Abundance Estimates of Three Wild Populations (Rethink Priorities)

Who should read this report?

Desk researchers attempting to Fermi estimate which wild animal interventions are cost-effective.
Field scientists who want to know how much value of information there would be in conducting research on the abundance of wild animal populations.
Grant evaluators who need to assess whether a proposed empirical study to estimate population size is adopting rigorous methodology.
Animal advocates who need practical guidance on whether to implement interventions when there is considerable uncertainty about the number of potential beneficiaries.

What we did

There is an interest in identifying cost-effective interventions to help wild animals. A major factor affecting cost-effectiveness is the number of target beneficiaries.
We searched the academic and gray literature for abundance estimates of populations for which there are already technically feasible interventions: Urban pigeons, urban rats, and birds who collide into windows.
For each population, we compare five different ways to respond to uncertainty about population size:
- Treating the best available estimates as though they were accurate.
- Adjusting existing estimates for known biases.
- Funding or conducting original empirical research.
- Exploring if there are more abundant wild populations that can be helped.
- Developing new interventions that can help a larger number of wild animals.

Findings for urban pigeons

It seems unlikely that there is any single ratio of pigeon population density to human population density that would be appropriate to apply to any given city: We found pigeon-to-human ratios ranging from .001 to 5.08.
- Methodological rigor may explain some of the variation– many studies did not empirically estimate detection probability.
High-quality studies of pigeon abundance should be relatively cheap and easy to conduct. Hence, it may be worthwhile to measure the size of a city’s population before deciding whether to actually intervene.
In the absence of reliable data, interventions should likely occur in older cities that attract tourists. This is because intentional pigeon feeding and old buildings are strong predictors of pigeon abundance.

Findings for urban rats

High-quality data on urban rat abundance is exceedingly scarce. The best studies we are aware of are of Baltimore in the 1950s.
A somewhat credible upper-bound estimate from New York City in 2011 suggests there are ~2 million rats (~.25 for every person).
Mark-recapture, the gold standard for estimating the populations of small mammals, is labor-intensive and at odds with the population control goals of municipal governments. It could be worthwhile to invest in improving and validating “resight” methods that do not require capture or handling.
The logistical barriers to accurately measuring urban rat populations will nevertheless remain formidable. An alternative to verifying that there are enough urban rats to help is to explore solutions that could also affect rodent population control in commercial settings (e.g., farms and food processing facilities).

Findings for bird-window collisions

Most studies of bird-window collisions are of just a few buildings that are known to cause a large number of collisions.
There are two large-scale Fermi estimates based on a 2010 survey of Edmonton residents. They suggest 16-42 million deaths in Canada, and 365 and 988 million deaths in the United States. Neither estimate the number of sub-lethal collisions.
To account for imperfect detection, these studies applied a correction factor to all buildings. A major uncertainty is whether there are any bird-windows at the majority of buildings with no observed strikes, especially low-rise buildings.
Given people’s interest in birds, it might be possible to cost-effectively coordinate an effort across universities located within cities to improve the evidence base.
Scientists know enough about the spatial covariates of bird strikes to develop spatial risk maps, which would ensure that interventions occur in high-risk areas.
Alternatively, reducing light pollution might help a large number of wild animals, though it would typically only modestly reduce bird-window collisions.

Limitations

We limited our search of abundance estimates to ~35 hours per population.
We did not attempt to measure the abundance of other populations that would be indirectly affected by interventions on behalf of the target populations.
Given that it is still early days for the wild animal welfare movement, it is unclear whether cost-effectiveness should actually be a major factor in determining whether to implement an intervention. Accumulating a track record of accomplishments may be more important at this stage, even if it is on behalf of relatively few wild animals.