Neighborhood Nestwatch Publications
This article appeared in a book about the general theory, practice and impact of citizen science programs.
Nearly 1 million acres is converted to urbanized landscapes each year in the United States, yet there is surprisingly little research to support the effects of urbanization on wildlife. There is a need, therefore, to increase the amount of research conducted on urban wildlife so landscapes can be managed to mitigate large-scale population losses. There is an equally serious need to bring this to the attention of the general public, particularly given the tendency of urban dwellers to become desensitized to the natural world. The Smithsonian Migratory Bird Center has addressed both of these needs by introducing a project called Neighborhood Nestwatch. This program engages people living in urban environments with the process of wildlife research by soliciting their help as citizen scientists. The program was created in 2000 in the Washington, D.C., region to conduct research into the effects of urbanization on birds while educating citizens about these effects and the process of science.
Nestwatch is unique among citizen science programs, because it revolves around annual face-to-face visits that occur directly in the properties of participants. Every breeding season a visit is conducted during which participants assist with capturing, measuring and color-banding birds within the program’s focal species group. A unique color band combination is placed on the birds’ legs so that participants and staff can track survival over time. Measurements provide information on individual bird health relative to the health of other individuals of the same species living in other areas of the study region. Feather and blood samples are sometimes taken, providing information on environmental toxins, blood parasites and genetic differences among populations. Time is also spent searching for nests which participants monitor.
Data on re-sightings of color banded birds and nests is submitted through regular mail or the program website. Throughout the visit participants are mentored on how to collect data, informed on general avian natural history and educated about the process of science. The program has been very successful with many general and scientific papers published. It has also proven the reliability and importance of citizen data as a vital resource in science-based conservation. Researchers found that Nestwatch participants were more engaged with the program relative to programs that are solely internet based.
A higher level of participation is due to the scientist-participant relationship that is cultivated during annual face-to-face visits. Through survey studies, it was found that Nestwatch was responsible for retaining specific ecological concepts related to avian conservation, making changes to their properties to enhance wildlife habitat and increasing awareness of their yard as a small piece in a larger puzzle. To date the program has provided training and career opportunities for 40 undergrad interns, two high school interns, two master’s studies, two doctoral theses and three postdoctoral studies. Overall Neighborhood Nestwatch reaches more than 1,000 people annually.Marra, P., and R. Reitsma. 2012. Neighborhood Nestwatch: Mentoring Citizens in the Urban Matrix. Citizen Science: Public Participation in Environmental Research, Pp. 43-49. Dickinson, J.and R. Bonney, eds. Ithaca: Cornell University Press.
Urban areas are quickly expanding throughout the U.S. which negatively impacts wildlife. Currently, little is known about how this increase impacts the productivity and survival of breeding birds within urban/suburban environments. This study focused on filling this knowledge gap by determining nesting and post-fledging success of one common suburban species: the gray catbird.
Nest success is defined as the probability that the young will survive both incubation and nestling stages. Post-fledging success determines how many individuals survive following the departure from the nest and for how long they persist. Taken together, these two variables set much of the stage for the overall survival of bird populations. Within an urban/suburban environment many factors influence fledging and nesting success, some natural and others not. Some of these factors include reductions of native predators, supplemental feeding from bird feeders, increased nest sites through nest boxes and threats from introduced contaminants. This research examined the impact of domestic cat predation on gray catbirds as an additional factor influencing nesting and post-fledging success.
This study determined the rate of nest and post-fledging success within three sites located in Nestwatch neighborhoods of suburban Washington, D.C. To do this, it examined how predation pressure, as well as the number and sex ratios of young in a nest, influence post-fledging success. Nests at each site were checked every 2-4 days during the incubation and nestling stages until the nests fledged young or failed. On the eighth or ninth day of the nestling stage individuals were banded with U.S. Fish and Wildlife aluminum bands. To determine post-fledging success, nestlings or newly fledged individuals were fitted with radio transmitters so they could be tracked using telemetry techniques. Lastly, in order to assess predator abundance surveys were taken at each location siting hawks, eastern chipmunks, gray squirrels, blue jays, domestic cats and American crows.
Nest success probability and estimates of post-fledging survival varied substantially among the three, yet all sites exhibited nests with similar brood sizes, sex ratios of young and fledglings. Differences became apparent when comparing sites according to post-fledging success. Overall, a significant trend showed greater numbers of domestic cats (fledgling predators) and gray squirrels (egg predators) with lower rates of post-fledgling success. All other predators were found to be similar across the sites. Hence, this research demonstrates that domestic cats, acting as introduced predators whose populations are subsidized with food and shelter, can largely affect the fledgling success of wild birds in urban/suburban environments, thereby adding to the many hurdles impeding avian survival.
Balogh, Anne L., Thomas B. Ryder, and Peter P. Marra. 2011. Population demography of Gray Catbirds in the suburban matrix: sources, sinks and domestic cats. Journal of Ornithology, 152.3: 717-726.
As urbanization increases so does noise. Noise can hinder birds’ effectiveness at attracting mates and defending territories, which could lead to decreased reproductive success and survival. We can think of any setting as having its “sound environment.” In the science of sound, frequency is a term synonymous with pitch. This study focuses on two factors prevalent in urban areas that impact this “sound space.”
One factor is impervious surface, such as the buildings associated with urban development. These structures serve as reflective surfaces and cause sound to bounce around or reverberate. It is the higher frequency sounds of birds that get distorted in this environment. Another characteristic of the urban sound space is a greater degree of ambient “hum,” or lower frequency background noise that originates from sources like automobile traffic. This noise can drown out the lower pitched portions of bird songs. When the full “message” that a bird intends to send through its song does not reach its intended target, that’s a problem. This study takes advantage of the “outdoor laboratory” of Nestwatch by closely examining how song structure of Nestwatch focal species may differ throughout the urban-to-rural gradient of the Washington, D.C., region.
There are three hypotheses in this study: 1) The minimum frequency sounds within bird songs will be affected by background noise level, and this effect will be stronger for species that have lower-pitched songs. Impervious surface will have little or no effect on minimum frequency. 2) The maximum frequency sounds will be affected by the amount of impervious surface, and this affect will be stronger for species with higher-pitched songs. Background noise level will have little or no effect on maximum frequency. 3) The frequency range of bird songs will be affected by the amount of urban development, and this effect will be stronger for species with higher-frequency songs. Noise level will have little or no effect on frequency range. Six Nestwatch focal species with varied song structure were included in this study. Detailed data on song structure and ambient noise were recorded in 28 Nestwatch neighborhoods.
In support of the hypotheses, one general trend of the study revealed that birds with lower-frequency songs (e.g., American robin) used higher-frequency elements as ambient noise increased. For these same species, their songs did not change in areas with increased reflective surfaces. A second general trend was that as reflective surfaces increased, those species with greater high-frequency elements in their song (e.g., gray catbird) lowered their respective frequencies. There was no change with increased low-frequency ambient noise. The results associated with the third hypothesis were mixed in that some species adjusted their song to both urban development and background noise regardless of the original frequency range of their song. In some cases, the response was different depending on whether a site was urban as opposed to rural. Furthermore, some species showed a predictable adjustment response in urban areas and not in rural areas. These results point to the possibility that some species may be more flexible with their ability to adjust song frequencies while others are more constrained, with their survival more at risk.
Dowling, J., Luther, D., and P.P.Marra. 2012. Comparative effects of urban development and anthropogenic noise on bird songs. Behavioral Ecology, 23 (1): 201-209.
The Neighborhood Nestwatch Program: Participant Outcomes of a Citizen-Science Ecological Research Project
Citizens living in metropolitan areas tend to become disengaged with science and technology. One unfortunate cost can be an increasingly uninformed society that is losing the ability to judge important issues when science is applied. The Neighborhood Nestwatch program was created in the hopes of bridging the gap between science and society by engaging citizens in mentored participatory research. Over a long period of time, Nestwatch participants collect data tha tcan help researchers understand the ecological and population complexities of eight species of birds along an urban-to-rural gradient in the Washington, D.C., area.
Based on surveys conducted with the Nestwatch participant base in Washington, D.C., this research sought to measure the impact of the program on gains in scientific knowledge, as well as increases in awareness of the natural environment. Through one-on-one interviews and paper questionnaires participants were asked about their past and present knowledge of avian ecology and conservation, as well as the significance of the project activities they and Nestwatch staff perform. Answers to these questions were used to measure quantitative and qualitative gains in scientific knowledge and environmental awareness.
Some of the major findings include 90 percent of participants gaining knowledge from the program, even among the most experienced birders. 83 percent of participants reported an enhanced perception of the surrounding natural environment in general. More specifically, the majority of participants made enhancements to their yard as habitat and gained a better perception of how their property fits within an urban ecological landscape. Many made improvements that provided shelter or food resources for animals other than birds.
Another significant yet unmeasured aspect mentioned by participants was the effectiveness of the face-to-face interaction between scientist and participant. Not only did this promote engagement in the scientific process, it also served to demystify science by breaking down pre-conceived barriers that exist between society and scientists. The article concludes by mentioning how programs like Nestwatch, which empower people to manage their properties as wildlife habitat, are becoming increasingly important.
This and the three articles that follow arose from a nationwide concern over the introduction and dispersal of West Nile virus in North America. With much of the attention focused on metropolitan areas, Neighborhood Nestwatch became a useful venue from which to embark on critical research on the issue.
Mosquitos are well known for being transmitters of infectious diseases. West Nile virus is a common disease spread from mosquito to different hosts, including birds. Dispersal of this virus through birds is dependent upon the receptivity of the host species and by the viability of the virus carried by mosquitos infecting them. In other words, the virus may be more viable if a mosquito gets its blood meal from one bird species rather than multiple species and if the mosquito carries a highly contagious strain. Dispersal of West Nile virus through birds also involves transmission through interactions among infected individuals and whether certain individuals or species groups carry natural resistance. By finding evidence for the mosquito-host interactions mentioned above, this pioneering research attempted to track the dispersal of West Nile virus through Nestwatch bird communities in the Washington, D.C., region.
In five Nestwatch neighborhoods, mosquitos were trapped and their feeding patterns observed. Mosquitos were identified to species when possible. If a blood meal was present it was tested for both the presence of West Nile virus and the identification of the animal providing the blood meal. Additionally, a bird census was taken at each site to get a sense for the abundance and diversity of species comprising the avian community. Finally, thousands of birds of many species were captured briefly and their blood sampled. Avian blood samples were analyzed for West Nile virus antibodies, which indicates positive virus transmission.
Culex pipiens, or common house mosquito, comprised 90 percent of mosquitos trapped. Perhaps the most alarming discovery of this study was that more than 43 percent of the blood meals found in trapped mosquitos originated from American robins, despite the fact that robins were uncommon or even absent from some study sites. This demonstrates that mosquitos preferentially select robins as a host despite a wide breadth of avian and mammal species to choose from. In addition, the blood meals in robins contained a highly contagious form of the virus that apparently is not lethal to them. This is in contrast to house sparrows, which were far more common than robins yet were far less likely to be identified in blood meals. This research points to the possibility that American robins serve as “super spreaders” for West Nile virus and act as a sort of reservoir where the virus can remain viable. These discoveries regarding West Nile virus transmission gain more importance considering the fact that the kinds of urban/suburban neighborhoods used in this study are among those having the highest recorded cases of West Nile virus in humans.Kilpatrick, A.M., Daszak, P., Jones, M.J., Marra, P., and L.D. Kramer. 2006. Host heterogeneity dominates West Nile virus transmission. Proceedings of the Royal Society B: Biological Sciences, 273: 2327-2333.
This study built upon the previous in that it focused on the likelihood of nesting young or adult American robins to contract West Nile virus and further contribute to the spread of the disease. The approach used infrared cameras on nests found by Neighborhood Nestwatch participants for a 24-hour period to detect body heat of mosquitos. This enabled researchers to count how many individual mosquitos landed on an adult or nestling. The assumption was that birds which were bitten more often were believed to hold a greater risk of contracting the virus. To complement the camera footage mosquitos and robins were closely observed in and around the nest to determine whether certain behaviors played a role in birds’ vulnerability to the virus.
Adults were bitten much more often than nestlings. Adult females received 2,000-6,000 bites during each of the incubation and nestling periods. The higher rate among adults makes sense, because they spend large amounts of time sitting during the brooding stage when they are either incubating or protecting young nestlings from predators. Adult birds that spent a greater time brooding did prevent nestlings from bites but were bitten more themselves and became more susceptible to the virus.
Not surprisingly, those nests with lower brooding time by adults showed higher rates of mosquito bites on nestlings. The study pointed out that through time, as the breeding season progresses, mosquitos become increasingly prevalent, which means birds that nest earlier in the season may have less risk of contracting the disease. Regardless of what particular time of year mosquitos frequent robin nests, it is clear that West Nile virus as a mosquito-borne illness is perpetuated through avian hosts, especially American robins and additional animal species.Griffing, S., Kilpatrick, A.M., Clark, L., and P. Marra. 2007. Mosquito landing rates and West Nile virus exposure of adult and nestling American robins. Vector-borne Zoonotic Diseases, 7(3): 437-443.
Each year, the number of West Nile virus cases in humans in North America largely increases by the end of summer and beginning of fall. Earlier in the season fewer cases are reported, and it has been suggested that this is a result of the American robin breeding cycle. The virus easily spreads from mosquito to American robins, as well as in the opposite direction, making the robin a capable host and disperser of the virus. The previous article showed how American robins are a preferred bird species host for common mosquitos along the east coast despite only making up a small part of the total bird diversity. However, following the breeding cycle in the spring and early summer, robins disperse and become less common leaving the mosquitos to look elsewhere for a food source. At this time, it is believed that mosquitos change their feeding patterns and shift focus to humans, effectively increasing the number of virus transmissions in human hosts.
To test this hypothesis and to understand the factors driving North American human West Nile virus occurrence, the feeding behavior, population changes and virus dispersal of mosquitoes, birds and humans was examined. At predetermined Neighborhood Nestwatch sites mosquito density was measured by collecting individuals which were later identified to species through DNA testing. The origin of blood meals found in trapped mosquitos were later identified and, finally, density and diversity of bird populations at the sites was recorded through censusing techniques. These data were collected over regular time periods throughout the avian breeding season.
More than 50 percent of blood meals tested from the mosquitos were that of the American robin, despite this species only making up 4.5 percent of the avian community. After four months of the breeding season the American robin population at the sites declined as breeding diminished and population dispersal began. This coincided with the seven-fold increase in amount of blood meals from humans. House sparrows remained very common throughout breeding and post-breeding period,s yet were not significant mosquito hosts. Thus, the shift in feeding from bird to human is not due to the lack of substitute avian hosts; rather, it is due to the decline in abundance of robins as their preferred host.
Robins breed in areas of high mosquito density, and they spend a lot of time sitting and brooding on nests (see previous article). Using breeding robins as hosts results in larger-scale amplification of West Nile virus than when compared with other birds and mammals. This is why the researchers surmise that if mosquitoes fed on humans more during the robin breeding season, there would be lower overall human disease transmission, but West Nile virus would not be amplified as much. Given this shift in host preference over time, there would be less alarm if that shift did not involve increased use of humans as hosts. Discovering these fine but critical details of West Nile virus transmission within urban ecosystems is vital toward developing solutions to these and other devastating diseases.Kilpatrick, A.M., Kramer, L.D., Jones, M.J., Marra, P.P., and P. Daszak. 2006. West Nile virus epidemics in North America are driven by shifts in mosquito feeding behavior. Public Library of Science: Biology, 4(4): 606-610.
Predicted and observed mortality from vector-borne disease in wildlife: West Nile virus and small songbirds
Tracing the spread and impact of diseases like West Nile virus in wild bird populations has many obstacles, not the least of which is finding diseased animals before their bodies have decomposed beyond the condition necessary to study disease transmission. West Nile virus has been examined in larger animals, such as crows, but not for smaller birds due to the aforementioned issues. Experimental injections of birds in captivity may offer an alternative to the use of dead birds. This approach, however, typically involves small, unconvincing sample sizes as well as a limited diversity of species. This study offers a method to trace the transmission network of West Nile virus within bird populations that depends solely on effort derived from field work without the need for injections. It uses results from captive injections but only as a means to verify what can be obtained from field work.
This study integrated field data from Neighborhood Nestwatch sites on mosquito feeding patterns, avian abundance and rates of avian West Nile virus infection. Collecting these data involved identifying the source of blood meals from mosquitos through DNA analysis, censusing birds at using line transects, and taking blood samples of thousands of birds and later analyzing them for the virus. All of these factors were used to create a mathematical tool to predict West Nile virus mortality rates. These predictions were then compared with actual rates from experimental infections. The tool incorporated variables such as the fraction of each population that had antibodies, the probability of survival, the original population size and the degree to which mosquitos feed more on one species versus another. Because this study involved experimental infections, only three species were used: the tufted titmouse and Carolina wren, because it had been reported in previous research that these species were experiencing large declines possibly due to West Nile virus; and the northern cardinal, because injection experiments had already been previously performed.
The prevalence of West Nile virus antibodies among wild populations was highest among cardinals, followed by wrens, then titmice. The presence of antibodies means natural defenses exist to combat an infection. So, cardinals have the greatest ability to fend off West Nile virus, titmice the least. This was in fact exactly the trend supported by the experimental infections, demonstrating the ability of field work in this context to create realistic outcomes. The study also highlights the fact that mortality among wrens, titmice and other small bird species may be greater than previously thought, because dead individuals go undetected. The article points out that actions to reduce the impact of West Nile virus on many bird species include habitat modifications to reduce mosquito larval habitat through removal of manmade containers (e.g., tires and clogged gutters).Kilpatrick, A. M. , Peters, R. J., Dupuis II, A. P., Jones, M. J., Daszak, P., Marra, P. P., and Laura D. Kramer. 2013. Predicted and observed mortality from vector-borne disease in wildlife: West Nile virus and small songbirds. Biological Conservation, 165: 79-85.
For a wild bird, inhabiting a metropolitan area can have its advantages and disadvantages. For instance, nest boxes can provide some cavity nesting species with more nesting sites. This study compared nesting success in rural and urban areas of house wrens, a small, migratory, insect-eating species which nests in tree cavities or nest boxes. Measures of nesting success included were the number of eggs, young and fledglings as well as underlying factors such as rates of feeding by adults, the quality of food provided by adults and the size and health of nestlings.
Over 200 nest boxes located in Neighborhood Nestwatch backyards were monitored closely during the egg and nestling stages as were the number of young that fledged. Nests that failed to produce any fledged young were a result of abandonment by the parents, predation, nest destruction by an outside source or exceedingly cold temperatures. Over prescribed time periods the number of trips made by parents to deliver food to the young and the time females spent brooding (protecting, insulating) young was recorded. In addition, nestlings were carefully weighed and wings measured at different periods.
Results showed that house wrens in both rural and urban areas commenced breeding at the same time and laid similar numbers of eggs. House wrens in rural habitats were less successful at producing nests that fledged at least one young, possibly due to greater numbers of predators, such as snakes and small mammals, in rural areas. But young reared in suburban areas weighed significantly less and had shorter wings (i.e., lower fitness) compared to those in rural areas. The number of feeding trips by parents in both areas was similar suggesting that different nestling fitness may be due to food quality rather than quantity. Inhabiting metropolitan areas, therefore, can be a mixed bag of benefits and traps depending on the food resources available within distinct locations.Newhouse, M., Marra, P.P., and S. Johnson. 2008. Reproductive success of house wrens in suburban and rural landscapes. Wilson Journal of Ornithology, 120(1): 99-104.
Lead has long been a concern to wildlife health. Its harmful effects led to actions, such as the eventual ban of lead sinkers used in fishing and lead bullets used in hunting. Actions such as these have taken place mostly in rural areas, but there has been relatively little revealed about the role lead plays in the health of wildlife in urban areas. When birds ingest lead it is stored in muscle tissue and levels build up over time creating physiological and behavioral abnormalities. In urban environments birds which mainly eat insects or invertebrates (e.g., worms) ingest contaminated food items. Additionally, ground-feeding birds (e.g., robins) inadvertently consume contaminated soil. Birds which feed on seeds and fruits may ingest lead due to the plant’s uptake of lead through the root system. The objective of this study was to quantify soil lead levels along an urban-to-rural gradient, measure blood lead levels of birds at these sites and examine the impact of lead on the body condition of these individuals.
A total of 53 Neighborhood Nestwatch properties were used to conduct the study. Three samples of soil were collected and evaluated at each site, and blood from up to seven different species was sampled and examined for lead content. Since nestlings are nourished only from food resources in the immediate area of the nest, blood samples were also taken from American robin and gray catbird nestlings to serve as a more accurate reflection of the environment in which they are raised.
There was a strong difference between urban and rural areas regarding soil lead concentration. Soil lead content in urban and suburban backyards averaged nearly nine times higher than that of rural backyards. Furthermore, adults and nestlings of all species inhabiting lead-contaminated environments had much higher lead levels compared to individuals inhabiting relatively uncontaminated rural areas. The Environmental Protection Agency’s “action level” for soil content in children’s play areas is 300 ppm. Most of the sites sampled were below this but were close. In addition, the Centers for Disease Control established a blood level of 0.1 ppm as being cause for medical attention in human children. A large number of urban/suburban adult and nestling birds, creatures much smaller than humans, showed blood levels above this threshold. This research has highlighted one of the many challenges birds must face in urbanized environments and is reflective of how an increasingly urban world will affect bird populations and other wildlife.Roux, K.E. and P. P. Marra. 2007. The presence and impact of environmental lead in passerine birds along an urban to rural land-use gradient. Archives of Environmental Contamination and Toxicology, 53(2): 261-268.
Quantifying avian nest survival along an urbanization gradient using citizen- and scientist-generated data
Since the early 1970s, the extent of urban area in the United States has more than doubled. This rapid increase in urbanization creates costs and benefits to wildlife depending on the species. Understanding the full picture of this issue requires long-term records on avian survival, not just in urban centers but also in outlying suburban and rural areas. Examining nest success within an urban-to-rural gradient over a long period of time enables a thorough look into how birds cope with urbanization.
Neighborhood Nestwatch has existed for nine years and through its network of citizen scientists has been collecting data on nest success and other variables important to bird survival. The goal of this study was to use Nestwatch data to examine nest success of five focal species across the Washington, D.C., gradient over the nine year period. In addition to discerning trends regarding bird survival, this study also statistically compared the reliability of citizen data versus data derived from scientists.
In addition to several years of nest data from both Nestwatch staff and participants, data from an artificial nest experiment using both quail and clay eggs was used as a method to characterize nest predators within the gradient. Data from 405 nests of American robins, northern mockingbirds, northern cardinals, gray catbirds and house wrens were included in this study spanning the years 2000-2008. Nests were monitored by either participants or staff. During repeated visits to nests, the number of eggs, nestlings and fledglings was recorded, as was the date of the last egg laid, the date of hatching and fledgling, and the date that the nest either fledged or failed due to abandonment, predation or other another cause. Artificial nests, small wicker baskets containing two quail eggs and one clay egg, were deployed in randomly selected Nestwatch backyards along the gradient. Nests remained for two weeks during which data were collected on disruption, destruction or removal of eggs and the origin of any markings left on clay eggs.
Overall, the results showed that predation is a central component determining nest survival and that the amount of canopy cover and impervious surface surrounding a nest influences predation risk. Somewhat surprisingly, four of the five species had higher nest success in urban areas compared to rural environments. Urban areas are sometimes referred to as “safe zones” in this respect because they contain lower numbers and diversity of predators. Artificial nest results showed that avian predators, such as American crows and blue jays, were more common than small mammals as the environment shifted from rural to urban.
More specifically, statistical tools showed that strong correlations existed when examining canopy and impervious cover at a 1,000-meter radius around a nest. This strong correlation is most likely due to the area requirements for nest predators. The house wren, which is not an open-cup nester but rather a cavity nester, had the highest probability of nest survival presumably due to the difficulty for predators to reach the nest. Finally, all data generated by citizen scientists were as reliable as that of Smithsonian scientists proving that citizen scientists provide a dependable and beneficial source of data for research and larger conservation programs.
The data on nest success suggests that urbanization may benefit some breeding birds due to reduced predation. But the researchers are quick to point out that even though urban environments may have less predation risk, they also contain unique threats to survival through factors such as exposure to contaminants, noise pollution, decreased fledgling success (all addressed in previous Nestwatch articles) and increased nest parasitism. So, once again, Nestwatch research has shown that life in an urban environment for birds is a mixed bag.Ryder, T., Reitsma, R., Evans, B., and P. Marra. 2010. Quantifying avian nest survival along an urbanization gradient using citizen-and scientist-generated data. Ecological Applications, 20(2): 419–426.
A consistent theme in many Nestwatch research projects focuses on the fact that developed land in urban areas is rapidly increasing and that this leads to a mixed bag of costs and benefits for wildlife. For birds, the nest boxes and bird feeders attributed to urban areas could be of benefit, while increased contamination and decreased vegetative cover represent potential costs. The long-term survival of any single population of bird species is affected by a combination of these and many other factors. Like the previous study on nest success, this research makes use of long-term data gathered by Neighborhood Nestwatch participants and scientists in the Washington, D.C., area. In this instance, all of the re-sighting data of color-banded birds from 2000-2012 was put to use to examine trends in the annual survival of thousands of individuals. These trends were then analyzed according to land-use features representing different levels of urbanization.
The field method upon which this research depends is called mark-recapture. During annual Nestwatch visits, focal species are captured, measured and color-banded. Each bird is given a unique band combination to distinguish one individual from another. Both participants and staff subsequently search for these birds through time and submit data to the project database. What results is a very robust database through which survival data from re-sightings can be examined according a wide variety of land use variables.
Seven of the eight focal species were included in the study (northern mockingbirds were excluded because insufficient numbers have been banded through the years). The results were very interesting, showing that some species seem to benefit from urban development while others do not. American robins, gray catbirds, northern cardinals and song sparrows showed increased survival as urbanization increased, while house wrens and Carolina wrens showed decreased survival with peak survival exhibited at rural sites. Carolina chickadees showed no change.
The four species with increased survivorship are within a group called habitat generalists. These species consume many different food types making them more adaptable to challenges associated with urban landscapes. Additionally, there is more supplemental feeding in urban areas through bird feeders. Also at play is that a considerable proportion of the diets of these species consist of fruit and insect types which are more abundant in urban/suburban environments.
The remaining three species — Carolina chickadees, house wrens and Carolina wrens — all had low annual survival rates, with the highest rate of any of them corresponding to the lowest rate of the northern cardinal. Explanation for their trend may lie in the fact that they utilize cavities for nesting. The decrease in natural cavities in urban areas plus the increased competition from cavity-nesting introduced species like starlings and house sparrows can only hamper survival. The “mixed bag” effect of urbanization on bird survival continues to reign. Once again, we see benefits and costs with explanations dependent on a more detailed examination of the activities and needs of each species.
Evans, Brian S., Thomas Brandt Ryder, Robert Reitsma, Allen H. Hurlbert, and Peter P. Marra. 2015. Characterizing avian survival along a rural-to-urban land use gradient. Ecology, 96(6), p. 1631-1640.
Previous Nestwatch articles focused on the spread of urbanization and how it affects general bird population trends. While it is important to acknowledge the severity of the rapid conversion of land, we need to go further and understand more specifically how various features of urban landscapes are creating the observed trends. In other words, through Nestwatch and other research we know generally what is going on, but we need to know why exactly it is happening. Only then can we address the specific needs of urban wildlife.
In this vein, the world of nonnative plants deserves attention. We know many of these plants as either invasive choking weeds (e.g., English ivy) or as ornamentals that adorn our yards (e.g., crape myrtle). Depending on where you are, nonnative plants can dominate the urban landscape. Not surprisingly, plants play a critical role in the web of life for birds. The research in this article is grounded in the fact that plants are the food for an enormous array of insects which, in turn, are a critical food resource for many nesting birds and their young. This is a study conducted in an urban landscape that represents one of the few instances where insect abundance and diversity found on native versus nonnative plants is examined while simultaneously gauging the health and breeding potential of a wild bird.
The study focused on the breeding and foraging activity of Carolina chickadees, a common cavity nesting species which makes use of nest boxes. Despite an adult diet dominated by seeds, chickadees almost exclusively depend on caterpillars obtained from tree foliage to feed their young. Researchers conducted meticulous caterpillar counts, bird censuses, foraging observations and vegetation characterizations in numerous Nestwatch yards located in the Washington, D.C., area. The goals were to determine the influence of native versus nonnative plants on food availability, chickadee tree preference and breeding territory selection.
Native plants were more likely to host a higher biomass of caterpillars compared to nonnative plants, and chickadees strongly preferred to forage in native plants that supported the most caterpillars. Not surprisingly, chickadees were more likely to breed in yards that had more native plants. In addition, there was a greater incidence of nest box use if the surrounding area had larger trees. Also, chickadee abundance declined as impermeable surface area increased and tree size decreased, suggesting that chickadees are hampered by increased land-use development. These results demonstrate that nonnative plants reduce habitat suitability for chickadees by reducing insect food available for breeding. Improving human-dominated landscapes as wildlife habitat should include increasing native plant species to bolster the food resources needed throughout the life history of insectivorous birds.Narango, Desiree L., Douglas W. Tallamy, Peter P. Marra. 2017. Native plants improve breeding and foraging habitat for an insectivorous bird. Biological Conservation, 213, 42-50.
Environmental filtering of avian communities along a rural-to-urban gradient in Greater Washington, D.C., USA.
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Evans, B. S., R. Reitsma, A. H. Hurlbert, and P. P. Marra. 2018. Environmental filtering of avian communities along a rural-to-urban gradient in Greater Washington, D.C., USA. Ecosphere 9(11):e02402. 10.1002/ecs2.2402