- Emily Bailes, Ann Bailey, Matheus Blezer Plumm, Tom McClellan-West, Lizzie Yarwood –
The enigmatic creatures of the sky, birds are creatures that are treasured by many around the world, by some as cultural symbolism, others a hobby to indulge in and some just find their presence a pleasant addition to a grey and largely urban world.
Human colonisation has had a huge impact on nature, birds included, with some able to utilise the new urban habitat to great effect; feral pigeons (Columba livia domestica) exploit waste products and buildings as a place to nest. However, some species have found it much harder to adapt to the threats that are present within the ever increasing urban environment.
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In recent reports, the conservation status’ of 246 bird species were evaluated of which 52 (21.1%) were placed on the Red list, 126 (51.2%) were on the Amber list and 68 species (27.6%) were placed on the Green list (Eaton et al. 2009). Over a period of 35-45 years, there were 28 species with statistically significant population declines of greater than 50%. One of these species is the Turtle Dove (Streptopelia turtur); according to the CBC/BBS trend it has showed a severe decline in abundance since the 1970s (Figure 1), and it is a priority species in the UK Biodiversity Action Plan. In the period of 2002-2012 the Turtle Dove had an 80% decline in population size in the UK.
Whilst there are many threats facing urban bird populations, there are four key threats that have been focused on in this review. Cat predation, window collisions, habitat loss and general human disturbance all have variable effects on bird abundance and have been discussed in the following paragraphs.
Predation by Felines
Whilst there are several high risk threats to urban bird populations, the most severe is that of cat predation. Cats have long been known to have a negative ecological impact. They are responsible for multiple cases of extinctions in ground nesting birds in island habitats, where they come into contact with species possessing little or no defence mechanisms against predation. This inevitably leads to extinction. The current global impact of free-ranging domestic cats on bird mortality is estimated to be up to 4 billion birds, annually (Loss et al. 2013). In the UK alone it is estimated that across a 4 month period, up to 100 million prey items are caught by cats, 24% of these being bird species (Woods et al. 2003). With cats bringing home an average of 13.4 prey items per year (Heezik et al. 2010), this can have a drastic impact on wildlife populations. One study that focused on the bird populations of a typical urban UK village found that 16% of birds caught in a year were House Sparrows (Passer domesticus) which are a red listed and highly endangered species (British Trust for Ornithology 2002) (Figure 2).
It was also found in the same study that cats were responsible for 30% of Passer domesticus deaths in that area. Cats are also responsible for the deaths of many other red listed species such as the Starling (Sturnus vulgaris) (British Trust for Ornithology 2002) (See fig 3). Another more worrying issue is that the number of bird deaths increased if that household was providing food for birds (Woods et al. 2003). This is especially troubling as cat owners trying to conserve endangered species may be doing more harm than good. These trends all point to cat predation being the number one threat to urban bird populations.
However, research has been conducted that suggests that the impact of cats on ecosystems may not be wholly negative. As an apex predator, an animal with no natural predators of their own, cats have free reign to hunt in UK ecosystems without fear of predation. As well as suppressing the abundance of birds they also frequently predate other animals and some mesopredators such as rodents. Whilst their impact on bird diversity and population is clear, they also play a key ecological role in the suppression of bird’s natural predators (Ritchie & Johnson 2009).
Window collisions are considered to be the second largest cause of bird fatalities in urban areas (Loss et al. 2014). This however may not be accurate due to the lack of quantitative estimates and research on other continents (Bayne et al. 2012). There have been few attempts to estimate the number of fatalities per year within populations, however the US and Canada have had some input. Klem JR (1990a) estimated a range of 100 million to 1 billion birds killed per year within the USA, which was later considered to be a conservative estimate (Klem 2006). Bayne et al. (2012) believed these estimates to be unlikely due to a lack of research in other geographical areas.
Windows pose a threat as birds do not perceive glass within their vision, and they will see windows and corridors as a route to other habitat areas (Klem et al. 2009). Reflections from windows are also perceived as a continuation of the sky-both of these things occur during light hours (Machtans et al. 2013). At night, windows become a source of light pollution which is particularly a problem for migratory birds (Loss et al. 2014). Upon collision, birds suffer fatalities due to head traumas, intracranial pressures and bleeding on the brain, rather than the “broken neck” explanation (Klem 1990b), fortified by the results found by Veltri & Klem (2005) in which 91% of birds found dead from window collisions had no skeletal or cervical fractures.
Urban environments, and the increase in buildings that feature glass, are proportionately linked to collision mortality rates (Hager et al. 2012). Glass is considered to be a non-selective lethal hazard by Klem (1990a) who found that sex, age and resident status does not affect the mortality rates of window collisions, neither is there a differentiation between smaller and larger species. However, this result may not be universal, with data only collected from two different buildings in differing locations. It would be favourable to take samples from a range of buildings in a number of locations around the world to see what impacts exist to which species, because although there is plausibly no difference between small and large species in terms of collision rate, those collisions impacts on individual species could be more profound.
Whilst being a threat to individual birds in large quantities globally, in the US there is a lack of research into the effects of this particular mortality along with other collisions on population dynamics, with one study going as far to suggest that collisions with man-made structures, including windows has ‘no discernible effect’ on the population trends in North America (Arnold & Zinc 2011).
Bayne et al. (2012) explain that more research needs to be done to obtain a reliable estimate for bird mortality caused by window collisions. Research can then be done to get a better perspective of what the mortalities are doing to the population numbers. This should also be done in other locations to give more clarity on the issue, and provide greater understanding moving forwards.
Habitat loss is becoming a big issue over the world due to urbanization from human development. On average in the United States, urbanised areas have as few as 35% tree coverage, with another 118,300km2 of forest land expected to be lost in the next 50 years. Forest land is important as not only does it provide shade, it also provides hidden nesting areas and food (Lerman et al. 2014). Habitat fragmentation and habitat loss not only causes a community diversity loss within the animal kingdom, it removes the nesting areas for these birds, leaving them vulnerable. Bird feeders that can be found in peoples gardens can help to reduce the lack of food caused by the removal of plants and trees, of which birds and their prey feed from. However this cannot prevent, or even counterbalance, the decrease in the diversity of species which are lost and replaced by non-native species (Shochat et al. 2010). In Malaysia, nearly half of their land has been urbanised and changed by humans, causing a loss in biodiversity. However they have created little ‘pockets’ within the urban areas for wildlife, such as nature reserves, local parks and plazas. Although, it has been noticed that exotic vegetation is not popular with the native insects and their prey-which impacts majorly on the bird population. Despite exotic plants not helping to increase native bird numbers, they are still being planted for ornamental reasons and as part of their local plants scheme so this does not help with the conservation of native birds (Idilfitri & Mohamad 2012).
Human disturbance is a threat to bird species in urban areas and can come in many forms ranging from pollution of waterways to introduction of non-native species into the environment. Habitat fragmentation and introduction of exotic species to urban environments has seen a decline in urban bird populations. Within urban areas there are a range of habitats, from parks containing primarily native tree species, to recently developed streetscapes that do not contain any mature tree species. Here, many of the tree species found are non-indigenous. Other habitats that can be found in an urban environment are streetscapes with only native tree species, and those with only exotic tree species (White et al. 2005). Higher species richness of birds and abundance is observed in habitats with more resources, such as parks, with the greater availability of food and nesting sites. In habitats with non-indigenous tree species, nectarivorous birds are less prolific as flowers that they specialize in are not present. Insectivorous birds are also less common in such habitats; native insects have evolved with native plants, so will be absent where these trees are absent, thus insectivorous bird species will be absent. Recently developed and exotic streetscapes often lack the shelter required for certain bird species, so these birds are driven out of potential habitats (Bhuller & Majer 2000).
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Bird species require specific flora species within distinct habitats that meet all of their requirements; some bird species are more tolerable of disturbed habitats with exotic species, whilst other species are dependent on the native flora and undisturbed habitats. Urbanisation may lead to an increase in non-indigenous bird species and the simplification of bird communities if exotic plants are planted and streetscapes continue to be created. Therefore native vegetation must continually be planted and parks protected in order to maintain complex bird communities and conserve native bird species (White et al. 2005).
The effects of light pollution from urban areas on birds are also of particular concern. A study conducted on European Blackbirds (Turdus merula) showed that birds exposed to 0.3 lux of illumination at night time had an irregular molt pattern with respect to individuals that were exposed to a dark night. Male European blackbird’s production of testosterone is stimulated annually by light, in a process called photorefactoriness. After the breeding season, the birds stop being photosensitive, and so the gonads are diminished whilst they are not in use. Gonads re-develop after photostimulation by shorter daylight hours in the autumn and the increase in day length thereafter. Following long periods of exposure to light at night time, the reproductive cycles of male birds stop, with testosterone concentrations in the plasma at a minimum. Thus, it can be suggested that artificial light could confuse birds, as they miss important photo-cues for photorefactoriness. Without these cues, testosterone concentrations remain at a minimum, gonads fail to re-develop and the bird will not reproduce come breeding season. The light pollution caused by urban environments is a threat to urban bird populations, as it leaves individuals unable to reproduce through confusing their photosensitive reproductive cycles. (Dominoni et al. 2013)
It is apparent that the threats mentioned in this review all have the potential to impact urban bird populations around the world and have all been reported in literature to have a negative effect on birds. However the threats mentioned have a great deal of complexity about them due to the interchangeable effects each threat can have on one another which may make distinguishing the effects of one individually difficult.
A great example of this is the provision of bird feeders in gardens. Whilst the benefits of providing food will be greatly received by a number of bird species, it can increase the frequency of predation by cats (Woods et al. 2003). It is also shown that providing bird feeders near urban buildings can increase the frequency of bird-window collisions (Klem 1990a). However, a dynamic could be present, where the ‘fear factor’ of cats is influencing the birds into quick flight paths that lead them to collide with windows. It is here that the two dynamics of the threats are interlinked.
Mortality rates of birds killed by window collisions with single building may be small, but deaths from window collisions with several buildings across the urban environment accumulate to give a high percentage of mortality. To reduce this high percentage of mortality, something must be done to the structure of buildings (e.g. urban residences) so birds do not confuse window panes as a pathway, thinking that it is clear for them to use (Loss et al. 2014).
Furthermore our constant introduction of exotic non-native species to parks and streetscapes changes the habitat and prey items available to urban birds which will further increase their population decline- some bird species have easily adapted to changing conditions, whilst populations of others have been disturbed with the loss of specific habitats.
Not only do the exotic species disrupt the local ecosystems, they also replace habitats that would otherwise be suitable for many native invertebrates that are an important source of food for birds. Whilst this threat isn’t as obvious or as directly damaging to birds, it is still a serious threat to be considered and further researched. Exotic plants are leading to declining native bird populations, but in spite of this they are still being planted for ornamental reasons (Idilfitri & Mohamad 2012).
Urbanisation may lead to an increase in non-indigenous bird species and the simplification of bird communities if exotic plants are planted and streetscapes continue to be created. Therefore native vegetation must continually be planted and parks protected in order to maintain complex bird communities and conserve native bird species (White et al. 2005).
All things considered there are a number of aspects that need to be addressed. Bayne et al. (2012) pointed out how there is a lack of quantitative estimates and lack of research in other geographical areas as reasons why window collision mortality and its effects on a population may not have particularly accurate data. The statement could be extended to various aspects of urban bird population. Threats mentioned above would benefit from extra research, however this is difficult to achieve when the practicalities of working in an urban environment, resources and time are taken into consideration.
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