With regard to their high adaptability to human settlements and global distribution, corvid birds (crows, ravens, jays, etc.) are good models to understand the impacts of urbanization on wildlife. At least 30 corvid species have become successfully accustomed or adapted to urbanized environments. The availability of easily accessible food and artificial nesting sites, coupled with low levels of predation, were found as the most important factors benefitting corvids in cities around the world.
1. Introduction
Urbanization is a spatio-temporal process of the development of cities and the increase in the concentration of populations in them, followed by a transformation of natural habitats into artificial ones
[1][2][1,2]. In general, urbanization is strongly associated with increased cover of imperious structures (e.g., buildings, streets) and human population density, as well as the fragmentation, degradation and loss of natural habitats. An urban development is an ecological modification that often alters the functions of a given ecosystem by affecting the structure of the food chain by removing or adding species, by encouraging human tolerance and adaptation, by increasing health risks for humans and wildlife and by modifying ecological processes in relation to ecosystem services
[3]. Urbanization leads to complex, diverse systems characterized by high levels of human disturbance, pollution and landscape and environmental changes
[1][2][4][1,2,4]. These changes can affect the biology, behavior, morphology and reproductive and survival traits of wildlife and can be responsible for the disappearance of native species and the appearance of non-native ones
[5]. Therefore, understanding these effects is essential for successful wildlife conservation and management in urban habitats.
The negative impact of human-made landscapes and infrastructures on wildlife has been detected in many studies
[6][7][8][6,7,8]. However, numerous studies have also described how certain species, such as corvids (e.g., crows, magpies), can benefit from these infrastructures, such as using buildings, poles and power lines as nesting sites
[9][10][11][9,10,11]. In addition, anthropogenic food resources and milder microclimate in cities might benefit many corvid species
[1][2][6][1,2,6]. Urbanization has been considered as an overwhelming evolutionary force acting on the life-history traits and population genetics of species
[8]. Currently, urbanization is still expanding at an accelerating pace
[12], unfortunately coinciding with a continuous increase in habitat loss. Although studies on the effects of urbanization on birds, at a community or individual species level, have been widely conducted, multi-species approaches with species belonging to the same family are still very scarce.
Corvidae is a family of mid to large-sized passerines. Many corvid species thrive in many types of urban environments, from the peripheral urban areas to highly urbanized urban core areas
[13][14][15][13,14,15]. Because of the wide distribution areas of many corvid species and good adaptability to many habitats, corvid species are often described as urban adaptors and even exploiters
[16][17][18][16,17,18]. Additionally, considering the high diversity and broad distribution of species of the Corvidae family, their spatio-temporal historical dispersal over numerous geographic and ecological areas may most likely contribute to the increase in taxonomic biodiversity
[19]. Thus, corvids can be considered as the ideal subjects for investigating the effects of urbanization on birds. Lowry et al. (2013) have stated that the first observed adjustment shown by wildlife species in a human-made environment is a modification of behavior
[20]. For example, numerous wild animals have been observed to alter their breeding, nesting and foraging patterns, diet composition, as well as vigilant behavior and vocalization, in response to human-made environments
[21][22][23][24][21,22,23,24].
2. Corvids in Urban Environments
Corvids have been widely successful in adapting to urbanized environments over recent decades over a worldwide geographic scale. At least 23% of the 130 corvid species in the world are reported to live in urbanized environments, attesting this group’s extreme flexibility in resource use and highly opportunistic and plastic behavior, often enabling them to reach high abundances in various cities around the world. In contrast, there is no evidence of occurrence in urban environments for 100 corvid species. However, this does not mean that these species avoid cities and towns; rather, it reflects the lack of detailed studies documenting the occurrence and breeding of corvids in urban environments.
2.1. Factors Explaining the Adaptation of Corvids to Urban Environments
Most corvids are generalist species (able to consume a variety of foods and survive in different types of habitats) with a high degree of behavioral plasticity that makes them easily adaptable to environmental changes [14][25][26][27][14,70,71,72]. It has been widely established that many corvid species have flexible survival modes in order to benefit increasingly from anthropogenic habitats in different areas of the world. In studies assessing movement patterns and/or nest site selection, the impact of anthropogenic food sources on individuals’ choices was explored. For example, while it has been reported that non-breeding Common Ravens adapt their space use patterns to benefit from human food sources over large areas [28][29][30][31][74,75,76,77], breeding pairs choose to nest in areas with good food availability nearby their nests [32][31][33][61,77,78]. Indeed, earlier studies have indicated that breeding Ravens forage mostly around their nest [34][35][79,80]. Moreover, movement patterns of the American Crow are strongly correlated with the abundance of anthropogenic resources [26][36][37][71,81,82], as easily accessible food in urban areas was described as the main cause for the regular movement of rural American Crow fledglings to the cities, thus resulting in annual increases of the urban populations [26][37][38][71,82,83]. Furthermore, food availability highly influenced the abundance and habitat choice of urban Carrion Crows and Jungle Crows (Corvus macrorhynchos) [39][40][41][42][43][84,85,86,87,88]. Even the Alpine Chough (Pyrrhocorax graculus), one of the least explored corvids, was observed to alter its foraging behavior and to increase its survival based on the availability of anthropogenic foods [44][25][45][55,70,89]. Winter bird feeding in cities and towns by humans also further increases foraging opportunities to corvids, such as for the Eurasian Magpie and Hooded Crow in northern latitudes [46][90]. The role of food availability is also supported by indirect evidence; for example, the decline of the Western Jackdaw populations in several European cities is partially caused by the lack of food [47][48][91,92].
The second most frequently mentioned factor influencing corvid presence in urban habitats is the availability of novel nesting sites in the cities, which, along with their highly behavioral flexibility, can also explain the colonization of cities by corvids. For example, the Common Raven is almost exclusively nesting on anthropogenic structures, such as electric poles, in cities [3][28][49][50][3,74,96,97]. Magpies and Hooded Crows were also reported to change their nesting habits in urban settings. For example, Magpies tend to nest on the highest trees available in response to high levels of disturbance [5][51][52][53][5,27,98,99]. However, with decreased persecution, Magpies can nest on a less preferred site and build their nests at lower heights [17]. Hooded Crows will also nest on non-preferred tree species and at lower heights as the urban population increases and preferred nesting sites become scarce [54][100]. Again, cavity nesters, such as the Western Jackdaws offer indirect evidence for the importance of nesting sites because jackdaw populations declined in some European cities mainly due to the loss of suitable nest sites caused by renovations and modernizations of buildings that had been previously suitable for cavity-nesting [47][54][55][56][91,100,101,102].
The third most frequently mentioned factor influencing urban corvids is their high adaptability in behavior, physiology and breeding biology. Corvids’ responses to environmental change have been shown to be highly flexible, and it is suspected to be correlated with specific biological traits shared by corvid species [21][22][57][58][59][60][61][21,22,28,53,104,105,106]. For instance, breeding biology parameters of several corvid species were reported to differ between urban and non-urban populations in several corvid species, including the Eurasian Magpie, American Crow, Common Raven and Hooded Crow, and to a lesser extent, the Steller’s Jays (Cyanocitta stelleri) and Jackdaws. In addition to earlier breeding, urban birds produce smaller clutches (fewer eggs) and rear their young longer until fledging, as well as produce smaller fledglings and fewer potential future breeders than non-urban birds [3][21][24][62][63][3,21,24,30,107]. While some of these differences appear to imply disadvantages of nesting in urban environments, they did not seem to affect the size or growth of the urban populations of these species. This discrepancy may be explained by two factors. First, the increasing population sizes of some urban corvids may be due to the immigration of young crows from adjacent rural populations into the city [36][81], as the higher breeding success in rural areas induced movements of rural dispersers into the city, creating a net immigration of crows into cities [37][82]. Second, Marzluff et al. (2001) demonstrated that urban crow populations tend to increase partially because survival rates are higher in urban than in rural corvid populations in North America [36][81]. It is important to note that most studies carried out during non-breeding seasons suggested that chances of survival are probably higher in cities than in non-urban habitats, as attested by large numbers of individuals that move from rural areas to cities for overwintering, at least in the Northern Hemisphere [64][65][66][67][108,109,110,111].
2.2. Corvids’ Responses to Urbanization
Since access to anthropogenic resources in the cities and high levels of adaptation to novel environments often translate into shifts in corvids’ activities, as stated above, many corvids change their behavior and get accustomed to human presence. The birds’ responses to human presence and proximity have been extensively studied, often in experiments, in the American Crow, the Carrion Crow, Torresian Crow (
Corvus orru) and Jackdaws. The various behavioral experiments aimed to explore the crows’ tolerance towards humans and their danger recognition ability in urban settlements
[68][45][69][29,89,114], while other studies aimed to investigate social learning in these birds
[70][71][72][73][115,116,117,118]. Decreased persecution in cities in the last few decades as opposed to rural areas appears to be an important factor promoting the corvids’ tolerance and habituation to humans and traffic; this tolerance is a pre-requisite of colonization of city centers as breeding habitat
[17][74][17,93]. Moreover, such habituation has been widely documented in the American Crow
[26][36][75][71,81,119].
In addition, several studies demonstrated that corvids can depend on social cues to learn about dangers in a given area and that they are able to communicate this information to each other
[76][29][77][65,75,120]. An extreme case of habituation to human presence has been observed in a Carrion Crow captured in Vienna Zoo, which did not show any sign of struggle during handling by researchers
[78][121]. Finally, studies based on measurements of crows’ flight initiation distance upon human approach have indicated that escape distances are considerably shorter in urban than in non-urban areas in many corvid species
[79][80][122,123].
2.3. Corvid Human Interactions
The increased abundance of corvids due to urbanization means that corvid-human interactions will also increase. Corvids have been reported to cause conflicts due to their disturbing noise, fecal droppings, garbage scattering, damages to infrastructures and aggressive behavior towards humans and pets. For that reason, corvids have often been perceived as nuisance birds and have intensively been persecuted or even hunted in many cases
[60][81][82][105,124,125]. However, in relation to wildlife and nature conservation, for example, the EU Birds Directive
[83][126], the disturbance of birds during their spring migratory and breeding periods is banned, as well as prohibiting the large scale and non-selective means of bird killing.
Because corvids often forage on communal waste found in household garbage dens, on commercial refuse in dumpsters, near outdoor restaurants and food stands, trashcans in parking lots, and landfills
[37][75][84][82,119,127], corvids can serve as vectors of disease transmission. In particular, communal roosts and the feeding location of corvids in cities greatly increase the chances of disease transmission, which is of great concern for human and animal health
[85][86][128,129]. Several studies addressed the potential role of wild birds as vectors and spreaders of pathogens of important zoonotic and other human-related diseases in urban areas. Most studies of zoonosis in crow species focused on the west Nile virus (WNV), which can cause high mortality in corvids, as demonstrated in the American Crow
[87][88][89][90][91][36,37,130,131,132]. Due to their high susceptibility to WNV, crows and ravens can thus be used as biosensors or early indicators of the presence of this virus in a given area. Similarly, corvids have been proposed or already been used to detect other pathogens of public health concern
[92][93][94][95][133,134,135,136].
In addition to diseases, contamination by human-made pollutants has been documented worldwide in various urban corvids. For example, increased lead concentrations and high levels of dioxins have been detected, and other environmental chemicals have been observed in Eurasian Magpies
[96][97][98][99][100][137,138,139,140,141], Common Ravens
[101][142], Rooks
[102][103][104][143,144,145] and Jungle Crows
[105][106][107][146,147,148]. While these studies emphasize the detrimental impacts of these pollutants on urban wildlife, their biological and physiological implications on the survival or reproduction of wild animals in urban areas are not yet fully understood. Finally, high corvid abundance in cities often leads to the homogenization and/or depauperating of the urban bird fauna
[108][109][149,150]. Some studies have indicated that the expansion of corvids, or their increase, in urban habitats have caused decreases in the richness or abundance of other species mainly via increased nest predation rate
[110][111][112][25,151,152].
Based on the artificial nest predation experiments, corvids are often perceived as efficient nest predators that directly impact the populations of other bird species
[113][114][153,154]. Despite this perception, relatively few experimental removal or population control studies have been performed in corvids
[111][115][116][117][151,155,156,157].
2.4. Management Efforts
Corvids in urban areas have often been considered as pests and sources of nuisance, thus have become the target of management efforts around the world
[82][118][119][125,162,163]. Human-corvid conflicts emerge because of the corvids’ garbage scattering, fouling infrastructures, roosting in high numbers on roofs and in parks, unpleasant vocalization, attacks on pets and humans during chick-rearing, use of sensitive infrastructure for nesting and predation on birds and other animal species dear to humans. These conflicts have initiated a large number of studies discussing the management of these birds in urban areas
[120][121][122][123][124][164,165,166,167,168]. Many studies focused on the House Crow, a highly invasive corvid originally from Southeast Asia (mainly Pakistan and India) that has recently colonized and been thriving in cities of the sub-Saharan region and in the Middle East, and on the Pied Crow (
Corvus albus) in Brazil. The wide range of these birds and their high flexibility makes them targets to persecution by shooting
[125][82][118][119][120][26,125,162,163,164].
3. Conclusions
Corvids have long been associated with the development of urban environments, and their worldwide distribution makes them the perfect model system to study the effects of urbanization on wildlife. A considerable proportion of species in the Corvidae family have already been shown to adapt to urban environments, and, with consideration to the geographical and taxonomical bias in our literature sample, it appears likely that many more species will be shown to be successful in the adaptation process. The primary traits of corvids that enable them to exploit new, urban environments are their high behavioral plasticity and flexible resource use. With easily accessible food being the most important resource attracting these birds into cities; influencing different traits of habitat selection (e.g., use of new nesting sites) and life history (e.g., earlier nesting, larger clutches, higher fledging success, reduced home ranges and territoriality), as well as behavior (increased tolerance of humans); understanding the relative importance of these changes in each species will be fundamental to better understand the adaptation process and human–wildlife interactions and to develop efficient management applications. While the effects of urbanization on numerous corvid species have been relatively well explored, there are important gaps in our knowledge, calling for a more diversified approach to study this process with different, complementary methods and a focus on the potential benefits of this process, such as the ecosystem services that corvid species provide. We encourage researchers to also address these aspects for a more balanced view of corvids in urban environments.
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