Bird migration and the impact of human activity on migratory activity
Migration is the movement made by animals between two separate locations on a regular basis for the purpose of accessing resources provided by these different sites (Alcock, 2013). Migration is a common process throughout the entire animal kingdom. For example, there is the migration of the Monarch butterfly across North America (Brown and Chippendale, 1974), the Atlantic salmons journey from river to ocean (Hansen, 1993), the marine turtles epic movement from feeding ground to breeding ground (Morreale et al., 1996) and finally there is our own species migration across national and international borders. The focus of this discussion however is the migration of Birds; most specifically looking at how, why and when they migrate and focusing on what exactly our impact as humans has on this very important phenomenon in the life cycle of many bird species.
Defining how migration first occurredÂ involves looking back over the history of migratory species and finding an evolutionary origin to the migratory lifestyle. In actual fact migration has evolved on several different occasions within the class Aves as shown in figure 1 which looks at the separate origins of migration within the family Turdidae. The multiple evolutionary origins of migration suggest that there are numerous reasons as to why adopting a lifestyle that involves moving between different areas is advantageous to a species (Outlaw et al., 2003). With any adaptation, there must be a selection pressure forcing a species to develop a new strategy to be successful (Williams, 2008). In the case of bird migration, the seasonal variability of primary resources is a major driving factor for migrating but the changing climate conditions throughout the year is also one of the main benefits of being able to migrate (Alerstam, 2003).
Birds can be long distance migrators, such as the Sooty shearwater, a sea bird that migrates approximately 64,000 km in 260 days with an daily travel distance of 900km (Schaffer et al., 2006). This heroic migration between North America and New Zealand is one of the largest migrations by a bird. In contrast the Spotted Owls of the Sierra Nevada have been observed to be altitude migrators, of short distance migrators, as they have a wintering site which is between 0.6-1.0 km closer to sea level than their usual nesting ground (Bias and Gutierrez, 1992). The comparisons between the journeys made by these two birds shows how two different environmental pressures can lead to the same solution. The long distance migrators were driven to travel across continents due to the need to find a sustainable food source in changing seasonal conditions while the altitudinal migrators are driven to move based on climatic conditions (Jenni and Kery, 2003). If migrating offers species a richer supply of resources and a more favourable environment to survive in then why do only some species of birds migrate? As with many things in the natural world migration has its costs as well as its benefits, the costs and risks associated with migration are so high that not every species is adapted, nor needs to undertake the challenges migration creates. In order to travel such distances individuals need to be able to store resources by carrying extra weight leading up to the migration. This store must then be accessed during the flight meaning the individual must be able to alter their metabolism to store and release fats efficiently (Jenni and Jenni-Eiermann, 1998). Finally the risk of death during this journey is high as there many factors at play in the success of making the trip.
The final aspect of migration is how species successfully manage to navigate from one location to another regardless of distance. There are three means of orientating observed in birds and they are pilotage, compass orientation and true navigation (Alcock, 2013). Pilotage is common in species such as homing pigeons where landmarks are used as navigational cues (Mehlhorn and Rehkämper, 2009). These landmarks could be environmental, such as rivers or forests but can also be artificial, in particular tall buildings and major roads. Migrating from one location to another without the use of cues requires a species to have a compass orientation and understanding of which direction it needs to travel to reach the location. An experiment done on the migratory European Robin showed that they can orientate using geomagnetic cues through a sense that is localised to the right eye (Wiltschko et al., 2002). True navigation is when a species has the ability to navigate towards the home location without any navigational cues and regardless of the direction (McFarland, 1999). Having summarised the integral mechanisms behind migration I will now spend the remainder of this discussion focusing on what our impact as humans is on the migration of birds.
Human interference on the animals we share the earth with is becoming an increasingly more serious problem. The effect we have on terrestrial migratory species may seem obvious as land use expands into new, wild territories but our effect on birds as they migrate through the power of flight is somewhat more complex. There are many factors that contribute to how we disrupt the migratory routes, territories and even the birds themselves. In parts of southern Europe and Northern Africa the illegal shooting and netting of birds has become an increasingly popular industry. In Egypt 700km of nets are put up along the coastline designed to trap thousands of migrating passerine birds such as willow-warblers, red-backed shrikes and barn swallows all of which are illegal to catch under the national law (Salama, 2016). Recently there has been a strong push for the local authorities within Egypt to enforce the regulations that are in place as these nets are positioned to intersect with one of the major migratory flyways from Africa to Eurasia as shown in Figure 2 where the area in question is within region 2. This area is not only targeted by nets in North Africa but many southern European countries such as France and Spain also engage in similar practices involving illegal netting.Â In Italy, particularly in the Sicily region, shooting of birds is a common sport when migrators are making their journey to warmer climates in Africa. Unlike the mist netting practices, game hunting is legal to permit holders and the Italian authorities allow the 710,000 hunters to shoot 30 birds a day over a 2 month period (CABS, 2015).Â A case against illegal hunting of birds has recently reached EU law regarding the trapping and commercial caging of birds in Malta. Many of the birds targeted are rare or protected species such as the endangered Pallid Harrier, the Purple heron and the Turtle dove (Raine et al., 2015). While the fight against illegal hunting and shooting of these birds is ongoing, the effects of these activities in undoubtedly having a negative impact on their success. Migrating to breeding grounds that are better equipped with resources and protection should be an adaptation for success in these species. Instead they are being illegally hunted, trapped and shot while en route over this area resulting in a much lower number of birds successfully reaching breeding sites. Undoubtedly this seriously damages population size of these European migrators and puts increasing strain on the challenging journey they make twice every year (Hirschfeld and Heyd, 2005). This particular study by Hirschfeld and Heyd (2005) reports that hunters are legally allowed to kill just 24 species of European native birds however a total of 82 different species were recorded to have been hunted that year. Despite the obvious illegal activity being done there is still a relaxed enforcement of these regulations across Europe and North Africa and widespread disagreement around how to resolve the problems we, as humans, are causing.
The current population of humans on earth is 7.5 billion with an annual growth so far, from January – March 2017 is nearly 17 million (World population clock, 2017). As the global populations increase so to do food demands and a pressure to convert unused land into cropland or an expansion of existing urban regions. This unfortunately leads to widespread deforestation as the need for space increases on a daily basis (Lambin and Meyfroidt, 2011). Deforestation is threatening the ecosystem in which many migratory birds depend on. The cloud forest in Central America is under threat due to increases in forest conversion. Many of the birds species in this region are altitudinal migrators and rely on the cloud forest for breeding (Cox, 2010). That is a very localised example of how deforestation is impacting migration but on a much wider scale the destruction of tropical and deciduous forests has a huge impact on birds migrating between the two tropics. The American Redstart, a small passerine bird, winters in the Caribbean, South America and Central America before migrating north to North America, and Canada to breed (Sherry & Holmes 1997). Breeding success in these birds depends on the quality of both the wintering and breeding habitat and disruption to tropical forests in South and Central America and to deciduous forests in North America has influenced the reproductive success of breeding pairs in these regions (Norris et al., 2004). Deforestation is not the only interference to habitat we are causing, our need to continuously change landscapes so as they are better suited to our increasingly more developed societies is impacting many important habitats for migratory birds.
Conversion of open land into man made developments can lead to many environmental disruptions. One case that I have a personal interest in is the proposed development of a new dual carriageway between Belfast and Londonderry in Northern Ireland. While there is widespread agreement that the existing A6 road has seen an increase in use over the past 10 years (Johnston, 2016) the proposed route of the new road has been highly controversial. The area in question isshown in Figure 3 and the proposed route passes straight through sensitive land that is used by Whooper Swans wintering in Northern Ireland. According to the RSPB Whooper Swans are an amber list species due to their small population and limited breeding areas. The swans migrate to the UK from Iceland to winter and this particular area in figure 3 is a highly important habitat for migrating pairs. The fields are flooded regularly by the bordering Lough Neagh and so are a rich feeding habitat for these migrating birds (RSPB). A number of local and national environment agencies and organisations are working with the the Department of Infrastructure to find a compromise that protects this nationally important habitat for the Whooper Swans. While the effect on these swans is yet to be seen as work on the new road is only reaching preliminary stages this year it will undoubtedly cause disruption to these wintering populations in the years to come.
While deforestation and urban development is a direct way in which we are destroying habitats of migratory birds an indirect way is through climate change. It is no secret that human activity is massively contributing to the speed at which climate change is occurring and some impacts of this is loss of habitats, reduced food availability and changing season lengths disrupting migration times (Travis, 2003; Bradshaw, 2006; Crick, 2004). In the UK we are seeing a lengthening of seasons meaning birds are arriving earlier and departing later (Sparks and Mason, 2004). While this can be considered a positive for UK bird numbers the migrating species arriving early could be disadvantageous to their need of resources. While the birds may be arriving early due to the changing home territory climate and the extended migratory territory season, food resources often do not sync with this change in habit. What is being observed is an increasingly unsuccessful first brood of early arrivers due to the food supply, whether it is insects or berries, not being available (Harrington et al., 1999). An example of a bird effected by this is the Waxwing, a Northern European and Scandinavian passerine bird that has increasingly been migrating to the UK during winter to access food supplies. This is because with climate change there have been fewer berries in the Northern and Boreal forests forcing the Waxwing to expand their migration further to the UK (Huntley, 2008). 
One of the most common migratory flyways is from Africa into Europe. Figure 4 shows the different routes the Montagu Harrier uses to get from the Sahel region of Africa across the mediterranean into Europe. While I have already discussed the dangers of migrating across North Africa and Southern Europe regarding hunting, it is the journey across the Sahara desert that I will now be focusing on. The Sahel region is dominated by the Sahara desert with most of the land area being dry, arid land with the outskirts and coastal regions being populated and used for agriculture (Zeng, 2003). However in the decade the Sahel region has been under drought conditions meaning a lot of the once arable, ‘green’ land is under going desertification, hence why there is a popular notion that the Sahara is expanding (Foley et al., 2003). Travelling across desert is not an easy journey to make. There is limited water, food, roosting sites and the climate is hot and dry. This means that birds such as the Montagu harrier (figure 4) or the Garden Warbler, both of which are summer migrants in the UK and Europe, need appropriate energy stores to attempt the journey (Jenni and Jenni-Eiermann, 1998). With this area of desert land expanding it makes the already challenging journey even more demanding on energy stores and as a result many of the smaller passerine birds such as the Garden Warbler are not as successful in migrating across Africa. Most species can not attempt the crossing in still air and require prevailing winds across the desert to aid in flight (Barboutis et al., 2011). With climate change set to continue the drought in the Sahel region migrating across this major ecological barrier successfully will become increasingly more challenging with a lot more pressure being put on stop off sites in North Africa and Southern Europe where there is already so much competition and hunting occurring (Trierweiler et al., 2014).
Stop off sites are crucial to the success of long distance migrations especially when birds are flying across challenging terrain or resourceless water. These stop off sites tend to be rich habitats that are used by migrants for intensive feeding to re stock fuel supplies for the next leg of the journey (Pomeroy et al., 2006). Common stop off sites are mud flats as they are rich in resources and often have low predation risks as migrants flock together on the shore. Species like the Sandpiper use coastal mudflats as stop over sites and a study observing site selection based on disturbances, predator presence and food abundance was conducted with results showing no preference of site when there are land disturbances or predator presence. The study by Pomeroy et al, (2006) did however provide significant results on site seleciton based on food supply indicating that this is the main purpose of choosing these sites for stop over points. One of the serious impacts of climate change is rising sea levels and so, low lying coastal areas such as mud flats will gradually becoming permanently flooded as a result (Church and White, 2006). Sadly this means that not only are destination sites being lost but so to are the stopover areas that are essential for long distance migrators.
Fighting climate change is one of the biggest environmental challenges we are undertaking. One method of cutting down the global carbon emissions, in an attempt to slow down the greenhouse effect and thus climate change, is increasing the production of renewable energy (Mitchell, 1989). Renewable energy offers clean production of resources that our expanding population demands and wind energy has made significant in roads in many countries. Unfortunately for migrating birds wind turbines are a serious hazard on their already difficult journey. Off shore wind farms are frequently causing disruption to flocks of nocturnal migrators flying across the baltic sea which has a large number of wind farms. A study taking observations of the collision frequency at a german offshore wind farm recorded a total of 442 birds of 21 different species were killed due to direct collision or subsequent death from injury over a one month period (Hüppop et al., 2006). While renewable energy is an important step towards fighting climate change, wind farm locations should be planned around major migratory flyways to reduce the impact they have on bird species.
It is easy to focus on how we negatively effect migrating birds but it is equally important to highlight the positives. While climate change has resulted in a change in migration times due to longer seasons it also has meant that, particularly in temperate latitudes, there has been an increase in the length of the breeding season (Sparks and Mason, 2004). Small passerine bird that migrate breed in the UK typically have two broods during the breeding season if the food availability and weather conditions are right. However the increase in length of this breeding season has seen a boost in successful 2nd broods witÂ some species such as the Reed warbler having as many as 5 broods over one season (Halupka, 2008). This increased success is due to the prolonged food supply allowing the fledglings to heighten their fitness and thus increase their chance of survival to adulthood. However as mentioned previously the success of the longer breeding period is dependent on the migratory species arriving at a peak time where resources and weather conditions are adequate to sustain a breeding population (Harrington et al., 1999).
Another negative human effect, that can also be deemed a positive, is urbanisation. Many species of migratory birds require navigational cues as means of finding flyways and locations. While in a natural environment these cues can be forests, rivers or mountains, man made structures can also prove to be very useful navigation aids (Kamil and Cheng, 2001). Studies into navigation and route choice of homing pigeons has shown that they choose to follow motorways and railway lines, using entrances and exits to navigate to their home territory (Lipp et al., 2004).
Finally, the recent initiatives of environmental agencies in the UK, such as the RSPB, to get involved and interact with garden birds has seen the number of households putting out regular bird food rise dramatically (Robb et al., 2008). While supplementary feeding provides our native garden birds, such as coal tits and blue tits, a year long food supply it also provides an invaluable resource for migrating birds. One species in particular that has started to expand its migration distance to include the UK is the Blackcap. While blackcaps were a rare site in UK gardens several decades ago they are now becoming a familiar winter addition to the suburban gardens. In the 1950’s ornithologists began to notice this change in migration route, instead of choosing a south westerly migration into Spain, blackcaps have evolved a successful new route into the UK (Plummer et al., 2015). As human activity continues to threaten not just the migration of birds but global biodiversity as a whole, the Blackcaps success story is a welcomed positive of our impact on the world around us.
Migration is an important adaptation in the life cycle of birds and allows many species to access resources that are necessary to survival. The evolution of a migratory life style was driven by environmental selection pressures and is now being inhibited by artificial pressures. As human activity becomes increasingly more intrusive on the environment the stress on migrating birds to navigate successfully, journey across challenging conditions and to find suitable territories is becoming ever more difficult. With stricter enforcement on hunting and capturing restrictions in Europe and North Africa the major African to Eurasia flyway could become a much safer route for migrating flocks. While that is something that can be controlled, the rate of population increase of the human race is getting higher and higher and so is the demand to provide resources, both land and food, for the population. If we changed our agricultural industry so that we are producing crops at a much more efficient rate, the need to cut down forests would lessen and in fact we would preserve rare habitats that are used not just by native species but also visiting migrators. While we are starting to make significant strides into tackling climate change by developing renewable energy sources there is still a lot of room for consideration of environmental preservation. Finally however, it is important to highlight the improvements we are making for migrating birds, while there are only a handful of positive impacts there is room to expand and create a more global effect.
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