If you spend enough time in a city, you’ll probably notice pigeons with injured legs or missing toes. You may have assumed the lost digits were the result of viruses or fights with other birds, but there may be another more inconspicuous agent to blame.
Instead, many birds get "stringfoot" – hair and string wrapped around their toes, cutting off circulation and resulting in necrosis and eventual loss. Furthermore, those with toe wounds on the mend often display marks similar to being strangled by a wire or cord, say the authors of the study published in Biological Conservation.
To kick off their city bird investigations, the team took to the streets of Paris, categorizing 46 sites based on habitat types such as density of people, foliage, and buildings nearby. They found 30 pigeons with mutilated toes between April and May 2013, jotting down the state of damage for each of them. They also recorded the color of the pigeons due to the link between coloration and cellular immune response.
In their sample, 1 out of 5 adult pigeons was mutilated. The team found no correlation between foot deformities and disease, and when one foot was hurt, the other foot was no more likely to be hurt than normal. Instead, they found that "toe mutilation in urban pigeons occurs in areas where pollution is high, identified here as air and noise pollution, and that mutilation are more numerous where human inhabitants are more numerous."
They suggests air pollution is not the direct cause of harm, but rather a proxy for how many people are in a given area. They also noted an increase in toe injury with higher density of strings and hairs on the ground.
"Indeed, we found that pigeons tended to be more frequently mutilated where hairdressers are more numerous. However, there are also more hairdressers where human population density is higher, leading to a potential indirect effect of human population density," write the team.
Pigeons in green spaces tend to have more toes, likely due to the fact that there are fewer people and less pollution, resulting in less of a chance for pigeons to get their feet tangled in hair or string on the ground.
"It also happens in other urban birds," study author Frédéric Jiguet, from the National Museum of Natural History (MNHN), told IFLScience. In fact, he recently "freed a crow with plastic strings tightened in its toes."
The team does not dismiss the notion that many urban pigeons are infected with viruses, but say evidence leans away from this being the reason for their missing toes. Instead, they call the pigeons "victims of urban-based pollution."
"Humans often accuse pigeons to be dirty animals, and pathogen reservoirs, so dangerous for humans, mutilated because they live in their dirt. But in fact they are the victims of human pollutions. They suffer far more from our pollution than we suffer from their presence," said Jiguet.
Other previously proposed hypotheses for the issue include foot infections from standing in their own excrement, infection by Staphylococcus bacteria, injury from chemical or physical deterrents like wire on buildings, and hereditary deformities.
However, the team says a close observation of the foot and toe can often reveal the “presence or former presence of a string or more generally human hair caught on digits.”
"The string or hair is 'captured' when the bird walks on the ground. Then it can drop off, or stay captured on a toe. If the bird tries to take it off, it can succeed or at the opposite tighten it on a toe," said Jiguet.
"It is important to pay attention to urban wildlife because such animals will inform us on the environmental conditions we live in – the One Health concept. Pigeons can be seen as the final receptor of our pollution, so we should pay attention to their health."