Sparrow populations are declining in many places. The cheerful little birds that just a few decades ago populated our streets and cheekily asked for a few crumbs of bread, are hardly seen today .
There are several factors that can cause the gradual disappearance of a species and this particular case is no exception. However, it is not a general disappearance. The feeling of absence that we perceive is due to a bias that most of us suffer from.
The bias of the urban point of view
The apparent disappearance of sparrows is due to a bias in our perception ; we see fewer sparrows in cities and deduce that they are disappearing. Actually, it is not about a disappearance but rather about the migration from the city to the countryside.
If global censuses are analyzed, it is observed that although there is a slight decrease in the last fifteen years, in reality the decline is very small -with obvious fluctuations typical of stable populations-, and seems to be recovering in recent years.
So if we want to know the causes of this apparent decline, what we need to find out is what has changed in cities so that they were once attractive to sparrows, and today they decide to escape from them.
Why do they leave the cities?
In recent decades there are several things that have changed significantly in cities, and that directly affect bird populations in general, and those of sparrows in particular.
On the one hand, the level of urbanization is decisive . Sparrows abound where they find food, they prefer open areas, with the presence of vegetation . Therefore, city spaces with a high level of urbanization, large crowds of people or heavy traffic are undesirable for these animals. The high level of pollution that some cities have also affects them very negatively.
On the other hand, it has changed the way buildings are constructed and managed , places where sparrows used to nest or rest. Buildings are getting taller and offer fewer resting areas for these birds. Also, the spiked metal structures that some people place on ledges and ledges prevent birds from perching on them . A sparrow that does not find areas to perch to rest will prefer to go to another place where it can.
But there is a more serious problem that is affecting sparrows in cities and we humans have introduced it.
The effect of invasive species
Among the invasive exotic animals that we have introduced in our cities, the Kramer’s parrot stands out. It is proven that these animals have generated impacts on the behavior of native species : they reduce the rate of feeding and increase the active vigilance of the birds, increasing their stress levels. Furthermore, cases of sparrows being directly attacked by invasive parrots have also been reported.
With that background, it almost makes sense for sparrows to migrate from cities to quieter places to live.
Finally, there has been talk of another cause for the disappearance of the sparrow , although, in reality, it has little foundation .
Electromagnetic radiation: it will be that no
Several scientific publications refer to a hypothetical electromagnetic contamination of mobile telephony associated with the detriment of sparrow populations. This hypothesis has several gaps .
For one, there is no causal connection . There is no known effect of non-ionizing radiation to indicate that it can somehow deter sparrows. Furthermore, if it existed , this effect would not be restricted only to mobile telephony, it should have been observed long before , with television and radio broadcasts.
Quite the contrary, what occurs is a temporal correlation of two independent events, which some people, following a cum hoc ergo propter hoc reasoning , mistakenly assume to have a causal connection.
Assuming that greater non-ionizing radiation from mobile phones must be the cause of the disappearance of sparrows because they coincide in time makes the same sense as assuming that the greater sale of ice cream on beaches causes jellyfish attacks .
Two independent unconnected events can coincide in time, moreover, two events can be produced by the same cause without being connected to each other . Just as in summer there are more people who buy ice creams on the beach, and also more bathers in the sea exposed to the attack of jellyfish, a higher level of urbanization may be causing, at the same time, a greater use of mobile telephony, and a displacement of sparrow populations to rural areas. Two events with the same cause, but independent of each other.
REFERENCES:
Bernat-Ponce, E. et al. 2018. Factors affecting the abundance of House Sparrows Passer domesticus in urban areas of southeast of Spain. Bird Study, 65(3), 404-416. DOI: 10.1080/00063657.2018.1518403
Covas, L. et al. 2017. Records of fatal attacks by Rose-ringed Parakeets Psittacula krameri on native avifauna. Revista Catalana d’Ornitologia, 33, 45-49.
Marzal, A. et al. 2018. Variation in malaria infection and immune defence in invasive and endemic house sparrows. Animal Conservation, 21(6), 505-514. DOI: 10.1111/acv.12423
Peck, H. L. et al. 2014. Experimental evidence of impacts of an invasive parakeet on foraging behavior of native birds. Behavioral Ecology, 25(3), 582-590. DOI: 10.1093/beheco/aru025
SEO/BirdLife. 2021, marzo 20. ¿Cómo le va al gorrión?
