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UPDATED: How Safe is Mist Netting? First Large Scale Study into Bird Capture Technique Evaluates the Risks


Capturing birds using mist nets to study behaviour, movement or the demographics of a species is one of the most common research techniques in ornithology, yet until now there have been no large scale studies into the risks mist nets pose to birds. Writing in the British Ecological Society’s Methods in Ecology and Evolution researchers from California used a dataset of over 345,000 records to evaluate the risks of mist netting.
Sooner or later, it happens to every ornithologist. A routine research protocol for a common practice such as mist-netting has sailed through review by the Institutional Animal Care and Use Committee (IACUC) dozens of times. This time, however, they ask you about the potential for injury and mortality. Oh, you respond, it is extremely low. The mortality rate for passerines caught by mist-netting under one per cent. “Really?” they say. “Can you provide any peer-reviewed literature to support that statement?”

Now, you can, thanks to biologist Erica Spotswood at U.C. Berkeley. Erica wanted to study seed dispersal ecology and plant invasions in French Polynesia. She used mist nets to capture birds to obtain fecal samples to characterize and quantify diets. The government of French Polynesia denied her permit request to capture fruit doves, which are a protected species in that country. She was told that the government wildlife agency was concerned that mist netting would injure or kill birds. She told him that the risk is very low, but he asked to see the study. Erica soon found that the study did not exist, so she decided to do that study herself.

She persuaded 22 banding organisations across the United States and Canada to share their injury and mortality data so she could assess the risk factors which could increase rates of injury or mortality including bird size, age, frequency of capture and the role of predators. Now you can tell your IACUC that an analysis of 345,752 birds capture in mist nets found that "the average rate of injury was 0·59%, while mortality was 0·23%."

Is that all you need to know? Definitely not. As low as these rates are, it is still important to take all reasonable measures to reduce the rates of injury and mortality. As responsible researchers who care about the impact of research on the birds we study, you should pay close attention to the recommendations that followed the analysis:
  • vulnerable species and individuals captured for the first time should be prioritized
  • banders should identify which species are most vulnerable at their own sites.
  • Personnel should pay attention to stress by using cues such as panting, lethargy, raising of feathers and closing eyes, and they should be particularly careful in recognizing stress cues for smaller birds.
  • Banders should also watch for signs of wing strain and tangling in smaller birds and internal injuries, leg injuries, cuts and predation in larger birds.
  • Training, training, and more training is key
As Spotswood said, “A lot of wildlife research requires that animals are captured and handled in order to study populations, demographics, behaviour or movement, yet the process can be stressful to animals. For both ethical reasons and the sake of the research, it is important to make absolutely sure any risks are minimized.”


“Our study fills in an important gap in understanding by evaluating one of the most commonly used methods in ornithological research. We have shown that when banders follow good practices, incidents are rare. We hope that the results of this paper will be widely read by the banding community and that it will help researchers minimize any risk of incident.”

The paper is available online:

E. N. Spotswood, K. Roesch Goodman, J. Carlisle, R. L. Cormier, D. L. Humple, J. Rousseau, S. L. Guers, G. G. Barton, How safe is mist netting? Evaluating the risk of injury and mortality to birds, Methods in Ecology and Evolution, Wiley-Blackwell, June 2011, DOI: 10.1111/j.2041-210X.2011.00123.x


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Chris Merkord
Jun 30 2011 07:11 PM

Summary
1. The capture of birds using mist nets is a widely utilized technique for monitoring avian populations. While the method is assumed to be safe, very few studies have addressed how frequently injuries and mortalities occur and the associated risks have not been formally evaluated.

2. We quantified the rates of mortality and injury at 22 banding organizations in the United States and Canada and used capture data from five organizations to determine what kinds of incidents occur most frequently. Analyses focused on passerines and near-passerines, but other groups were included. We evaluated whether body mass, age, sex, mist net mesh size, month and time of day or frequency of capture are related to the risk or type of incident. We also compared the recapture histories over time between birds that were injured and those that were never injured for 16 species.

3. The average rate of injury was 0·59%, while mortality was 0·23%. Birds captured frequently were less at risk to incident. Body mass was positively correlated with incident and larger birds were at greater risk to predation, leg injuries, broken legs, internal bleeding and cuts, while smaller birds were more prone to stress, tangling-related injuries and wing strain. Rates of incident varied among species, with some at greater risk than others. We found no evidence for increased mortality over time of injured birds compared with uninjured birds.

4. We provide the first comprehensive evaluation of the risks associated with mist netting. Our results indicate that (1) injury and mortality rates below one percent can be achieved during mist netting and (2) injured birds are likely to survive in comparable numbers to uninjured birds after release. While overall risks are low, this study identified vulnerable species and traits that may increase a bird’s susceptibility to incident that should be considered in banding protocols aimed at minimizing injury and mortality. Projects using mist nets should monitor their performance and compare their results to those of other organizations.