Bird migration perils
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.[1][2] During spring migration, birds return to their breeding sites to exploit the temporary superabundance of food, allowing them to raise more young.[3]
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.[4] Identifying and understanding the processes and perils can allow us to implement effective management and conservation strategies for these species.[5]
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.
Mortality on breeding grounds: small, insect eating birds contribute to the majority of post-arrival deaths, but many other birds including
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.[1]
Hunting
The passing of
Malta, an archipelago of small islands along the Mediterranean, is a very important migration flyway for birds.[6] 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.[6]
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.[7]
Major foraging areas for vulnerable
Stopover habitat loss
Birds use stopover sites to feed, rest and refuel during their migration period.
Increased predation at stopover sites could lead to drastic declines in migratory bird populations.[9] 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.[9]
Collisions and confusion at oil platforms
Over 40 million seabirds are negatively affected by oil platforms.
Poisoning by pesticides
Since there are very little regulations regarding
In their study on Dickcissels and crop damage in Venezuela, Basili and Temple (1999), found that the population of Dickcissels declined by 40% between the years of 1960 and 1980. The declines were primarily due to direct killings by humans. Dickcissels migrate to Venezuela in winter and they tend to gather in large colonies (millions of birds) to feed and sleep. Farmers in Venezuela thought that Dickcissels were pests that fed on rice and cereal crops, so they aerially sprayed the region with pesticides to kill of the birds. The dickcissels consumed only 0.37-0.745% of the grains produced. If the farmers had been better informed of how small of an impact Dickcissels had on their crops, the population declines of these birds could have been prevented.[11]
Night skies are obscured by artificial lights in many cities around the world.[13] These lights are illuminated from buildings, roads and other human structures. When flying across the city, migratory birds could become attracted to artificial lights in the sky. These birds tend to follow light beams and fly continuously in circles, dying from exhaustion or predators as the result. Increased illumination due to artificial lighting could also disrupt foraging behavior of diurnal birds, making these species forage at night, instead of the day. The negative effects from artificial lights are particularly evident in bad weather and when stars are covered by clouds, because birds that migrate at night use light beams for navigation.
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.
Lights on tall structures can disorient
Collisions and disruption from wind farms
Contributory factors
Deforestation
Deforestation leads to fragmented forest habitats and nest predators tend to be more abundant in these fragmented landscapes.[22] If the fragmented area is long and narrow, it will have greater predation rates because it can easily be reached by nest predators from other areas. Compared to rural woodlots, nest predation rates were higher in suburban areas due higher densities of nest predators such as Blue Jay, Common Grackle, raccoons, dogs, cats and rats. Deforestation thus affects population cycles of birds by changing predator-prey relationships and making the birds more susceptible to predators.[22]
Oil developments at the
Climate change
Migratory birds are seriously affected by climate change because they cannot assess changes in spring weather from their wintering grounds.[23] Higher spring temperatures can lead to earlier increases in insect abundance, but many bird species were not able to advance their arrival dates. For example, pied flycatchers timed their egg hatch cycles with subsequent increases in food to raise as many young as possible. Spring migration based on day length had allowed flycatchers to arrive on time, and their egg laying times correlated with insect abundances. However, due to climate change, the flycatchers are now forced to lay eggs earlier, which leaves these birds not enough time to prepare their nests properly.[23] Climate change poses a serious threat to long distance migrant birds because they arrive at inappropriate time to exploit environmental opportunities, and face higher competition with resident species.[2][23] Birds such as the pied flycatcher can start nesting earlier, but their arrival time at the breeding grounds does not change because birds cannot remotely sense temperature changes on breeding grounds from their wintering grounds.[2][23] The birds cannot depart their wintering grounds unless they have enough energy and fat reserves to support their migration journey, and since early arriving birds usually get the best resources, most species face intense competition for early arrival and early departure. For example, in American redstart, individuals with better phenotypic qualities arrive and mate first.[1][24]
See also
References
- ^ a b c d e Newton, I. (2007). Weather related mass-mortality events in migrants. British Ornithologists' Union, 149, 453-467.
- ^ a b c 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.
- ^ Newton, I. (2006). Can conditions experienced during migration limit the population levels of birds?. Journal of Ornithology, 147(2), 146-166.
- ^ a b Hutto, R. L. (1988). Is tropical deforestation responsible for the reported declines in neotropical migrant populations Archived 2010-12-30 at the Wayback Machine. American Birds, 42, 375-379.
- Ecological Applications, 20(2), 398-418.
- ^ a b Falzon, M. A. (2008). Flight of passion hunting: Ecology and politics in Malta and the Mediterranean. Anthropology Today, 24(1), 15-20.
- ^ a b 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.
- ^ a b c 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.
- ^ a b 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.
- ^ a b 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.
- ^ .
- ^ a b Cox, C. (1991). "Pesticides and Birds: From DDT to Today's Poisons". Journal of Pesticide Reform. 11 (4): 16.
- ^ a b Longcore, T., & Rich, C. (2004). Ecological light pollution. The Ecological Society Of America, 2(4), 191-198.
- ^ a b c d Klem, D. (1990). Collisions between Birds and Windows: Mortality and Prevention Archived January 3, 2014, at the Wayback Machine. Journal of Field Ornithology, 61(1), 120-128.
- S2CID 227519492.
- S2CID 213571293.
- S2CID 11925316.
- ^ Drewitt, A. l., & Lagston, R. H. (2006). Assessing the impacts of wind farms on birds. British Ornithologists' Union, 148, 29-42.
- ^ a b Wells, J. S., Lefkowith, S. C., Chavarria, G., & Dyer, S. (2008). Impact on Birds of Tar Sands Oil Development in Canada's Boreal Forest. Natural Resources Defense Council, 1, 1-39.
- ^ a b c Rappole, J. H., Ramoa, M. A., & Winker, K. (1989). Wintering wood thrush movements and mortality in Southern Veracruz. Nature, 106, 402-410.
- ^ Norris, D. R., & Taylor, C. M. (2006). Predicting the consequences of carry-over effects for migratory populations. Biol. Lett., 2, 148-151.
- ^ a b Wilcove, D, S. "Nest predation in forest tracts and the decline of migratory songbirds". Ecology 66.4 (1985): 1211-1214.
- ^ a b c d Both, C., & Viesser, M. E. (2001). Adjustment to climate change is constrained by arrival date in a long-distance migrant bird. Nature, 411, 296-298.
- ^ Kokko, H. (1999). Competition for early arrival in migratory birds. Journal of Animal Ecology, 68, 940-950.