultures resulted in an increase in rabies and possibly an outbreak of bubonic plague in India.5 In the 1990s, livestock were treated with diclofenac (an anti-inflammatory); diclofenac was later found to cause kidney failure in vultures, and its use has led to the near extinction of some species.5,9,10 As vulture numbers declined, the increased availability of carcasses led to an increase in feral dogs and rats.5,10 These alternate scavengers act as reservoirs for diseases that affect humans. Consequently, the loss of vultures has been associated with an increase in human rabies deaths (spread by feral dogs) and may be responsible for an outbreak of plague (spread by rats).5 The economic impact has been high: 48,000 human rabies deaths between 1992 and 2006 cost the Indian economy $34 billion, and a plague outbreak in 1994 cost $2 billion.5
Supporting services include processes such as nutrient cycling, soil formation, and ecosystem engineering.5,11
Nutrient cycling and soil formation
As one might imagine, an exact count of the number of individual birds on the planet is impossible to come by. However, it is clear that birds are abundant; estimates place the number between 200 billion and 400 billion.12 Due to their abundance, birds constitute important players in ecosystem processes and functioning. Billions of birds consuming many billions of tons of food digest that food and transform it into simpler components. Digestive wastes passed into the environment undergo further decomposition and become available to plants as soil nutrients. The feathers that birds molt and the inedible components they leave behind when feeding (seed hulls, fur, bones, etc.) also enter nutrient cycles. This abundance of organic material enriches soils and contributes to the process of soil formation.
Additionally, birds are mobile links between systems, moving nutrients between different habitats and over great distances.11 Fish-eating birds move nutrients (in the form of fish) from aquatic environments to terrestrial ones. During migration, food may not be fully processed until a bird is hundreds of miles from the place where the food was consumed. This tremendous mobility makes birds extremely important in global nutrient cycling as well as local nutrient cycling.
Ecosystem engineering is the process by which organisms alter the environment by creating structure, and this structure impacts the organism(s) that created it and/or other organisms in the environment.13 The structure made can be a deliberate construct on the part of the organism, such as a beaver’s dam, or it can be the consequence of the growth of the organism(s), such as a coral reef. Plants, fungi, and microbes, as well as animals, impart structure to their environments. Birds act as ecosystem engineers through the construction of nests, tree holes, and burrows – structures that are used by other organisms when abandoned by birds.
Cultural services encompass the place of birds in human society in art, religion, and leisure activities.5
From the sacred ibis of ancient Egypt to the doves, sparrows, and eagles prevalent in Christian theology, birds have long played a role in religious iconography. In some traditions, birds are central characters. Among the Lenape (Native American), for example, the sacrifice of Many-Colored Crow teaches the value of selflessness and service. In other traditions, birds are companions or guides. In Nordic mythology, Odin, the father of the gods, is accompanied by two ravens, Hugin (Thought) and Munin (Memory); the ravens fly out into the world each day and bring back all the news of what they have seen to Odin. In still other traditions, birds serve as symbols. In ancient Greece, the phoenix symbolized renewal, while the owl – companion bird of the goddess Athena – embodied wisdom. The role of birds in religion may also be of a more practical nature, such as the sky burials or blood sacrifices practiced in some cultures.
For as long as humans have been creating art, birds have been providing inspiration. Birds are represented in paleolithic carvings and cave paintings. Leonardo DaVinci’s notebooks contain exquisite studies of bird wings that he drew during his investigations of flight. Many notable painters included birds in their work – Van Gogh’s Wheat Field with Crows and Monet’s The Turkeys at the Chateau Rottembourg are just two examples. A few artists have devoted their lives to capturing birds on canvas. John James Audubon, the 19th century painter (and namesake of the Audubon Society) is likely the most famous of these. A double elephant folio (a really BIG book – 39.5 inches by 28.5 inches) of Audubon’s The Birds of North America sold at auction in 2010 for $10.3 million.14
In addition to being commemorated on canvas, birds have made their mark in literature. Jonathan Livingston Seagull (the title character in Bach’s novella), Hedwig (Rowling’s Harry Potter series), Thorondor (Tolkien’s Lord of the Rings), and Owl (Milne’s Winnie the Pooh) are just a few of the better-known avian characters. Birds are elemental characters in numerous poems (Poe’s The Raven, Keats’ Ode to a Nightingale, and Henry’s Pigeons, to name a few), serving as inspiration, symbol, and allegory. Avian themes also abound in music (Tchaikovsky’s Swan Lake, Stravinsky’s Fire Bird), film (The Birds, Fly Away Home, March of the Penguins), and even video games (Angry Birds).
Leisure and tourism
The most easily economically quantified cultural services provided by birds are the leisure activities they support – hunting and birding – both of which support tourism, as people often travel specifically to engage in theses pastimes.
Birds hunted for sport include both terrestrial birds (such as pheasant and grouse) and waterfowl (ducks and geese). According to data from the US Fish and Wildlife service, in 2011, waterfowl hunting by ~ 1.5 million hunters in the United States generated ~$1.4 billion in direct economic activity (retail expenditures) and ~ $3.0 billion in total output (includes other economic activity supported by hunting), as well as and state and federal revenues of $202 million and $234 million, respectively.15 Interestingly, these estimates do not appear to take into account the actual value of the meat of the waterfowl hunted. For this reason, bird hunting in the United States is more accurately categorized as a cultural service (leisure activity) rather than a provisioning service (supporting basic need for food).
The economic benefits of bird watching as recreation far outstrip the economic benefits of bird hunting. In the United States in 2011, 47 million people engaged in birding – most of them (41 million) from their own homes.16 Birders generated ~$41 billion in direct economic activity (retail expenditures) and ~ $107 billion in total output (includes other economic activity supported by birding), as well as state and federal revenues of $6 billion and $7 billion, respectively.16 Birding is an activity that is relatively evenly distributed across economic classes and genders, though there is a marked tendency for participation to increase with age and level of education.16 Birding has the benefit of being a hobby that, due to the increasing availability of online bird identification guides and smart phone apps, can be enjoyed with relatively little financial cost – though birding aficionados can, and obviously do, spend a great deal of money on equipment to watch birds, books to identify birds, cameras to photograph birds, food/feeders to attract birds, and travel to see birds.
Economic losses due to birds
Though humanity derives great benefit and pleasure from birds, birds can cause economic harm. These “dis-services” include collisions with aircraft, befouling of public areas and bodies of water, spreading disease, and agricultural losses.
Bird-plane collisions are not frequent, but they do occur, causing an estimated $1.2 billion annually in damage to aircraft worldwide.17 Though such collisions are typically fatal to the bird, loss of human life or serious damage to the aircraft is rare.17 Collisions occur most frequently during takeoff or landing, and the culprits are typically flocking birds, such as geese, seagulls, and starlings. The (in)famous incident (now a major motion picture) in which the pilots of US Airways Flight 1549, Sullenberger and Skiles, made a forced landing on the Hudson River was precipitated by multiple bird strikes (Canadian geese) at low altitude.17 The potential problems/dangers/costs posed by birds at airports are sufficiently high such that some airports and military bases have resorted to a variety of measures to deter birds and/or reduces bird population size. These control measures include modifying the habitat around the airport to make it less attractive to birds, employing sonic canons and other noise generators to scare away birds, and using trained raptors or dogs to harass/frighten birds.
Birds as public nuisances and vectors of disease
Where birds gather in large numbers, their feeding and excrement can damage the area and may pose health risks. Large congregations of ducks and geese in parks with ponds (or other areas sporting grassy expanses with water, such as the manicured landscapes around many corporate buildings) are an example of this. Defecation on land while feeding (geese are the major culprits here) creates odor and obstacle hazards on lawns and paths that reduce enjoyment of park-goers. Defecation in (or near) the water contributes to nutrient loading that promotes algal blooms (eutrophication) that reduce water quality in ponds and lakes. Humans promote congregating behavior by feeding waterfowl. “Feeding the ducks” is a popular pastime, despite being harmful to the birds and contributing to environmental degradation. Fortunately, many municipalities are beginning to take steps to discourage the feeding of waterfowl.
Transmission of disease is more likely when birds congregate. Some of the diseases carried by birds affect only other birds. Duck viral enteritis (duck plague) and avian botulism are examples of this type of disease. Some diseases generally affect only other birds, but there is a possibility that they could become zoonotic and pass to humans. Bird flu is an example of this type of disease. Birds can also act as reservoirs for diseases that are vectored by parasites (e.g. mosquitoes). West Nile virus is an example of this type of disease.
Bird damage in agricultural settings
The scarecrow as a popular symbol of farming suggests that birds are often viewed as being destructive to crops. This is particularly true of fruit production. For example, aggregate losses/damages inflicted by birds to select fruit types (honeycrisp apples, wine grapes, blueberries, and cherries) across five states (California, Oregon, Michigan, New York, and Washington) were estimated at $189 million.18 To reduce bird-related losses, a variety of techniques have been employed, including netting, scare tactics (including visual deterrents, such as streamers and scarecrows, and noise makers, such as sonic canons, bangers, and bird distress cries), chemicals, and, more recently, trained raptors and dogs. Efficacy of these methods varies greatly.
Interestingly, most information concerning crop losses due to animal depredations come from surveys sent to farmers, as opposed to actual controlled experiments and measurements in the field. It has been suggested that farmers tend to over-estimate crop losses to birds: surveys indicate that, while farmers believe they lose 25% of their crop to birds, actual bird-related losses are less than 1%.5
Birds viewed as detrimental to agriculture may (or used to) be hunted as vermin. “Vermin” is a designation reserved for animals that are deemed dangerous to livestock or crops, carry disease, or that damage cropland/pastureland. Birds historically labeled as vermin include crows, ravens, starlings, and most raptors. Prior to their extinction, passenger pigeons and Carolina parakeets were also considered vermin (a label that contributed to their eventual extinction). At one time, state and local governments paid bounties for species designated as vermin. Though many states still label certain species as vermin, and have hunting seasons for these species, the practice of paying bounties is in decline. Birds are largely protected from vermin hunting under the MBTA.
Nearly 200 species of birds have gone extinct since the 1500s.19 Very broadly, the mechanisms causing avian extinctions can be grouped into two categories: predation and habitat loss.
Predation includes factors such as hunting/trapping and the introduction of invasive species. Problematic invasives are sometimes other bird species that compete with native birds, or plant species that displace the vegetation that native birds rely on. Most often, however, the invasive species that cause bird extinctions are introduced predators; among the worst offenders are domestic cats (that become feral), red foxes, and brown tree snakes.
Habitat loss includes any mechanism by which habitat is rendered unsuitable for use by the bird species in question. This includes conversion of natural areas to non-natural landscapes (agriculture, logging, residential/commercial development, etc.), as well as factors that change the character of an area in such a way that the landscape is no longer hospitable to the bird (pollution, climate change, fire/increased fire frequency, etc.).
Invasive species, hunting/trapping, logging, and agriculture were the main drivers of historic species loss, contributing to the extinctions of 122, 77, 45, and 33 species, respectively.20 Today, more than 1300 species of birds (13% of all bird species worldwide) are under threat of extinction,21 and agriculture now represents the major threat to birds.20 Both past and present, avian extinctions have been particularly prevalent on islands. (The why behind that statement is the topic for another blog.)
A century ago, commercial exploitation of birds for meat and feathers threatened many avian species. Passage of the Migratory Bird Treaty Act made it unlawful to pursue, hunt, take, capture, kill, or sell birds listed therein. The MBTA was one of the country’s earliest pieces of environmental legislation, and it expanded on the protection provided to a limited number of bird species under the Lacey Act of 1900. The Endangered Species Act of 1973 came later; its goal is to rescue imperiled species, and a species must be classified as endangered or threatened throughout its range to receive protection. Triggering the protection of the endangered species act requires that a species be well studied – substantial data is needed in order to make the determinations about changes in population size.
Currently, more than 1000 bird species fall under the protection of the MBTA.1 The advantage of the MBTA is that, as an international treaty, it acknowledges the value/importance of birds, recognizes that bird populations transcend national boundaries, and broadly protects migratory bird species. Protections have, historically, been extended, not just to the birds themselves, but also to the habitats that sustain them; there is concern that this interpretation will change. The National Audubon Society web site has a good article explaining the MBTA and how it may be at risk.
Conservation and citizen science
There are many ways to become involved in bird conservation besides establishing feeders in your yard and/or supporting organizations devoted to bird/habitat protection (both worthy endeavors!). Getting involved in bird-related research is a third (and very enjoyable) option. Organizations, such as the Cornell Lab of Ornithology and the National Audubon Society sponsor a variety of programs that use observations submitted by cadres of Citizen Scientists. These projects are not one-and-done events: some initiatives have been going on for decades. The long-term datasets produced are used to track bird populations, look for changes and trends, and help develop management plans for vulnerable species. The data collected address questions about important population parameters such as geographic range size, numbers, timing of breeding and migration, and prevalence of disease.
The Great Backyard Bird Count
One such Citizen Science initiative is taking place soon. The Great Backyard Bird Count (GBBC) is a four day event that occurs every February over President’s day weekend (it runs from the Friday before through the Monday of President’s Day). The first GBBC took place in 1998, and in 2013 the event went global. GBBC statistics for 2017 show participation on all continents (including Antarctica), over 180,000 checklist submissions, 6261 species sightings, and 29.6 million individual birds counted.
The GBBC is open to birders of all ages and skill levels. IUPUI’s Center for Earth and Environmental Science will be participating in the GBBC this year, leading novice birders as part of the Center’s service learning program, as well as doing bird counts at the Lilly ARBOR.
“If you take care of birds, you take care of most of the big problems in the world.”
Thomas E. Lovejoy, Tropical Conservation Biologist1
Birds matter. They are important contributors to healthy, functional ecosystems. It behooves us to learn how their populations are changing and how changes in the environment affect them. So get out there and count some birds! Do it for fun! Do it for the birds! Do it for Science!
And if you are trying to get young people excited about bird watching . . . . . just stress the whole feathered dinosaur aspect . . . . . . or tell them that it is like Pokémon Go, but for real.
- Cornell Lab of Ornithology: 2018 Declared Year of the Bird and Citizen Science project descriptions
- Great Backyard Bird Count 2018 press release
Citizen Science projects
- Great Backyard Bird Count
- Project FeederWatch
- Celebrate Urban Birds
- Christmas Bird Count
- Habitat Network
For help with bird identification
The MBTA does not discriminate between live birds, dead birds, or bird parts (skins, bones, claws, beaks, feathers) or constructs (eggs, nests). The Fish and Wildlife Service oversees enforcement of the MBTA and has the authority to issue permits allowing activities normally prohibited by the MBTA, such as the hunting of game birds. Permits are also issued for a variety of other reasons, including taxidermy, falconry, scientific and educational purposes, depredation control (e.g. killing geese that become a hazard at airports), and religious ceremonies of Native American tribes. So – technically – unless you have a permit, don’t collect that nest or pick up that cool feather – you’ll run afowl of the law!
- A group of vultures is called a kettle (if in flight), a committee (if on the ground or roosting in trees), or a wake (if feeding on a carcass).
- A group of crows or ravens is a murder; a common ornithological joke is that two crows together constitute attempted murder.
- Angry Birds is the most popular freemium game series ever produced, with over 3 billion downloads.
- Owls fly silently.
- A forced water landing, made by a plane not suited for landing on water, is termed ditching – which sounds so much less horrible than crash landing. The ditching of US Airways Flight 1549 is described as the most successful ditching in aviation history.17
Not so fun facts: leading causes of accidental bird mortality
A casual survey of available data indicates that the major causes of accidental bird mortality (attributable to humans) include collisions with buildings (window strikes), collisions with vehicles, collisions with power lines, and cats (not necessarily in that order). Other important sources of mortality are poisoning (usually indirect), collisions with communications towers, electrocution (a different category than smacking into power lines), oil pits, hunting, fishing bycatch, and collisions with wind turbines. Although most sources list similar causes of bird deaths, deaths attributed to a particular cause – and hence importance as a source of mortality – differ considerably. For instance, the number of deaths attributed to “cats” each year ranges from 2.4 billion (US Fish and Wildlife) to 500 million (Sibley) to 118 million (Bird Almanac). A closer inspection of the details reveals telling differences: the US Fish and Wildlife data (from 2016) and the Sibley data (from 2003) refer to deaths attributed to all domestic cats (both house pets and feral cats), while the Bird Almanac data (from 2004) refers specifically to bird deaths caused by house cats. Other sources of human-caused bird mortality, such as habitat destruction, are considered important, but actual number of deaths attributable to these causes are unknown.
Here are a few things that the average person can do to help reduce accidental bird deaths:
- Keep pets indoors; cats are predators – they will hunt if given the opportunity.
- Spay/neuter your cats (both “indoor cats” and “outdoor cats”). Growing populations of feral cats are becoming problematic in many areas; don’t let your cats contribute to the increase.
- Install windows screens. By reducing reflection, screens can reduce the number (and severity) of bird window strikes.
- Minimize pesticide use. Chemicals marketed for agriculture and home gardening may be lethal to birds. Raptors are susceptible to collateral damage inflicted by poisons directed at rodents; snap traps or live traps are alternatives to mouse/rat problems outside the home. Remember: a raptor will eat thousands of rodents over its lifetime – they are far better at removing rodents than we are! If a hawk/owl eats a rodent that has been poisoned, the bird may die as well.
1National Geographic. Why Birds Matter. January 2018, vol. 233(1). Goldberg S. (editor in chief).
2National Geographic Press Room: http://press.nationalgeographic.com/2018/01/05/national-geographic-annou…
3Franzen J, Sartore J. 2018. Why birds matter. National Geographic 233(1): 30-57.
4Costanza et al. 2017. Twenty years of ecosystem services: How far have we come and how far do we still need to go? Ecosystem Services 28:1-16.
5Sekercioglu CH. 2017. How much is a bird worth? Living Bird 36(3): 18-20.
6Wikipedia article: guano: https://en.wikipedia.org/wiki/Guano
7Abramsky Z, Rosenzweig ML, Subach A. 2002. The cost of apprehensive foraging. Ecology 85: 1330-1340.
8Roggenbuck et al. 2014. The microbiome of new world vultures. Nature Communications 5:5498. doi: 10.1038/ncomms6498
9Becker, R. 2016. Cattle drug threatens thousands of vultures. Nature. doi:10.1038/nature.2016.19839
10Ogada DL, Keesing F, Virani MZ. 2012. Dropping dead: causes and consequences of vulture population declines worldwide. Annals of the New York Academy of Sciences 1249: 57–71. doi:10.1111/j.1749-6632.2011.06293.x
11Sekercioglu CH. 2006. Increasing awareness of avian ecological function. Trends in Ecology and Evolution 21(8): 464-471. doi: 10.1016/j.tree2006.05.007
12Gaston KJ, Blackburn TM. 1997. How many birds are there? Biodiversity and Conservation 6(4): 615-625.
13Jones CG, Lawton JH, Shachak M. 1997. Positive and negative effects of organisms as physical ecosystem engineers. Ecology 78(7): 1946-1957.
14The Economist: Most expensive book titles sold at auction. https://www.economist.com/blogs/dailychart/2010/12/books
15 Carver E. 2015. Economic impact of waterfowl hunting in the United States: Addendum to the 2011 national survey of fishing, hunting, and wildlife-associated recreation. US Fish and Wildlife Service. Report 2011-6.
16 Carver E. 2013. Birding in the United States: A demographic and economic analysis. US Fish and Wildlife Service. Report 2011-1.
17Wikipedia article: Bird strikes: https://en.wikipedia.org/wiki/Bird_strike
18Anderson A, et al. 2013. Bird damage to select fruit crops: the cost of damage and the benefits of control in five states. Crop Protection 52: 103-109.
19Wikipedia article: List of recently extinct bird species: https://en.wikipedia.org/wiki/List_of_recently_extinct_bird_species
20Conant E. 2018. Birds on the brink. National Geographic 233(1): 24-25.
21IUCN Red List: birds: https://www.iucn.org/theme/species/our-work/birds