Guest blog post by Dr Daria Dadam, Research Ecologist, British Trust for Ornithology (formerly at the Institute of Zoology, Zoological Society of London).

Once a ubiquitous sight and sound of our cities, the house sparrow (Passer domesticus) is now absent from many urban areas. This small passerine bird has declined in England by 70% in the past 40 years, according to data from the British Trust for Ornithology.

This familiar but charismatic species has attracted a lot of recent scientific attention  and several hypotheses have been put forward to explain its decline. Predation by Sparrowhawk (Accipiter nisus) and cats (Felis catus) did not reach definitive conclusions, with pollution from particles in diesel engines and related immunosuppression also proposed  as potential explanations but without firm evidence. The finding that lack of invertebrates in the chicks’ diet reduced fledging success in a study in Leicester informed a large-scale feeding experiment in London, which resulted in increased fledging success but it did not boost the breeding population the following year. However, overwinter survival of juveniles was identified as a potential problem, but no mechanisms had been found. In all this work, one aspect had been overlooked – the potential role of parasites.

Photo: House sparrow by Ben Andrew (rspb-images.com)

Parasites are known to have the capacity to affect animal populations by affecting  host survival and/or reproductive success. Infectious disease emergence poses a threat to wildlife conservation and several parasites have been demonstrated as having impacts on birds at population level across the world: avian malaria in Hawaii, West-Nile Virus and Mycoplasma gallisepticum in North America and Trichomonosis in Great Britain, where the parasite has been identified as a cause of a marked decline of the greenfinch (Chloris chloris) population. Could parasites be implicated in the decline of the house sparrow?

New study finds that 74% of London’s house sparrows carry avian malaria

A team of researchers from the ZSL Institute of Zoology, in collaboration with the RSPB, set up a three-year study to investigate whether parasites could be behind the demise of the sparrow in London. The “cockney sparrow” has declined by 70% in the last 20 years, once a familiar sight in parks in the centre of the capital, now many areas are devoid of the chirping sound of this iconic species.

Almost 400 individual house sparrows were trapped using fine mist-nets and were marked with colour-rings to allow us to track their survival through the winter. We also collected, under Home Office Licence, a tiny blood sample, as well as a faecal sample, from each individual. We then tested for blood parasites as well as bacteria from faecal samples from private gardens across 11 colonies scattered across the capital, seven of which had a negative population trend, and four a stable or increasing population.

We found that 74% of house sparrows, higher than previously recorded in any other wild bird population in Northern Europe, tested positive for Plasmodium relictum, the parasite that causes avian malaria, and that the population decline was greatest in those colonies having the highest infection intensity (the number of parasites in an individual), especially in juvenile sparrows. We, therefore, had a link between parasites and the decline but how much impact was it really having? Did birds really die from it?

To answer this we undertook some survival analyses, in collaboration with the BTO, to see how big the effect might be. This showed that birds that carried the highest infection of malaria, whether they were juveniles or adults, had lower survival probability than those with less severe infections. Now we had a possible mechanism: avian malaria was linked to population decline through reduced overwinter survival of juveniles.

A further line of evidence for the importance of malaria came when we looked at another blood parasite, Atoxoplasma, which was present in about one-third of birds. Crucially, individual sparrows with these survived as well as their uninfected compatriots, suggesting the avian malaria was key, not simply birds being in poor health.

Figure: Relationship between intensity of infection with malaria parasite Plasmodium relictum and survival probability in juvenile house sparrows.

What about the other possible, less-well studied causes, such as the impact of air pollution and immunosuppression? There was no relationship between  NO2 air pollution and either population trend or parasite infection rates across our 11 colonies.  There was also no evidence of immunosuppression, because whilst we did find other parasites, these were found at much lower frequency than avian malaria, and they did not show a negative relationship with population trend nor survival.

Why would avian malaria, which has probably been present  in  the UK before the onset of the house sparrow population decline, now be affecting this species in urban areas? We are not sure, but house sparrows may now be exposed to more malaria-carrying mosquitoes than before, perhaps driven by warmer and wetter conditions. It has already been hypothesised that for birds in Northern Europe infection rates of avian malaria will increase due to climate change.

A study on i’iwi (Vestiaria coccinea), a brightly-coloured honeycreeper native to Hawaii, showed that the intensity of malaria infection was related to the number of bites from infected mosquitoes. We also know that resistance to Plasmodium has a genetic basis in house sparrows, and given the well-recorded sedentary nature of the species, it is possible that loss of habitat within cities might have led to isolated populations which are not very diverse genetically. Low gene diversity can lead to a less-effective immune system and higher mortality, which, combined with low recruitment of new birds into the colonies, can lead to population decline.

Whilst the underlying causes of house sparrow declines remain unknown, and it is likely due to a combination of factors, we suggest that avian malaria may be implicated. Further research is needed to understand exactly how avian malaria may be affecting urban house sparrows. Improved disease surveillance and continuing monitoring of the species at national level are essential for effective conservation management.

Dadam, D., Robinson, R.A., Clements, A., Peach, W.J., Bennett, M, Rowcliffe, J.M and Cunningham, A.A. 2019. Avian malaria-mediated population decline of a widespread iconic bird species, Royal Society Open Science.

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