The house sparrow mystery

The Big Garden Birdwatch results for 2016 have now been published, finding that the house sparrow is the number one visitor to our gardens. This is fantastic news because, as many of you know, house sparrows are a red listed species in the UK due to declining by half in rural areas and by over 60% in urban areas. However in recent years the UK house sparrow population decline is believed to have stabilised.
But why? What caused this bird to decrease so drastically in the first place? What has changed preventing further declines?
House sparrows occur in two very different habitat types therefore it is likely there is more than one factor causing their declines. The main factors that have been researched are predation, nest site availability, habitat loss, decrease in food supply, disease and health issues.
Predation by sparrow hawks has been researched by various biologists with mixed results. Some studies have found that sparrow hawk presence negatively affects house sparrow numbers at feed stations. However other studies have found no significant effect of sparrow hawk presence on house sparrow numbers. Predation by domestic cats has also been researched. Predation by domestic cats has been found to be at such a high rate it could cause negative effects to local populations, however only on urban areas of Bristol.
House sparrows mainly nest in roof cavities of buildings, under tiles or around the soffits and fascia’s. When wood fascia’s rot they create holes for house sparrows to nest in. Except now wooden fascia’s are being replaced with plastic ones or replaced and repaired much sooner, therefore reducing nest site availability for house sparrows. However declines have occurred in areas where roof replacement is less widespread.
Fledging success of urban house sparrows was found to be higher in areas where there is a high proportion of deciduous shrubs and little concrete. Paving and concrete are replacing native shrubs reducing habitat and increasing exposure of foraging birds which causes a higher risk of predation, as off street parking is highly desired especially within wealthy areas. In rural areas the changes to populations has been strongly linked to the intensification of agriculture.
Due to stricter hygiene laws requiring food to be protected to minimize risk of contamination and reduce pests, foraging opportunities for the house sparrow have decreased. In urban areas the streets are much cleaner than previously and the use of horse drawn carriages has ceased, reducing foraging opportunities again.
A reduction of aphids is believed to be a main factor causing house sparrow declines. Aphids are fed to house sparrow chicks for the first few days after hatching and therefore vital for chick survival. However other birds such as tit species that feed on aphids are not decreasing in numbers, therefore is it availability of aphids at the peak of the house sparrow breeding season? Potentially weather could play an important factor. For example it has been found that warmer weather is favourable during the incubation period, fledging and nestling development.
Disease could be the cause of declines but no disease has been found presently.
Due to little movement between colonies house sparrow populations are becoming increasingly isolated therefore inbreeding is more likely, resulting in reduced variation which leads to lower fitness and population viability. Although it was observed in a controlled environment house sparrows selected against inbred females.
Overall there are endless possibilities as to why house sparrows declined but no particular cause has solved the mystery of the house sparrow. But hopefully through more research and our help the mystery can be solved and the house sparrow ceased decline can carry on and perhaps numbers can start to increase.

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Applied Animal Studies BSc (hons) degree graduate from the University of Northampton. RSPB volunteer at Fen Drayton lakes reserve. Passion for conservation and nature, main interests are British species in particular birds of prey.

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