City blackbirds are sicker, but longer lived

Blackbirds in urban areas are in poorer health than their country cousins but, paradoxically, are living longer.

Common blackbirds (Turdus merula) are a familiar sight in city parks and gardens throughout Europe, but, says researcher Juan Diego Ibáñez-Álamo from the University of Groningen, Germany, “they also live in their original forest areas, which makes them ideal candidates for a study of the effect of city life on health”.

Ibáñez-Álamo partnered with Simon Verhulst, also from Groningen, and other researchers, to compare the health of urban blackbirds with those living in natural forests. The study, published in the journal Royal Society Biology Letters, uses the length of telomeres as biomarker of blackbird health.

Telomeres are areas of repeated nucleotide sequences at the end of each chromosome, which protect the chromosome from deterioration. They are often described as protective caps. Shortened telomere length (TL) is associated with increased mortality in numerous vertebrate species, such as wild birds, lizards and humans.

“There are many indicators of health, like the presence of parasites or the immune response, but these will vary over time,” Ibáñez-Álamo explains. He regards TL as a robust and unchanging biomarker, and therefore a good candidate for the comparison of urban and forest birds.

The field work involved the assistance of local biologists to capture blackbirds in mist nests, in five pairs of urban-and-forest populations across Europe. The birds were divided into two age classes: yearlings and adults, then blood samples were taken. After analysis in the laboratory, the biologists discovered that both yearling and adult urban blackbirds have shorter telomeres than those in forests.

But, somewhat paradoxically, the proportion of older birds was higher in the cities.

“This means that mortality is lower in the cities, so the advantages of city life compensate for the negative health effects,” Ibáñez-Álamo says.

The absence of predators along with increased prey availability in urban areas could be compensating for the negative effects of the shortened TL.

Genetic factors may also be important, as TL is highly heritable, says Verhulst. “This could be present at birth or develop in the first year, as cities are an unhealthy environment. And it could even be that birds with short telomeres end up in cities and thereby create a population with shorter telomeres.”

The study concludes that the findings illustrate even the most successful of city-dwelling birds and other animals may be paying a price for living in human-dominated habitats.

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