There is a very large global movement to feed birds and I want to address why this is a human success story and could be an ecological disaster. These two alternatives follow from two divergent views of the role of humans in the world’s ecosystems. The first is the dominant view that humans are the most important species on Earth, and that we can design the world to maximize our wellbeing without concern of the ecological consequences. The second is a view that we are the custodians of the Earth and that our aim must be to conserve the Earth’s biodiversity and protect its ecosystems. The second view is gaining more visibility with the conservation movement, but if it is to become dominant, there are many ecological problems that deserve our attention. One of the most obvious ones is bird feeding. There are at present no global policies on feeding birds and views on feeding are controversial (Baverstock et al. 2019).
Humans feed birds because they like to look at them and because they have a general belief that feeding in winter or severe weather prevents bird deaths (Brock et al. 2021, Clark et al. 2019). If that is correct, we would expect to see that if we had one large area where birds were fed in winter, and another in which birds were not fed, there should be a difference in population size in the two areas in the following spring. I have yet to see any study that shows this differential effect. Consider an alternative hypothesis that feeding does indeed improve bird survival in winter, but this merely feeds more predators that now have a larger prey base, so the improvement is largely in the predator populations. It is certainly true for some migratory bird species that if they are fed they can overwinter in more northerly areas or in cities and towns, so geographic winter ranges can expand.
Perhaps the most obvious impact of feeding birds and providing water is the transmission of diseases associated with feeding stations and bird baths. Lawson et al. (2017) explored this problem with bird feeding in Great Britain and found emerging diseases over a 25-year period, focusing on protozoan, viral, and bacterial diseases with contrasting modes of transmission. They also considered mycotoxin contamination of food residues in bird feeders, which present a direct risk to bird health. Rogers et al. (2018) described a mortality event in a declining population of band-tailed pigeons in California with a loss off about 18,000 pigeons associated with tricomonosis in a drought in which birds visited artificial water sites like bird baths. Purple et al. (2015) have demonstrated that the protozoan parasite Trichomonas gallinae could persist in bird baths.
There is a certain irony in the general belief that feeding improves the survival of wild birds. I am reminded of an old story about the English ornithologist David Lack who in the 1930s was talking to a local bird club about his long-term study for a life table of the English Robin. He reported from his banding studies that the life expectancy of the robin was about one year. After his talk, an elderly woman came up to him and started beating him over the head with her umbrella. Once she calmed down, she challenged him because she had a robin singing in her back yard for the last 10 years, so she assumed it was the same robin.
There are other consequences of feeding birds. One is the attraction of squirrels to bird feeders, and the subsequent displacement of birds. One study in southern England showed that grey squirrels occupied the feeders nearly half of the time they were in service (Hanmer et al. 2018). Another consequence of feeders is feed spilled to the ground which can attract rats and other less desirable species in urban settings. Many of these problems are not unique to bird feeding. Fležar et al. (2019) used cameras to investigate sites where European brown bears were being artificially fed year-round on plant-food and carrion from road kills in Slovenia. Over one year they detected 23 vertebrate species at the feeding sites in about 68,000 photos, most frequently brown bears, red foxes and European badgers, but also about half of the species coming to the feeding sites were birds. Roe deer also used these bear feeding sites, even though it is technically illegal to feed roe deer in this jurisdiction because deer feeding on corn and other plants materials can lead to fatal metabolic diseases. The key point is that feeding stations can attract a variety of non-target species with largely unknown consequences for the local wildlife community.
It will take a brave set of ecologists and veterinarians to define and test the critical hypotheses that arise from feeding wildlife of any kind if only because of the vested interests of the bird seed producers along with so many humans who ‘know’ that feeding is ‘good’ for wildlife. The irony of all this in the end is that many people in parts of the Earth suffer from poor nutrition and starvation while in the first world we use agricultural products to feed birds and other wildlife.
Baverstock, S., Weston, M.A., and Miller, K.K. (2019). A global paucity of wild bird feeding policy. Science of The Total Environment 653, 105-111. doi: 10.1016/j.scitotenv.2018.10.338.
Brock, M., Doremus, J., and Li, Liqing (2021). Birds of a feather lockdown together: Mutual bird-human benefits during a global pandemic. Ecological Economics 189, 107174. doi: 10.1016/j.ecolecon.2021.107174.
Clark, D.N., Jones, D.N., and Reynolds, S.J. (2019). Exploring the motivations for garden bird feeding in south-east England. Ecology and Society 24, 26. doi: 10.5751/ES-10814-240126.
Fležar, Urša, Costa, B., and Krofel, M. (2019). Free food for everyone: artificial feeding of brown bears provides food for many non-target species. European Journal of Wildlife Research 65, 1. doi: 10.1007/s10344-018-1237-3.
Hanmer, H.J., Thomas, R.L., and Fellowes, M.D.E. (2018). Introduced Grey Squirrels subvert supplementary feeding of suburban wild birds. Landscape and Urban Planning 177, 10-18. doi: 10.1016/j.landurbplan.2018.04.004.
Lawson, B., Robinson, R. A., and Cunningham, A. A. (2018). Health hazards to wild birds and risk factors associated with anthropogenic food provisioning. Philosophical Transactions of the Royal Society, Biological Sciences 373 (1745): 20170091. doi: 10.1098/rstb.2017.0091.
Purple, K.E. and Gerhold, R.W. (2015). Persistence of two isolates of Trichomonas gallinae in simulated bird baths with and without organic material. Avian Diseases 59, 472-474. doi: 10.1637/11089-041115-Reg.1.
Rogers, K.H., Girard, Y.A., Woods, L.W., and Johnson, C.K. (2018). Avian trichomonosis mortality events in band-tailed pigeons (Patagioenas fasciata) in California during winter 2014–2015. International Journal for Parasitology: Parasites and Wildlife 7, 261-267. doi: 10.1016/j.ijppaw.2018.06.006.