Many urban wildlife biologists face unique challenges when trying to capture their study organisms. Traditional methods that are used in remote areas are often not available to urban wildlife biologists, either because the method requires traps that the public tampers with (though this problem does happen to non-urban biologists as well), because urban animals are more likely to evade traditional capture methods, or because the method simply isn’t practical for the urban environment.
In my own work with pigeons, I spent months trying to figure out how to capture pigeons across cities before finding a method that actually worked (which you can read about here). Henry Adams and coauthors came across a similar problem when trying to catch urban American White Ibises (Eudocimus albus).
Capturing birds in urban areas
Many species of wading birds, including American White Ibises, are living in urban environments, leading to differences in behavior and resource use. To better understand some of the differences from conspecifics living in more natural environments, researchers needed to capture urban ibises and take measurements. Traditionally, mist-nets, clap traps, and cannon nets are used to capture birds wading in or nearby shallow water. These methods are effective in more rural areas because vegetation provides cover for researches and camouflages the net. However, these traditional methods are not practical in anthropogenically altered habitats such as golf courses, zoos, urban parks, and manicured lawns, all of which contain high levels of human activity.
Moreover, when working in urban areas, researchers need to consider public safety and how the public will respond to seeing animals captured. With this in mind, Adams and coauthors designed two capture methods — a manual leg lasso and a modified manually operated flip net. Both these capture methods exploited the habituation of urban ibises to humans and allowed the researchers to successfully capture ibises in urban areas.
Capture methods
Leg Lassos: Leg lassos were designed using fishing line and were similar to snares. One end of the lasso was tied to a dowel and the other end contained a slipknot. Researchers baited the flock with bread, encouraging the flock to move towards the lasso. When a bird stepped in the loop, a different researcher would pull on the anchor, securing the lasso around the bird’s foot.
Flip Nets: Flip nets were designed using mist-nets attached to two poles. The poles were anchored to the ground and, when triggered, would flip over, capturing multiple birds at once. Researchers used this method in open spaces where flocks were habituated to people. Once the net was set up on the ground, three people would then work together to capture the ibises — two stationed on either end of the net, and a third baiting the ibises and luring them towards the net. When at least three ibises were within range, the two people on the end would simultaneously pull the ropes attached to the net causing the net to flip over the birds.
Mist-nets: Nylon mist-nets are often used for catching smaller birds that fly into the net and get tangled in one of the pockets. However, because of their size in this study researchers set up mist-net in a V, N or triangle formation. The researchers used plastic lawn flamingos that were painted white to act as decoys and set these decoys up around the mist-nets. Two people then hid behind a blind waiting to untangle individuals who got stuck. If ibises landed near the mist-nets, researchers would attempt to flush the birds into the net.
What did they find?
Researchers captured 361 American White Ibises at 20 locations. Leg lassos captured 126 individuals, flip nets captured 102 individuals, and mist-nets captured 133 individuals. Based on active capture time, researches were most successful with the flip net, capturing on average 6 birds per hour. The leg lasso captured an average of 1.6 birds per hour, and the mist net captured 0.5 birds per hour. The researchers did find some bias in which ibises were captured with each of the different methods. With the leg lasso, ibises with a greater mass relative to tarsus length were captured and more males were captured than using the other two methods. The researchers also captured more juveniles with the mist-net than the other methods. Perhaps unsurprisingly, the researchers captured more habituated individuals using the leg lasso and flip net, both techniques that involved baiting and luring the individuals towards the capture device.
Overall, each method advantages and disadvantages including cost, difficulty of assembly, ability to capture habituated and non-habituated birds, and the number of individuals captured at once.
wetlands and urban areas. Table from Adams et al. (2019).
Have you encountered problems when sampling animals in urban areas? What methods have you used? Let us know in the comments below.
Read the full manuscript here:
Featured image: “American White IbisII” by Terry Foote is licensed under CC BY-SA 3.0
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