Behind the paper: returning home to study how box-nesting birds fend off their parasites

Blog written by Sarah A. Knutie, Assistant Professor at the University of Connecticut, USA. Read the full paper here.

As a first generation college student, I had little knowledge of the college process and what it entailed. At the University of Minnesota, I started as a computer science major because my mother showed me a newspaper article promoting women in the field of Information Technology (IT) and I enjoyed using HTML to create unofficial fan websites for celebrities (you’re welcome Leonardo DiCaprio).

After several years of coursework, I decided that computer science was not my passion so I started exploring all other possible majors. I enrolled in a few ecological field courses at the UMN’s Itasca Biological Station in northern Minnesota and after a few days of the courses, I had found my career path. Over the next decade, I decided that I wanted to return to Minnesota someday to establish a long-term research program to inspire and train undergraduates just like myself. Therefore, while pursuing my graduate degrees on unrelated projects, I spent all my “free time” during the summer months building connections with local residents, including Christmas tree farmers, at and near Itasca Biological Station.

Banding birds in northern Minnesota in 2008. Photo by Erin Feichtinger.

After finishing my PhD, for which I studied the effect of introduced parasitic nest flies on birds in the Galapagos Islands, I wanted to find a local system in the US to understand the variation in host defenses against native parasites. The perfect system finally caught my eye: box-nesting birds and their parasitic nest flies Protocalliphora sp.

Nest box in a field in northern Minnesota. Photo by Sarah Knutie.

Not only are these flies similar to the parasites that I studied in the Galapagos (they live in the nests and the larval (maggot) stage feeds on the baby birds), but I also could experimentally manipulate the parasites and environmental conditions to establish links among host, parasites, and their environment in Minnesota. Protocalliphora flies parasitize a wide range of bird species, but some of the most commonly-studied bird hosts are the eastern bluebirds and tree swallows.

Adult eastern bluebird. Image at the top of the blog shows an adult tree swallow. Both pictures by Jeremy Cohen.

In 2014, my father and I started building wooden bird nest boxes for eastern bluebirds and tree swallows. Over the next 4 years, we built and established close to 150 new nest boxes near Itasca Biological Station, which added to the existing 50 or so boxes that had already been set up by local residents.

My father, Steve Knutie, with the first few nest boxes that he built in my grandmother’s garage. Photo by Sarah Knutie.

The first step of the project was to determine the effect of the parasite on hosts and whether hosts could defend themselves against the parasite. I had already established working relationships with local residents, including co-author John Hurlbert from Bemidji’s Christmas [Tree] Forest. I recruited University of Minnesota undergraduate students, including first author Kirstine Grab and co-authors Allie Parker and Dasha Pokutnaya, to help with the fieldwork. I also collaborated with Dr. Brian Hiller from Bemidji State University and his undergraduate student McKenzie Ingram. I was fortunate to receive two small grants from the Minnesota Ornithologists’ Union and North American Bluebird Society to get this work started.

Former Christmas tree farmer John Hurlbert shows his granddaughter the tree swallows in his nest box. Photo by Sarah Knutie.
Dr. Brian Hiller and McKenzie Ingram at a bluebird nest box. Photo by Sarah Knutie.

My team first reviewed published studies on the relationship between the parasitic nest fly and bluebirds and swallows. We found that across space and time, bluebirds had over twice as many parasites compared to swallows but both bird species did not suffer a negative consequence of the parasite on their survival. These results suggested that bluebirds and swallows are both well defended against the parasite, but might defend themselves in different ways.

Recently hatched tree swallow nestlings. Photo by Sarah Knutie.

Two field seasons went by in Minnesota before we had enough pairs of birds occupying the boxes for the study. In 2016-2017, we started our study by experimentally manipulating the nest parasite (in other words, hosts were either parasitized or not) to determine the effect of the parasite on the growth and survival of the nestling birds. We found that the parasite did not affect the survival of either species. Therefore, both hosts were tolerant to the effect of the parasite at their respective parasite numbers. However, swallows had half as many parasites as the bluebirds. These differences were likely because swallow nestlings produced an immune response to the parasite, which reduced parasite numbers; in contrast, bluebird nestlings did not produce a detectable immune response to the parasite. We could not identify the specific way that bluebirds defend themselves against the high number of parasites to which they are exposed, but the birds are likely investing in tolerance measures to compensate for energy lost to the parasite or effectively repair damage caused by the parasite.

Tree swallow parent visiting a nest box. Photo by Jeremy Cohen.

After gathering these two years of data, I decided that it was time to publish the work. Since undergraduate education continues to be a theme in this research, I invited Kirstine Grab to spearhead the paper, from analyzing the data to writing the first and final drafts. This was Kirstine’s first scientific paper, and writing the paper with her was an incredibly rewarding experience for both of us. I am so proud of and encouraged by these early-career scientists.

First author Kirstine Grab holding baby bluebirds. Photo by Sarah Knutie.

Since 2017, my team has continued monitoring the effect of the parasite on bluebirds and swallows to determine whether there is annual variation in host-parasite relationships over a long time scale. The team has also been working hard to determine what environmental factors, such as food availability or temperature, affects host defenses against parasites. We have also established a citizen science project to determine the spatial distribution of nest parasites across the entire range of bluebirds and swallows throughout the United States. Please get in touch if you are interested in participating (nestparasitestudy@gmail.com).

University of Connecticut undergraduate Alyssa Addesso with a nest from a citizen scientist. Photo by Sarah Knutie

I am very fortunate that I have been able to come full circle in my career. The past 15 years – from taking classes at the Itasca Biological Station as an undergraduate to mentoring students of my own at that same station – has been a long, but extremely rewarding, journey. My advice to those who are interested in pursuing a career in field research is to think long term. Where do you see yourself in 10-15 years? Can you start setting up your system now? Often times, these side projects just need a few small steps at a time but once you’ve taken enough of these steps, you might just create the center piece of your research program!

Sunset over Lake Itasca at Itasca Biological Station. Photo by Laura Domine.

I would like to respectfully acknowledge that the fieldwork was conducted on the ancestral territory of the Chippewa Nationwhere Clearwater and Beltrami counties, Minnesota, are located.

Are sparrowhawks causing declines in UK garden birds?

Blog written by Ben Swallow. Read the full paper here.

Birds (amongst other ecological taxa) are consistently subjected to a variety of external environmental factors. Some species are responding to those external pressures more successfully than others, with trajectories varying across species as well as across spatial and temporal scales. In order to understand which of these factors are the principal drivers of those changing population trajectories, and hence implement beneficial conservation measures, it is important to develop statistical models that are able to account for those varying factors across spatial and temporal hierarchies.

Our paper expands on the statistical methodology developed in Swallow et al. (2015), applying the approach to a range of 10 avian species commonly found in UK gardens and contrasting two slightly different model structures to data from the British Trust for Ornithology’s (BTO) Garden Bird Feeding Survey (GBFS). The GBFS counts maxima of each species feeding on provisioned food in each of 26 weeks over the winter period. The aim of the methodology is to link observed changes in each of 10 species of birds to a higher range of environmental variables than has previously been accomplished. Statistically speaking, we wish to test for impacts of as many potentially relevant variables as possible, with the caveat that only a subset of these would be expected to impact any given species.

The modelling framework is conducted in a hierarchical Bayesian framework, accounting for correlations between and within both sites and years in the statistical model. We utilise a relatively underused statistical distribution, the Tweedie distribution, which is able to account for the excessive exact zeros inherent in the data. The reversible jump algorithm deployed allows us to move between models with different subsets of covariates, spending a larger proportion of iterations in models with covariates and associated slope parameters that match the observed counts of birds at that site and year. Subsequently we get a quantitative measure of how much evidence we have to support an impact of a given variable on each species.

In addition, we wished to test the impact of a change in model formulation, namely whether conclusions were similar when we looked at the rate of change in environmental variables instead of the absolute value of those variables. Conclusions from both frameworks were largely similar but also highlighted some additional interesting hypotheses.

One particular recurring hypothesis associated with declines observed in some small songbirds has been a return of Eurasian sparrowhawks to regions they had been wiped out from due to pesticide use during the 1950s and 60s. We found a consistent negative impact on three species, namely house sparrow, blue tit and starling. Our results also suggested an association between the level of impact of sparrowhawks on a given prey species and the prevalence of that species in the sparrowhawk’s diet. It is important to note, however, that while a statistically important impact of sparrowhawks on house sparrow and blue tit was detected, the practical difference in terms of reduction in numbers of these species is far from enough to account for the observed declines. It is also difficult to determine in this case whether the associated impact is a causal reduction in overall numbers or relates to these two species avoiding feeding areas where higher densities of sparrowhawks were observed. Furthermore, even if the former is the true case, previous large-scale studies of breeding densities (e.g. Newson et al. (2010)) failed to find significant impact of sparrowhawks on their prey species. This may suggest that the observed impact on house sparrows and blue tits detected in our paper corresponds to those individuals that would not subsequently form part of the breeding population due to other limiting factors.

Our results highlight a variety of other potential drivers of population changes. Ground frost was consistently found as an important factor in numbers of birds visiting gardens. Higher amounts of ground frost were associated with higher numbers of birds visiting garden feeders, but this became a negative association when annual change in ground frost was used. This may suggest that whilst birds will be encouraged to visit garden feeders when weather is poor, very severe changes between years can cause negative population impacts.

Whilst this research adds to the understanding of wide-scale impacts of environmental factors on garden birds, further developments in this area are inevitably to involve the creation of joint multispecies models, where species that share the same environment niche are modelled simultaneously. Nature does not exist in isolation and hence ideally its components should not be modelled independently. Our previous work on these data published in the same journal (Swallow et al. 2016) introduced a formal multispecies extension to the model presented in this paper, whilst in Jones-Todd et al. (2018), we developed a joint spatio-temporal model for house sparrow, collared dove and sparrowhawk. One of the principal difficulties with these types of models is the increasing complexity they introduce, which from a statistical point of view equates to an increased computational demand. There are therefore two complimentary priorities for research in this area. Firstly, we must construct increasingly realistic models to infer the complex and high-dimensional interactions inherent in these communities, harnessing the increase in data that we have observed over the past decade. With this, we also need to develop increasingly efficient statistical methodologies and inferential frameworks that allow us to gain the necessary understandings of these ecosystems in realistic computational timeframes.

References:

  • Jones-Todd, C. M., Swallow, B., Illian, J. B. and Toms, M. P. (2018) ‘A spatio-temporal multi-species model of a semi-continuous response.’ (JRSS(C)67(3), 705–722)
  • Newson, S.E., Rexstad, E.A., Baillie, S.R., Buckland, S.T. & Aebischer, N.J. (2010) Population change of avian predators and grey squirrels in England: is there evidence for an impact on avian prey populations? (Journal of Applied Ecology, 47, 244–252).
  • Swallow, B., King, R., Buckland, S. T. and Toms, M. P. (2016) ‘Identifying multi-species synchrony in response to environmental covariates.’ (Ecology and Evolution6(23), 8515–8525)
  • Swallow, B., Buckland, S. T., King, R. and Toms, M. P. (2015) ‘Bayesian Hierarchical Modelling of Continuous Non-negative Longitudinal Data with a Spike at Zero: An Application to a Study of Birds Visiting Gardens in Winter.’ (Biometrical Journal Special Issue, 58(2), 357–371, Special)

A Microscopic Worm Produces More Babies in Response to Parasite Attack

Blog written by Victoria L. Pike. Read the full paper here.

Virtually all organisms are being constantly attacked by parasites. Hosts can deal with it in a variety of ways, but they ideally want to limit the damage caused by the parasite. One strategy that can limit damage to host reproductive success is ‘fecundity compensation’ (Minchella, 1985; Vale and Little, 2012), a response to infection exhibited by species across the animal kingdom including fish, snails, planktonic crustaceans and mice. Fecundity compensation involves an increase in the number of offspring produced when hosts are exposed to parasites, allowing the host to counteract any reduction in lifespan caused by infection.  

Despite the widespread occurrence of fecundity compensation, understanding how hosts can manipulate their reproduction in response to the threat of parasites remains largely unknown. Previous work has shown that potential links between other damage limiting responses (e.g., physically evading pathogens), stress responses, and the immune response (Gleichsner et al., 2016; Schulenburg and Ewbank, 2007).

In our study, we investigated whether fecundity compensation was linked with the stress hosts experience when faced with parasite infection. We used a microscopic worm, Caenorhabditis elegans, as the host in our experiment and a parasitic bacterium, Staphylococcus aureus, that produces toxins that destroy gut cells when inside the worm. This animalwas a good choice of host for this experiment as it has a rapid life cycle which it completes in around 3.5 days (Hope, 1999), meaning that experiments can be replicated quickly. Additionally, the worm’s diet is made up of microbes, so it can be easily infected by eating the parasite. Moreover, mutants of this animal have been made which allow us to knock down or ramp up the host stress response relative to a wild-type worm.  

Image of Caenorhabditis elegans on a lawn of Escherichia coli food.Image by Anke Kloock.

In our study, we used three different mutants of the wormwith either a normal, suppressed or heightened stress response to explore links between stress response and fecundity compensation upon parasite attack. We also maintained control groups of each worm mutants only given food to eat, not parasites. We then counted the number of offspring mutant and wild-type worms produced once exposed to parasites or only fed food.

Our results showed that fecundity compensation is linked to host stress in C. elegans worms. Once they survived parasite exposure, both wild-type worms and those with the heightened stress response, increased their reproductive output in the presence of the parasite. Worms with a suppressed stress and dampened immune response, however, did not produce more offspring relative to the control after parasite exposure.

These results have helped to reveal a valuable system for future research on fecundity compensation. The worm’s rapid lifecycle makes it suitable for experimental evolution approaches, as experiments can take place in large populations and over multiple generations quickly. Using this model, one could thus investigate the conditions under which fecundity compensation might evolve and be maintained. Biologists can also further investigate the mechanism of fecundity compensation in the worms. We have uncovered a potential mechanism linking fecundity compensation to stress, but more experiments with a greater diversity of mutants, targeting different pathways of the stress response, could help to clarify this mechanism. Furthermore, we only conducted our experiment with one type of parasite – it would be interesting to see whether fecundity compensation varies with different parasites that vary in the harm they cause.

Parasites are everywhere. With global change increasing the contact organisms have with infections (Altizer et al., 2013; Harvell, 2002) and ramping up the environmental stress hosts can experience (Gleichsner et al., 2016), it is crucial we understand the ways parasites can alter host reproduction. In our study, we have uncovered a link between enhanced offspring production and the host’s stress response. This research provides a springboard for further investigation into how fecundity compensation works.

References:

Altizer, S., Ostfeld, R.S., Johnson, P.T.J., Kutz, S., Harvell, C.D., 2013. Climate Change and Infectious Diseases: From Evidence to a Predictive Framework. Science 341, 514. https://doi.org/10.1126/science.1239401

Gleichsner, A.M., Cleveland, J.A., Minchella, D.J., 2016. One stimulus—Two responses: Host and parasite life-history variation in response to environmental stress. Evolution 70, 2640–2646. https://doi.org/10.1111/evo.13061

Harvell, C.D., 2002. Climate Warming and Disease Risks for Terrestrial and Marine Biota. Science 296, 2158–2162. https://doi.org/10.1126/science.1063699

Hope, I.A., 1999. C. elegans : a practical approach. Oxford University Press.

Minchella, D.J., 1985. Host life-history variation in response to parasitism. Parasitology 90, 205. https://doi.org/10.1017/S0031182000049143

Schulenburg, H., Ewbank, J.J., 2007. The genetics of pathogen avoidance in Caenorhabditis elegans. Molecular Microbiology 66, 563–570. https://doi.org/10.1111/j.1365-2958.2007.05946.x

Vale, P.F., Little, T.J., 2012. Fecundity compensation and tolerance to a sterilizing pathogen in Daphnia. Journal of Evolutionary Biology 25, 1888–1896. https://doi.org/10.1111/j.1420-9101.2012.02579.x

On the search for rabbits: where citizen science meets the scientist’s needs

Blog written by Emilie Roy-Dufresne. Read the full paper here.

Where are they and where could they go? Those are the main two questions asked by conservation scientists when it comes to invasive species. Originally from other regions and introduced accidentally or intentionally for recreation and commercial purposes, introduced species often migrate into new areas where environmental conditions differ from their native range. When these new environmental conditions are favourable, introduced species can thrive in their new habitats and become invasive. In most cases, invasive species upset the ecological balance of their new host ecosystems, resulting in the extinction of native species and a myriad of socio-economic impacts. Understanding how new environmental conditions can affect an invasive species’ population dynamics and distribution are critical so that resources can be deployed to the right time and place to support effective management and environmental protection.

Many scientific approaches are used to characterise invasive species’ distributions and their interaction with their new host environments, with monitoring and field experiments the most commonly understood and applied. While field studies are invaluable resources of detailed ecological information regard invasive species, their application at large scale can be difficult as they are very time consuming and costly, leading to delays in efficient and effective management actions. An alternative, pro-active approach is to use statistical models known as correlative-SDMs (short for Species Distribution Models), to predict where in a new habitat an invasive species may establish and persist. These sites can then be targeted by landscape managers for control or eradication programs.

Correlative-SDMs are powerful and flexible methods, as they only require known presence and absence locations of the species, and information on the environmental conditions recorded at these sites (e.g. maximum and minimum temperature, rainfall, index of green vegetation, etc.). This information is statistically analysed to generate maps of ‘environmental niche preference’ for the species – that is, areas where the species is likely to establish and persist. A regional comparison can then be used to identify regions with the highest potential risk of being invaded.

Correlative-SDMs are heavily reliant on good data to make good predictions. The best models are built from data covering the entire range of environmental conditions suitable for any given species. While collecting these data is not as complex as directly monitoring a species abundance, it remains challenging because invasive species can be widely distributed in their non-native habitat, and so data may be missing from regions which have not been surveyed due to logistical or financial constraints. One way to overcome this data limitation is to supplement the data collected by experts with data collected by volunteer citizen scientists (e.g. through phone apps device).



European rabbit (Oryctolagus cuniculus), Freemantle, 2015. Photographer: Michael Graham

In our study, we used the case of the European rabbit (Oryctolagus cuniculus) in Australia to explore the advantages and disadvantages associated with the use of citizen science data within correlative-SDMs. The European rabbit (Oryctolagus cuniculus) was introduced into Australia in 1788. Rabbits are now considered a significant pest of agricultural and environmental ecosystems, being listed as a Key Threatening Process to Australian ecosystems and biodiversity in 1999. They compete with native fauna and local livestock by overgrazing both native and introduced plants, which can lead to soil erosion. In the past 50 years, the presence and abundance of rabbits have been monitored through extended management programs led by expert scientists across Australia (Roy-Dufresne et al., 2019). In 2009, a citizen science app was developed as a vigilance program for the rabbit (Feral Scan Data, 2016). This allowed us the opportunity to investigate the pros and cons associated with the use of citizen science data within correlative-SDMs, by comparing the models’ performance when expert and citizen science data were used separately or together.

Front page of the Rabbit Feral Scan website, 29/9/19

We found that there were massive advantages to using both expert and citizen science datasets when formulating correlative-SDMs, with great improvements in model performance. Addition of the citizen science dataset doubled the spatial coverage of expert-only derived occurrence data used to build our models – adding an additional total area equivalent to a third of the total landmass of Australia. On top of this boost in data coverage, citizen science data also provided new and critical information on the environmental conditions associated with the ecological niche of rabbits in Australia. Together, these insights drastically improved model performance and reliability, and highlighted the invaluable value-add of using citizen science when studying and managing invasive species.

Rabbits at Quinyambie Station, NE Australia. Photographer: Peter Bird

Citizen science can therefore be a crucial value-adding component to the development and implementation of more effective monitoring programs for invasive species on a national scale. Our work shows that collaborations between experts and citizen scientists can be a valuable tool, when addressing data deficits associated with low levels of monitoring in areas which are difficult and costly to access. Although it remains important that fieldwork led by experts persists, especially when evaluating the accuracy of data collected by citizen scientists and the level to which the invasive species impact their host habitats, directing the activities of citizen scientists towards areas with missing information can provide an effective solution to collect data quickly and cheaply so that management decisions can occur on a more productive and effective timescale. Frameworks and networks such as the one we used can easily be extended to further other collaborative actions. For example, the collection of dead rabbit carcasses by citizen scientists can help scientists to survey more widely in order to understand diseases dynamics in rabbit populations across Australia, data which can then be used to understand and improve the effectiveness of biological control programs across the country.

Rabbit warren, characterised by heavy grazing and soil erosion, Barossa Valley, 2015

In an attempt to improve the data accessibility in science, we published our complete datasets in an open online publication (Roy-Dufresne et al., 2019).

References:

Feral Scan Data. (2016). https://www.feralscan.org.au/

Roy-Dufresne, E., Lurgi, M., Brown, S. C., Wells, K., Cooke, B., Mutze, G., … Fordham, D. A. (2019). The Australian National Rabbit Database: 50 yr of population monitoring of an invasive species. Ecology, 100, e02750.

A new approach for ecosystem-scale manipulations of bird abundance and species richness

Blog written by Chelsea L. Wood. Read the full paper here.

For generations, ecologists have relied on manipulative experiments to explore the dynamics of ecological communities. Some of the most influential studies in the ecology canon are experimental manipulations – think Bob Paine’s experimental exclusion of the keystone predator Pisaster ochraeceous, Stephen Carpenter’s whole-lake manipulation of nutrients, Dan Simberloff and EO Wilson’s island biogeography experiments in the Florida Keys, or Gene Likens’ forest-clearance experiments at Hubbard Brook. Correlational and comparative studies have their place for detecting and exploring the generality of patterns, but experimental manipulations are needed to understand the causal relationships that underlie ecological patterns.

But despite the value of manipulative experiments, they can be exceedingly difficult to execute, particularly when the focal community contains large-bodied, vagile species. For example, several studies point to the ecological importance of birds as predators, nutrient importers, hosts, seed dispersers, pollinators, and scavengers. But progress in understanding the ecological role of birds at the ecosystem level has been hampered by the difficulty of performing experimental manipulations of bird abundance across large spatial extents.

Our paper, recently published in Ecology and Evolution, presents a new method for experimentally increasing the abundance and richness of birds at the scale of entire aquatic ecosystems, with minimal cost, risk to wildlife, and need for maintenance. This approach involves the use of attractants that encourage birds to use a particular site, instead of deterrents that discourage birds from using that site or physically preventing their access to the site (approaches whose efficacy often attenuates over time). Our approach was effective at increasing the abundance and species richness of water‐associated birds at central California ponds.

Figure 1. Map of study sites in the East Bay region of central California. Eight experimental ponds were located in Joseph D. Grant County Park (circles) and eight were located on San Felipe Ranch (triangles). Of these, eight were randomly assigned to the bird attractant treatment (blue) and eight were randomly assigned to the control treatment (red)

We worked at 16 small ponds located on two adjoining properties in the East Bay area of central California (Figure 1). To attract birds to attractant‐treatment sites, we added perching habitat, nesting habitat, two mallard duck decoys (one male, one female), and one floating platform to each pond (Figure 2). We then assessed bird abundance by monitoring ponds with trail cameras. We compared the change in bird species richness and abundance from before the manipulation (i.e., 2014) to two years after manipulation (i.e., 2017) in control versus attractant treatments (a before–after–control–impact or BACI design).

Figure 2. Attractant manipulations installed at Glorious Pond, Joseph D. Grant County Park. bb = bird nesting boxes, fp = floating platforms, yellow arrows indicate added perching habitat

We found that our bird-attractant treatments augmented both bird species diversity and bird abundance. Bird species richness declined over time in both treatments, probably due to the effects of a prolonged drought that affected California during the time period of our experiment, but the decline in bird richness was less pronounced in the attractant compared to control treatments. Total bird abundance (across all species) increased in the attractant treatment while it declined in the control treatment. The bird species in which the attractant treatment had the most positive influence on abundance were American Robins, Black Phoebes, California Quail, Western Kingbirds, unidentified passerines, raptors, and waterbirds; together, these taxa accounted for 83% of total bird detections.

Our results suggest that simple, inexpensive modifications to existing pond habitat can produce a substantial change in bird abundance and richness – providing a way forward for field experiments that effectively quantify the ecological role of birds. It is especially notable that the manipulations were effective two years after their implementation; this allows for experiments with long temporal durations, a key feature for assessing ecological processes that occur on long time scales.

The manipulations we implemented were inexpensive, easily maintained, and unobtrusive. We estimate that our attractant treatments cost approximately US$103 per pond ($60 for wood duck box, $25 for generic bird box, $2 for fence posts to mount bird boxes, $6 for duck decoys, $10 for materials to construct floating platform), and required fewer than two person‐hours to install. In addition to their low cost, our manipulations were durable and easily maintained: despite the presence of large mammals (e.g., deer, pigs, coyote, cows) that might trample or otherwise compromise attractants, we observed no negative wildlife interactions. Manipulations required very minimal maintenance; we checked on ponds once per year and spent ~15 person‐minutes per pond per year re‐positioning floating platforms or duck decoys, supplementing shoreline perching habitat, or (for only one pond over the two‐year experiment) re‐mounting a fallen bird box. Importantly, the manipulations were unobtrusive and inconspicuous. This low visibility minimizes the chance that the treatments will be noticed by human visitors, reducing the likelihood of vandalism, theft, and objections by neighbors, park users, landowners, or land managers concerned about the aesthetic value of ponds. In fact, one of the land managers we worked with was enthusiastic about these manipulations, which she hoped would contribute to the conservation value of wetlands under her stewardship. The low cost, ease of maintenance, inconspicuousness, and conservation benefits of our approach allowed us to maximize the size and number of manipulated ponds, increasing statistical power and biological realism.

There are numerous potential applications of our approach to manipulating bird abundance and richness. We plan to use this method to perform a large‐scale, long‐term bird manipulation experiment in central California ponds. Our aim is to quantify the effect of increases in local bird abundance and richness on the composition of pond communities, and particularly on the transmission of parasites within ponds. Birds play a variety of roles in these pond ecosystems: as dispersers of parasites, predators of hosts, and hosts for vectors and the pathogens they transmit. Manipulative experiments are therefore necessary to disentangle the potential effects of change in bird biodiversity on disease processes and to discover the net effect of bird biodiversity loss on the prevalence of disease in ponds. Our method of bird augmentation might also be useful for scientists working on other questions about the ecological roles of birds, or in other ecosystems. Most bird manipulation experiments to date have investigated the role of birds as predators using bird deterrence, and bird exclusion is a suitable approach for assessing the impacts of bird predation on community composition at small spatial scales. However, because our approach can be deployed across larger spatial scales than traditional caged or netted bird exclosures, it can also be used to investigate processes that occur at large spatial scales: for example, nutrient export/import, seed dispersal, and scavenging/decomposition. Our approach could also be easily adapted to augment birds across large plots in other relatively open ecosystems—for example, grasslands, meadows, open woodlands, tundra, marshes, wetlands, dunes, and beaches.

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