New York, July 17 (IANS): Birds that live in suburban areas exhibit significantly higher levels of territorial aggression than those in the countryside, says a study that shed light on the effects of human population expansion on wildlife.

"A possible reason for this is that these birds have less space but better resources to defend," said Scott Davies, a post-doctoral associate at the Virginia Tech University in the US.

The suburban birds were also found to be more territorial, showing that their increased aggression persists throughout a breeding season.

"Living near humans provides better food and shelter, but it also means more competition for these limited resources," Davies added in the work published in the journal Biology Letters.

For the study, the team measured territorial aggression in 35 urban and 38 rural male song sparrows at three rural and three urban sites in the New River Valley during the spring of 2015.

The researchers played a recording of a male song sparrow and observed how the territory-holding birds responded to a simulated intrusion from a neighbour.

These birds showed a higher level of aggression -- they approached and remained near the speaker, flapped their wings furiously, engaged in loud singing and then began to produce 'soft song' -- a term that researchers use to describe the quiet, garbled noise that a bird makes, which is predictive of an impending attack.

Though rural birds still responded to a song intrusion but not as vigorously as they usually do.

The world population is projected to reach 9.6 billion by 2050 and increasing by more than two billion people, according to the United Nations.

Though many animals avoid habitats that are impacted by humans, some species can adjust and live in suburban and even urban habitats.

"Suburban sprawl is a primary form of human habitat change and though many species can survive in our backyards, their behaviour and physiology may change to cope with shifts in resources and with new disturbances," said Kendra Sewall, Assistant Professor at Virginia Tech University.

However, “we need to understand the widespread behavioural differences between various species of urban and rural bird populations to get an idea of how urbanisation will affect their survival and diversity in the future,” said Kiki Sanford, an American science communicator, who was not involved in the study.