Not all blackbirds are totally black. Some have a few white feathers, or even a lot. In the first systematic study of leucism (the total or partial lack of black and brown pigments in feathers) in blackbirds, biologists from the University of Groningen show that it is more common in urban than in forest populations. They have a number of explanations for this phenomenon, some of which could have consequences for human city dwellers. The study is published by the Journal of Avian Biology on 3 October.
Previous studies on leucism focused on just one location, explains Juan Diego Ibáñez-Álamo, postdoc at the Groningen Institute for Evolutionary Life Sciences. He studies the health of blackbirds in different European locations in Spain, France and Finland, comparing populations in cities and forest areas. During his studies, he recorded leucism in the birds he caught. This gave him the opportunity to test previous suggestions that leucism was more frequent in cities. ‘But we also used additional methods to try to respond to this question.’
Apart from observing birds caught for his research project on blackbird health, a master student observed birds on transects through five Spanish cities and geographically paired forest areas. Furthermore, an internet search provided some 1,200 blackbird photographs from all over the world. Using standard procedures that were developed for studying bird coloration through this type of ‘citizen science’, Ibáñez-Álamo and his colleagues looked for leucism in the pictures.
This led the biologists to a number of conclusions, says Ibáñez-Álamo: ‘First, we confirmed that leucism is more frequent in urban areas, a general finding supported by all methods used’. Other findings are that leucism is more frequent in males compared to females and, as in the case of humans, it increases with age.
As the study was purely observational, it is not possible to draw firm conclusions on the cause for leucism in these birds. A number of explanations have been suggested. Ibáñez-Álamo: ‘One explanation is the free radical theory of ageing, which says that damage by reactive oxygen species increases with age and this could induce programmed cell death in the melanocytes that produce the dark pigment.’ Increased leucism in older birds – on both urban and forest locations – is congruent with this explanation.
Other suggestions focus on a lack of tyrosine in the diet of urban birds, or an increased exposure to mutagens in cities. Both could reduce the production of dark pigments. Finally, white patches may cause increased predation. ‘The number of predators is higher in forest areas, so this might explain why we find less birds with leucism there. And nesting females might be more prone to predation, which could explain why we see less females with leucism.’
The importance of this observational study is that the data on leucism are now firmly established. Work in progress suggests similar data on leucism for many other bird species. ‘Leucism is associated with cities, a man-made environment, therefore we are probably the cause of this phenomenon’, says Ibáñez-Álamo. The conclusion is relevant for urban ecology, and might even have implications for humans: ‘If leucism is caused by increased exposure to mutagens or oxidative stress in cities, this is likely to affect our health as well.’
Reference: Izquierdo, Lucía; Thomson, Robert; Aguirre, José Ignacio; Díez-Fernández, Alazne; Faivre, B.; Figuerola, J.; Ibáñez-Álamo, Juan Diego: Factors associated with leucism in the common blackbird (Turdus merula). Journal of Avian Biology Doi:10.1111/jav.01778
See also: Blackbirds in the city: Bad health, longer life
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