Prey populations: the only thing to fear is fear itself

In reference to the Great Depression, Franklin Delano Roosevelt is famously quoted as stating during his 1933 inaugural speech “the only thing we have to fear is fear itself.” Roosevelt was no biologist, but his words could equally apply to a different type of depression – the decline of animal populations that can be caused by fear.


Roosevelt’s inauguration in 1933. Credit: Architect of the Capitol.

Ecologists have long known that predators can depress prey populations by killing substantial numbers of their prey. But only in the past two decades or so have they realized that predators can, simply by their presence, cause prey populations to go into decline. There are many different ways this can happen, but, in general, a predation threat sensed by a prey organism can interfere with its feeding behavior, causing it to grow more slowly, or to starve to death. As one example, elk populations declined after wolves were introduced to Yellowstone National Park. There are many factors associated with this decline, but one factor is fear of predators causes elk to spend more time scanning and less time foraging. Also, elk tend to stay away from wolf hotspots, which are often places with good elk forage.

Liana Zanette recognized that ecologists had not considered whether predator presence can cause bird or mammal parents to reduce the amount of provisioning they provide to dependent offspring, thereby reducing offspring growth and survival, and slowing down population growth. For many years, she and her colleagues have studied the Song Sparrow, Melospiza melodia, on several small Gulf Islands in British Columbia, Canada. In an early study, she showed that playbacks of predator calls reduced parental provisioning by 26%, resulting in a 40% reduction in the estimated number of nestlings that fledged (left the nest). But, as she points out, Song Sparrow parents provision their offspring for many days after fledging; she wondered whether continued perception of a predation threat during this later time period further decreased offspring survival and ultimately population growth.

Song sparrow

The Song Sparrow, Melospiza melodia. Credit: Free Software Foundation.

Zanette’s student, Blair Dudeck, did much of the fieldwork for this study. The researchers captured nestlings six days after hatching , weighed and banded them, and fit them with tiny radio collars. They then recaptured and weighed the nestlings within a few hours of fledging (at about 12 days post-hatching) to assess nestling growth rates.


Banded sparrow nestling with radio antenna trailing from below its wing. Credit: Marek C. Allen.

Three days after the birds fledged, Dudeck radio-tracked them, and surrounded them with three speakers approximately 8 meters from where they perched. For one hour, each youngster listened to recordings of calls made by predators such as ravens or hawks, followed, after a brief rest period, by one hour of calls made by non-predators such as geese or woodpeckers (or vice-versa). During the playbacks, Dudeck observed the birds to record how often the parents visited and fed their offspring, and whether offspring behavior changed in association with predator calls. This included recording all of the offspring begging calls.


Blair Dudeck simultaneously uses a tracking device to locate Song Sparrows and a recorder mounted to his head to record their begging calls. Credit: Marek C. Allen.

Fear had a major impact on parental behavior. Parents reduced food provisioning vists by 37% when predator calls were played in comparison to when non-predator calls were played. They also fed offspring fewer times per visit, which resulted in 44% fewer meals in association with predator calls.


Mean number of parental provisioning visits (in one hour) in relation to whether predator (red) or non-predator (blue) calls were played. Error bars are 1 SE.

Hearing predator calls had no effect on offspring behavior – they continued to beg for food at a high rate, and did not attempt to hide.

Some parents were much more scared than others – in fact, some parents were not scared at all. The researchers measured parental fearfulness by subtracting the number of provisioning visits by parents during predator calls from the number of visits during non-predator calls. A more positive number indicated a more fearful parent (a negative number represents a parent who fed more in the presence of predator calls). The researchers discovered that more fearful parents tended to have offspring that were in poorer condition at day 6 and at fledging.


Offspring weight on day 6 (open circles) and at fledging (solid circles) in relation to parental fearfulness.  Higher positive numbers on x-axis indicate increasingly fearful parents.

Importantly, more fearful parents tended to have offspring that died at an earlier age. Based on this finding, the researchers created a statistical model that compared survival of offspring that heard predator playbacks throughout late-development with survival of offspring that heard non-predator playbacks during the same time period. They estimated a 24% reduction in survival. Combined with their previous study on playbacks during early development, the researchers estimate that hearing predator playbacks throughout early and late development would reduce offspring survival by an amazing 53%.

This “fear itself” phenomenon can extend to other trophic levels in a food web. For example recent research by Zanette and a different group of researchers showed that playbacks of large carnivore vocalizations dramatically reduced foraging by raccoons on their major prey, red rock crabs. When these carnivore playbacks were continued for a month, red rock crab populations increased sharply. This increase in crab population size was followed by a decline of the crab’s major competitor – the staghorn sculpin, and the crab’s favorite food, a Littorina periwinkle. Thus “fear itself” can cascade through the food web, affecting multiple trophic levels in important ways that ecologists are now beginning to understand.

note: the paper that describes this research is from the journal Ecology. The reference is Dudeck, B. P., Clinchy, M., Allen, M. C. and Zanette, L. Y. (2018), Fear affects parental care, which predicts juvenile survival and exacerbates the total cost of fear on demography. Ecology, 99: 127–135. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Blinded by the light: victims of the night

In late October, the municipality of Buenavista del Norte on the Canary Island of Tenerife, celebrates the day of the Virgin of Los Remedios, including, among other features, a big light display. As a child, Airam Rodríguez noticed that many shearwaters would also drop in (literally) for the festivities, attracted by the bright lights, but unable, in many cases, to get back in the air. Many of these shearwaters died from a variety of causes, including the impact of flying into the ground, dehydration, predation and poaching. As an adult, Rodríguez collaborated with researchers around the world to evaluate the scope of light-induced shorebird fallout.


Fallout victim: grounded Short-tailed Shearwater. Credit: Airam Rodríguez

The researchers began their work by searching a science citation index – the Web of Science – for articles on light-induced seabird mortality. They used references from these articles to find additional articles. In addition, they used the internet and social media to find programs in which citizens are encouraged to report grounded birds, and contacted people associated with these programs to get qualitative and quantitative data.

Rodríguez and his colleagues discovered light induced seabird fatality on 47 islands, three continental locations and across all of the world’s oceans. Of 115 species of burrow-nesting petrels, 56 have been reported as grounded by light. Several other groups of birds, including puffins, auklet and eiders also suffer from light-induced fallout, and it is very likely that more species are unreported.


Numbers of reported grounded seabird fledglings across the globe.  Circle size = numbers of birds  reported. Numbers = number of species affected. Circle color = IUCN (endangerment) category for each species as follows: CR = critically endangered, EN = endangered, VU = vulnerable, NT = near threatened, LC = least concern.

Of deep concern is that 24 species are globally threatened. In addition, fallout has been reported at sea, induced by lights used for fisheries and by lights on oil platforms. All of the studies of light-induced fatalities on land documented the highest mortality in fledglings that are grounded during their first flights from their nests toward the ocean.


Numbers of species of threatened seabirds that were rescued across the globe.  Numbers were not available for species with ? symbol.

Researchers don’t know why birds are attracted to lights. Perhaps birds view lights as a source of food; for example some species eat bioluminescent prey. Alternatively, as cavity-nesting birds, the only light these chicks see is from their burrow entrance, particularly when their parents bring in food, so the fledglings might confuse light with a food source. Lastly, artificial lights might override any celestial light cues the birds normally use for navigation, confusing them and causing them to crash to the ground. Supporting this hypothesis, seabirds generally don’t crash into lights, which might be expected if they mistook a light for bioluminescent prey.

Cory's shearwater fledgling at their nest at Tenerife Canary Islands. Photo by Beneharo Rodríguez

Fledgling Cory’s Shearwater first sees the light of day after emerging from its burrow at Arona on southern Tenerife Island. Credit: Beneharo Rodríguez

So what can be done about this problem? Accurate data are hard to come by, as many estimates of fallout-induced mortality come from relatively untrained volunteers, who are less likely to report dead birds. As one example, on Kauai, surveys from a general public rescue program for Newell’s Shearwaters identified 7.7% mortality, whereas later systematic surveys by trained researchers indicated 43% mortality. In some rescue operations, birds are banded and released, which, in theory, allows researchers to estimate the survival rate of rescue birds, but, in practice, these data are usually insufficient for accurate estimates

Rodríguez and his colleagues recommend a multipronged approach to combat seabird fallout. Individuals grounded by artificial lights can be rescued so they don’t succumb to the common causes of death – dehydration, predation and vehicle collision. In many cases the general public takes birds to designated rescue stations, where they are cared for until judged to be ready to release. The first rescue program was set up on Kauai in 1978; since then, people working for 16 rescue programs have released over 40,000 birds.

Release of a grounded shearwater. Photo Nazaret Carrasco (1)

Beneharo Rodríguez releases a Cory’s Shearwater from a cliff at Buenavista del Norte on Tenerife Island. Credit: Nazaret Carrasco.

The birds would be best served if humans behaved in ways that minimized fallout. Researchers need to learn more about why birds are attracted to artificial lights so engineers can develop outside lights that don’t attract them. Existing lights can be turned off when not needed, and dimmed when they are essential. Special accommodation can be made for unusual cases; for example in Cilaos, Reunion, Indian Ocean, streetlights are turned off during the fledging period of Barau’s Petrel. Lights can also be shielded so they illuminate an area for humans, but minimize the light visible to birds. Degraded nesting and breeding habitat can be restored to help compensate for birds that are lost to fallout. Lastly, conservation efforts should benefit the local economies so that residents will be more likely to support conservation initiatives, such as reduced evening lighting, that they might otherwise oppose.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Rodríguez, A., Holmes, N. D., Ryan, P. G., Wilson, K.-J., Faulquier, L., Murillo, Y., Raine, A. F., Penniman, J. F., Neves, V., Rodríguez, B., Negro, J. J., Chiaradia, A., Dann, P., Anderson, T., Metzger, B., Shirai, M., Deppe, L., Wheeler, J., Hodum, P., Gouveia, C., Carmo, V., Carreira, G. P., Delgado-Alburqueque, L., Guerra-Correa, C., Couzi, F.-X., Travers, M. and Corre, M. L. (2017), Seabird mortality induced by land-based artificial lights. Conservation Biology, 31: 986–1001. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.

Biological control: birds vs. (insects vs. insects)

We all know that birds eat crop-destroying bugs, so we might think that farmers would welcome insectivorous birds to their fields with radiant rakes or happy hoes. But not so fast! Research by Ingo Grass and his colleagues alerts us to the reality that not all insects are created equal. Some insects eat crops, but some insects eat insects that eat crops.

Aphids are one of the worst scourges of the agricultural world. They suck the phloem sap from many plant species; this action can kill the plant directly, and also cause infections by plant pathogens and viruses. Fortunately for farmers, many animals enjoy eating aphids, including birds such as the Eurasian Tree Sparrow, Passer montanus, and insects such as ladybird beetles and hoverfly larvae.


Hoverfly larva consumes an aphid while a second aphid looks on. Credit: Beatriz Moisset.

Grass and his colleagues knew that sparrows eat both aphids and hoverflies, but they did not know how the effects of bird predation on these insects cascaded down to the oats and wheat crops grown near Gottingen, Germany. Their research tested the hypothesis that sparrows eat so many hoverflies that aphid abundance actually increases (despite also being eaten by sparrows), and oat and wheat abundance decreases (top food web in the diagram below). If so, they reasoned that removing the birds would increase hoverfly abundance, thereby decreasing aphids and increasing grain abundance (bottom food web).


Agricultural food web with (top) and without (bottom) sparrows. Arrows show consumption, with dashed arrows indicating weak effects, and solid arrows and doubled organisms indicating strong effects.

The researchers set up an experiment with 11 nest boxes strategically placed between an oat field and a wheat field. Each box was equipped with a camera, so the researchers could see what the parents fed to their nestlings. In addition, Grass and his colleagues set up eight 4 X 5 meter plastic mesh exclosures which excluded birds, but allowed insects free access. They periodically surveyed 50 plants in each exclosure and in equal-sized control plots for hoverflies and aphids over the course of the sparrow breeding season. Because these birds can have three broods, this project kept them (the sparrows and the researchers) busy from early May to late July.


The birds fed very little on the two grain fields during the first brood, but towards the end of their second brood, they turned their attention to feeding on insects from the two grain fields, and later to eating the ripening grain. One important finding is that bird predation severely reduced hoverfly abundance. By early July hoverfly abundance was about 1 per 50 shoots when birds were present, and more than 3 per 50 shoots when birds were excluded (top graph below).



How did hoverfly consumption translate to aphid abundance? As you can see from the bottom graph, by early July, aphid abundance without birds was considerably lower than aphid abundance in the presence of birds. Taken together, these findings indicate that European Tree Sparrows consume hoverflies, which ultimately leads to an increase in aphid abundance.

Grass and his colleagues conclude that insectivorous birds can interfere with natural pest control of cereal production in central Europe. When birds were experimentally excluded, aphid densities declined 24% in wheat and 26% in oat crops. European Tree Sparrows were doubly bad for the crops, as they also harvested substantial quantities of grain from these fields to feed their third brood. The researchers argue that management of biological control systems for agriculture requires a broad food-web perspective that accounts for trophic cascades, such as the interactions that occur among sparrows, hoverflies, aphids and various types of economically important grain crops.

note: the paper that describes this research is from the journal Ecology. The reference is Grass, Ingo, Katrin Lehmann, Carsten Thies, and Teja Tscharntke. 2017. Insectivorous birds disrupt biological control of cereal aphids. Ecology 98 (6): 1583-1590Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Golden Eagles: no tilting at windmills

Todd Katzner and several other scientists were puzzled by a vexing problem. They knew that the wind turbines at Altamont Pass Wind Resource Area (APWRA) were killing large numbers of Golden Eagles that flew into their spinning blades. Yet the population of Golden Eagles in the area had stayed relatively stable over the years despite this unnatural source of mortality. The researchers considered two possibilities. First, this population of eagles may have had unusually high birth rates or unusually low death rates from other sources to compensate for the high windmill-induced mortality. Alternatively, immigrant Golden Eagles might be replacing those killed by turbines.

Altamont Pass Wind Farm

Altamont Pass Wind Farm, California. Credit: Todd Katzner.

This question has important implications for conservation biologists. If immigrant Golden Eagles are replacing those killed by windmills at APWRA, then the apparent stability of the local Golden Eagle population may be at the expense of other populations that are providing APWRA with these immigrants. So, even though APWRA’s windmills are not directly causing local eagle populations to decline, windmills at APWRA (and other windmill sites) may be indirectly leading to a decline in other populations. So Katzner and his colleagues did genetic and molecular analyses of tissues remaining from these killed eagles to learn as much as they could about these eagles and where they came from.

Golden Eagle in flight

Golden Eagle in flight. Credit: Michael J. Lanzone.

The researchers used tissue samples from 67 eagles that were killed at APWRA between 2012-2014. They subjected these tissues to a variety of genetic tests to determine the sex and age of each individual, and to evaluate the genetic differences between individuals killed by the windmills.

In addition, Katzner and his colleagues used stable isotope analysis to evaluate whether the killed individuals were local birds, or immigrants from afar. For this analysis the stable isotope ratio is the ratio of a rare and nonradioactive isotope of hydrogen (2H) found in the sample (feathers of killed birds) in relation to the common isotope (1H). A feather’s stable isotope ratio is very tightly correlated to the stable isotope ratio of the water the bird drinks. The last important point is that different regions of the world have different characteristic stable isotope ratios in rainwater. So if you can determine the stable isotope ratio of a bird’s feather, you can compare it to the world stable isotope ratio map, and determine where the bird most likely spent the previous year (once birds molt, their new feathers assume the stable isotope ratio of their new location). This approach will underestimate the number of immigrants, because some distant locales have a similar stable isotope ratio as APWRA, and birds from those regions will be incorrectly scored as being local.

Stable isotope map

Map of May-August stable isotope ratios (of 2H in rainwater).  Same colors represent similar stable isotope ratios, ranging from relatively high ratios (deep orange), to relatively low ratios (dark blue). I don’t discuss the meaning of the circles and triangles in this blog post.

Based on this analysis, more than 25% of the dead eagles were immigrants to the area, with some birds originating from more than 800 km away. The researchers point out that APWRA might be particularly attractive to eagles looking for a home because it provides two types of resources that are important to these birds – visually open feeding grounds with easily-located prey, and a consistent updraft to facilitate relatively effortless flight.

Fig 3a

Probability that an eagle killed at APWRA was local.  If the probability was less than 0.5 the researchers scored it as immigrant; if greater than 0.5 the researchers scored it as local.

About half of the immigrants that could be sexed were juveniles or subadults. The researchers argue that the apparent stability of the population in the APWRA region is achieved by young immigrants replacing those birds that are killed by windmills.


Percentage of local vs. immigrant (nonlocal) Golden Eagles by age.

Katzner and his colleagues are concerned that APWRA functions as an ecological sink that attracts eagles, primarily from nearby western states, to replace those killed by windmills. High death rates are particularly problematic to slow-growing populations, such as Golden Eagles, which usually lay only two eggs, with generally only one surviving chick per breeding season (the larger chick often kills its sibling). The researchers also point out that windmills also kill many other animals, including numerous bat species, which also have slow-growing populations. They encourage the renewable-energy industry to develop technology that will reduce windmill-induced death. Such efforts are already underway, and there is preliminary evidence that newer generation turbines are reducing Golden Eagle mortality rates.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Katzner, T.E., Nelson, D.M., Braham, M.A., Doyle, J.M., Fernandez, N.B., Duerr, A.E., Bloom, P.H., Fitzpatrick, M.C., Miller, T.A., Culver, R.C. and Braswell, L., 2017. Golden Eagle fatalities and the continental‐scale consequences of local wind‐energy generation. Conservation Biology31(2): 406-415.Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.

Highly disturbed birds

About 50 million years ago, the fast-moving Indo-Australian plate crashed into the Eurasian plate, giving rise to the Indian peninsula, and beginning a process of faulting and folding that ultimately formed our present day Himalaya Mountains. This process continues today, with the Himalayas still rising about 5 mm per year. The region is very variable, with tremendous glaciers and snowfields at high elevations, and forests and grasslands at lower elevations.


The lead Author, Paul Elsen, stands in front of the Tirthan Valley.  The highest peaks range up to 4900 meters.

The variation in elevation, climate and soils make the Himalyan region in northern India a mecca of biological diversity, hosting over 10,000 identified plant species and about 1000 bird species. As in most of India, human population growth is putting enormous pressure on the forested regions, partly as a source of wood for heating and cooking, which has led to extensive deforestation. In concert, substantial forested areas are being converted to farms or pastures to feed the growing population. Paul Elsen and his colleagues wanted to know how these transformations of forests to cropland and pastures were affecting bird population across the region. They were particularly interested in how birds survived the winter, a period of climatic stress and food scarcity, when many of the birds descend from their high elevation breeding grounds to lower elevations that are nearer to human populations.


Chestnut-headed Tesia, an altitudinal migrant found in high elevation forests in summer, and in forests and agricultural lands in winter. Credit: Prashant Negi.

The researchers set up three transects across four different landscapes (total of 12 transects), representing four levels of disturbance. The undisturbed landscape was primary forest in the Great Himalayan National Park. A second disturbance type – low intensity – retained a mixture of community forest used for timber and fuel, and also included some small agricultural plots. A third disturbance type – medium intensity – had small wooded areas, but was dominated by mixed agriculture including orchards and a variety of crops such as grains, beans and garlic. The final disturbance type – high intensity – was used as pasture, had mostly grasses and very few trees or crops.


Four land-use types. Credit Paul Elsen

The basic research protocol was literally a walk in the woods. Elsen walked (slowly) along the same trail in each transect three times during the winter season, and identified and counted all of the birds. Other researchers identified, measured and counted the plants growing along the transects.


Lead field assistant, Lal Chand (left), and co-author Kalyanaraman Ramnarayan (right) conduct plant surveys near the top of the world.


Elsen was stunned by what his team discovered. Before beginning this study, he had spent about a year in the Himalayas within intact forests doing other PhD-related research. His travels into surrounding villages showed significant bird activity, but he assumed these birds were primarily species associated with humans or more open habitats. He expected decreasing bird diversity and abundance with increasing agricultural intensification, where the bird communities in intact primary forest would be teeming with species in high densities, and the areas with mixed agriculture and intensively grazed pastures would have just a few species. The data below paint a contrasting picture.


Mean and standard error of (a) bird abundance and (b) number of bird species per site across the four land-use types.

Primary forest hosted the fewest number of birds and the fewest species of birds. Among the three disturbance levels, low- and medium-intensity had greater abundance and diversity than did the high-intensity disturbed sites. At least in the winter, low- and medium-intensity disturbed landscapes can be beneficial to bird populations. Elsen suggests that birds are attracted to the tremendous amount of food available in the agricultural lands, such as fruiting trees and shrubs, even in winter. Some birds can consume these fruits, while other birds consume the yummy energy-rich insects that are attracted to the fruit. There are also plenty of seeds available for granivorous birds. But high-intensity disturbed landscapes lack these benefits, leading to fewer forest-adapted bird species, which are replaced by open-country or generalist bird species.


Pastoralist and his goats in a high-intensity disturbed site. Credit: Prashant Nagi.


The researchers caution that we still don’t know have a clear picture of how birds use different landscapes during the breeding season, although preliminary data indicate that more species are unique to primary forests during breeding season than in winter, and that fewer species inhabit intensively grazed pastures during breeding season than in winter. Consequently, Elsen and his colleagues recommend a holistic conservation approach, which recognizes the importance of conserving large portions of intact primary forest, while at the same time preserving landscapes with low- and medium-intensity agriculture.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Elsen, P. R., Kalyanaraman, R., Ramesh, K., & Wilcove, D. S. (2016). The importance of agricultural lands for Himalayan birds in winter. Conservation Biology 31 (2): 416-426. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.

E-lek-trical death to migrants

Leks have been described as singles bars for birds, though with all the singing and dancing that can go on there, a Karaoke bar might be the closest human analog. Male birds, such as the Great Bustard, Otis tarda, get together at traditional display grounds (leks) and strut their stuff, providing no material resources for females, and being visited by females solely for the purpose of mating.


Three male Great Bustards on a lek in Central Spain. Credit: Carlos Palacin.

After the mating season concludes, some male great bustards in central Spain fly further north while others remain near the lek area. Migrants benefit from cooler and moister environmental conditions, and, in some cases, greater food availability. But migrants flying to a new area consume calories, and more recently, run the risk of flying into power lines, thereby injuring or killing themselves.


Newly erected power lines in central Spain. Credit: Carlos Palacin.


Carlos Palacín and three other researchers used radio-tracking technology to follow the behavior of 180 male bustards over the course of 16 years. They knew that some bustards died from collision with power lines, but they didn’t know whether these collisions were affecting migrants and non-migrants (sedentary birds) differently, nor if these collisions were changing the migratory behavior of bustards in the 29 breeding groups they studied. So they tracked their birds by ground and by air and determined whether each bird was migrant or sedentary, how long each bird survived, and when possible, the cause of death. For migrant bustards, the researchers measured when and where they migrated, and whether they remained migrants their entire lives.

Palacín and his colleagues discovered that birds migrated away from the lek primarily in May and June, and returned to the breeding grounds over a much more prolonged time period during the autumn and winter.


About 35% of the birds were sedentary, while 65% migrated an average of 89.9 km, with the longest migration of 261 km. Migrants had much higher mortality rates; for example among 73 birds captured and marked as juveniles, migrants survived an average of 90.6 months (post marking), while sedentary males survived an average of 134.7 months, almost 50% longer! The same pattern follows for 107 birds that were captured and marked as adults. The lesson here is that migration kills.


So why migrate? Well it appears that before humans (and in particular, before power lines), migration was a much more beneficial strategy. The researchers identified three causes of bustard mortality: collision with power lines in 37.6% of the cases, poaching (9.1%) and collision with fences (2.6%). The bustard forensic team was unable to determine mortality in the remaining cases, so these percentages may underestimate human impact. Importantly, the researchers discovered that death from power lines was more than three times greater in migrants than in sedentary birds.


This study clearly demonstrates that human infrastructure can shape the migratory behavior of a population. Over the time period of the study, the percentage of sedentary birds has increased sharply even though food availability actually decreased near the breeding grounds as a result of urbanization.


The decrease in migration may be compounded by a finding that juveniles learn to migrate (or not) from adults during their first three years of life. So if there are more sedentary adults to serve as role models for juvenile behavior, more juveniles will develop into sedentary adults. But sedentary behavior can have several drawbacks. A greater number of sedentary males will increase competition for food and other resources. Also, birds may overheat during particularly hot summers near the breeding grounds. In addition, sedentary birds may have higher inbreeding rates and lower genetic diversity, which in turn can make a local population more susceptible to disease and other environmental changes, ultimately making it more prone to extinction.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Palacín, C., Alonso, J. C., Martín, C. A., & Alonso, J. A. (2017). Changes in bird‐migration patterns associated with human‐induced mortality. Conservation Biology 31: 106-115Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.