The ubiquitous Eurasian tree sparrow (Passer montanus) is a globally distributed perching bird. It's original range was in Eurasia but was introduced to Southeast Asia where it is a commensal with humans. It has modified its behaviour and life history to exploit human habitations. Where humans build their houses, the birds thrive.
But that was when people had some gardens with their houses. The bird has been declining in numbers in much of Europe . In England, populations have crashed by 90% prompting calls to put the birds on the national endangered species list.
In Europe the birds are largely rural rather than urban. Changes in farming practices are regarded as the main reason why populations have crashed. The birds need hay and winter stubble to make it through northern Europe's cold winters. Farmers left stubble when they sow in Spring. Since many farmers have shifted to autumn sown crops (partly due to warming winters), the birds lost a food resource. Another factor is agricultural specialization and the overuse of pesticides. While the birds are largely seed eaters, they shift to insect prey when they breed.
In the Philippines where the birds are primarily urban, we have noticed a declining population especially in Metro Manila for the last 5 years. We estimate the decline at 30-40%. It could be that the declining area of greenspaces and gardens in the city are responsible for the decline. Metro Manila is developing skywards (with skyscrapers) and this may affect the birds. Birdwatchers say that it was Makati City (most urbanized in the Metro) that first showed fewer and fewer sparrows.
However another hypothesis has been proposed. The birds are declining since they no longer can stand the urban heat! As the climate becomes warmer, the birds are stressed, can't nest and thus their populations decline.
Nonetheless whatever the reason for their decline, the tree sparrow is becoming the proverbial canary in the coal mine for urbanites. The decline of the birds should be a warning.
The birds figure so much in the popular imagination and culture. The birds have been the metaphor for the poor and downtrodden from the time of Jesus Christ to today.
Simon and Garfunkel's poignant song "Sparrow" may represent the bird's plight
"Who will love a little Sparrow?
Who's traveled far and cries for rest?"
"Who will love a little Sparrow?
Will no one write her eulogy?
"I will," said the Earth,
"For all I've created returns unto me,
From dust were ye made and dust ye shall be"
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