Migrating birds face many perils as they travel between breeding and wintering grounds each year.
Migration is a dangerous part of a bird's life cycle, with many trade-offs; birds receive benefits from wintering and breeding in better quality habitats, at the price of higher predation risks and greater energy expenditure.
Hazards during migration include collisions with manmade objects such as glass windows and railings on buildings, power lines and communication towers, predation by pet and feral domestic cats, collisions with vehicles, and lack of stopover habitat where birds can refuel. Contrary to popular belief, collisions with wind turbines represent a relatively small proportion of bird mortality compared to other primary sources of collisions. The risk of starvation is increased when stopover sites are lost through climate change or loss of habitat to development or agriculture. Mortality on both breeding and wintering grounds may be increased for similar reasons.
Context
Migrants tend to travel away from polar and temperate zones in the winter because of low temperatures and shortage of food in their breeding areas.Newton, I. (2007). Weather related mass-mortality events in migrants. British Ornithologists' Union, 149, 453-467.Buskirk, J. V., Mulvihill, R. S., & Leberman, R. C. (2012). Phenotypic plasticity alone cannot explain climate-induced change in avian migration timing. Ecology and Evolution, 2(10), 2430-2437. During spring migration, birds return to their breeding sites to exploit the temporary superabundance of food, allowing them to raise more young.Newton, I. (2006). Can conditions experienced during migration limit the population levels of birds?. Journal of Ornithology, 147(2), 146-166.
Many populations of migratory birds are in serious decline. Anthropogenic reasons for this include deforestation and habitat loss, hunting, pesticide uses, urbanization, and climate change.Hutto, R. L. (1988). Is tropical deforestation responsible for the reported declines in neotropical migrant populations . American Birds, 42, 375-379. Identifying and understanding the processes and perils can allow us to implement effective management and conservation strategies for these species.Faaborg, J., Levey, D. J., Johnson, D. H., Holmes, R. T., Anders, A. D., Bildstein, K. L., et al. (2010). . Ecological Applications, 20(2), 398-418.
Immediate perils Bad weather In-flight mortality: poor weather conditions can significantly decrease bird populations, especially during migration. Most of weather-related in-flight mortalities are due to heavy storms, mist or rain. Passerines and other small sized birds are particularly affected by adverse in-flight weather conditions, but larger birds such as eagles and swans could also be killed.
Mortality on breeding grounds: small, insect eating birds contribute to the majority of post-arrival deaths, but many other birds including waders and waterfowls are also distressed by weather changes on breeding grounds. Since young birds are inexperienced, they are more vulnerable than adults to extreme weather conditions.
Mortality on wintering grounds: Unreasonably cold temperatures on the wintering grounds kills thousands of birds, resulting in 30-90% population declines of migratory birds. For example, between 27000 and 62000 ducks, mostly tufted duck and common pochard, starved to death during a very cold winter in March 1986.
Hunting
The passing of Migratory Bird Treaty (US, 1916) and Migratory Birds Convention Act (Canada, 1917) made it illegal to kill or capture migratory birds. Even though migratory bird acts were passed in the early 20th century, many countries still have no laws or programs to protect migratory birds. International bird trade is a multibillion-dollar industry and hundreds of exotic birds are captured and then sold all over the world.
Malta, an archipelago of small islands along the Mediterranean, is a very important migration flyway for birds.Falzon, M. A. (2008). Flight of passion hunting: Ecology and politics in Malta and the Mediterranean. Anthropology Today, 24(1), 15-20. Throughout the years, hunters killed hundreds of millions of birds each year as they migrate over the island of Malta. To protect resident and migratory birds, BirdLife international had been organizing special raptors camps since the late 1990s. Even though hunting is a part of Maltese people's culture, the interference from birdwatchers all over the world has led to decreased killings of birds.
Bycatch from commercial fishing
While hunting kills millions of terrestrial birds, the bycatch from commercial fisheries is responsible for the majority of human caused mortality of migratory birds. Scientists have estimated that between 2679 and 45586 birds are killed each year as fisheries by-catch. Dredging, gillnetting bottom otter trawling and longline's are some of the main methods fisheries use to catch fish. Gillnets are responsible for the majority of seabird bycatch, followed by longline and bottom trawling. To catch tuna and other fish, long-line fishing boats drag many kilometers of hooked lines behind them. Seabirds try to catch the fish and accidentally get trapped in hooks.Ellis, J. I., Wilhelm, S. I., Hedd, A., Fraser, G. S., Robertson, G. F., Rail, J. F., et al. (2013). "Mortality of Migratory Birds from Marine Commercial Fisheries and Offshore Oil and Gas Production in Canada". Avian Conservation and Ecology, 8(2), 4-10.
Major foraging areas for vulnerable seabirds (albatrosses and shearwaters) tend to overlap with world's richest fishing grounds, thus increasing the proportions of accidental bycatch of birds. Fisheries could also indirectly affect the trophic structure and foraging methods of seabirds. Since most sea birds are long-lived and have low reproduction rates, even a small increase in bird mortality could cause significant population declines.
Stopover habitat loss Birds use stopover sites to feed, rest and refuel during their migration period.Catry, P., Encarnaca˜o, V., Arau jo, A., Fearon, P., Fearon, A., Armelin, M. and Delaloye, P. 2004. Are long-distance migrant passerines faithful to their stopover sites? Journal of Avian Biol. 35, 170-/181. Many of the stopover sites may be already compromised or are threatened owing to increased urbanization, agriculture, oil gas or mineral exploitation, fisheries, tourism and many other anthropogenic activities. In one study the researchers found that birds with high phenotypic plasticity can adapt their behavior and skip low-quality stopover sites. Migratory birds such as swans, geese and waders show high site fidelity (they are loyal to their stopover and cannot change them), while long distance passerines have much lower site fidelity. Passerines have low site fidelity because they can be flexible in their habitat selection. Since they do not migrate in flocks, migratory passerines do not have a fixed migration route or stop-over site sequence and they can change their stopover sites based on wind selectivity or habitat quality. Even though many birds can change their stopover sites, birds such as swans and waders depend on wetland stopover sites to 'refuel' on migration. The destruction of these sites could therefore be detrimental to bird populations.
Increased predation at stopover sites could lead to drastic declines in migratory bird populations.Lank, D. B., & Ydenberg, R. C. (2003). Death and danger at migratory stopovers: problems with "predation risk". Journal of Avian Biology, 34(3), 225-228. The study done by Lank and Ydenberg (2003) examined the effects of predators on migratory birds at stopover sites. The researchers found that predation risk is higher for heavier birds (due to decreased take-off ability) and leaner birds (increased exposure due to higher feeding needs). Many birds also developed anti-predator behaviors to lower the probability of mortality. Since anti-predator behaviors are energetically costly, the migrants with lower energy reserves allocated less time to anti-predator behaviors.
Collisions A considerable threat to migratory birds is the threat of collision posed by man-made structures along migration routes. Such structures may include buildings, oil platforms, and wind turbines.
Collisions and confusion at oil platforms Over 40 million seabirds are negatively affected by oil platforms.Weise, F. K., Montevecchi, W. A., Davoren, G. K., Huettmann, F., Diamond, A. W., & Linke, J. (2001). "Seabirds at risk around offshore oil platforms in the North-west Atlantic". Marine Pollution Bulletin, 42(12), 1285-1290. Seabirds tend to aggregate around oil rigs, attracted by artificial lighting, flares, food and other visual cues. Seabirds often collide directly with oil platforms or circle around oil rigs and flares for days, eventually dying of starvation. Birds such as storm petrels, dovekies and shearwaters migrate across the Grand Banks and hydrocarbon development near the oil platforms significantly decreases the populations of these birds.
Collisions with buildings
Artificial light sources can attract millions of birds to lighthouses, broadcast towers and other buildings, resulting in direct mortality of birds at night. There are less artificial lights during the day, but millions of birds still die due to direct collisions with various human made structures.Klem, D. (1990). Collisions between Birds and Windows: Mortality and Prevention . Journal of Field Ornithology, 61(1), 120-128. Birds often fatally strike the glass because they cannot differentiate between real sky and reflection of a sky in a window. Any object that increases bird density near windows can potentially lead to higher death rates. Reflective windows are particularly dangerous as birds are often attracted to them. Placing bird attractants such as bird houses, water and nutritious vegetation near windows also increases the number of birds killed. Birds may also be impacted by bright lights at nights as they have extra-retinal photoreceptors that are disoriented by the reflection of light from these buildings. Mitigating the amount of light emitted from glass surfaces at night, such as windows, can reduce the amount of fatal bird collisions with buildings and structures. To decrease the impact of artificial lighting, many cities had implemented lights out program, in which people turn off or dim the lights in tall buildings during migration season.
Lights on tall structures can disorient migrating birds leading to fatalities. An estimated 365-988 million fatal bird collisions with buildings occur annually in North America, making human-made structures a large contributor to the decline in bird species. To reduce bird strikes, it is suggested to remove all bird attractants near the windows or to partially cover the windows. For new buildings, scientists have recommended installing windows in a way that panes reflect the ground instead of the sky.
Collisions and disruption from wind farms