House sparrow populations have declined sharply by an estimated 71% since 1977
Those birds who live in town and cities have a poorer diet because they rely on what humans eat and this tends to be junk.
Their bodies also have weaker defences, unable to fight off harmful disease causing free radicals triggered by pollution.
And the findings could have health implications for humans too.
The noisy and gregarious birds are the ultimate avian opportunists exploiting man's rubbish and wastefulness.
If our cities are unhealthy for birds, then as their neighbours we should be concerned because we are exposed to the same environmental stressors as house sparrows
But worryingly, there has been a severe decline in the UK house sparrow population, with an estimated 71 per cent drop between 1977 and 2008.
Now Spanish scientists found urban house sparrows are particularly stressed during the breeding season, which could have a huge impact on their numbers.
Amparo Herrera-Dueñas at the Complutense University of Madrid, said: "We find that house sparrows living in the city are suffering from more stress than those living in the countryside, and we link this to differences in air quality and diet.
"It is particularly bad for urban birds during the breeding season when they are torn between allocating resources towards fighting the toxic effects of pollution or towards laying healthy eggs, both of which aren't helped by their poor diet.
Urban pollution and unhealthy diets are to blame for the recent decline in bird numbers
"If our cities are unhealthy for birds, which is what our study is suggesting, then as their neighbours we should be concerned because we are exposed to the same environmental stressors as house sparrows."
The study took blood samples from hundreds of the birds from rural, suburban and urban areas around the Iberian Peninsula.
It analysed the blood for signs of oxidative stress, which can be used to measure how much an environmental stressor, such as pollution, is weakening the bird's natural defenses.
Ms Herrera-Dueñas added: "Air pollutants or an unhealthy diet can promote the formation of free radicals.
Birds living in urban areas suffer more stress than those living in the countryside
"These molecules are the by-product of a normal functioning body, so our cells have developed a mechanism to counteract them.
"However, under demanding conditions, the production of free radicals can overwhelm these antioxidant defences, causing oxidative stress.
"When this happens, free-radicals can accelerate the ageing of cells.
"In humans this has been linked to respiratory diseases, such as asthma, as well as cardiovascular disorders and cancer."
The study found urban sparrows suffered higher levels of free-radical damage and were trying to fight off these damaging molecules, but compared to their rural counterparts, their natural defences had a lower capacity to do so.
She concluded: "We need to work hard to improve the quality of the urban environment, for example, air quality and the design of green areas.
"Even the leftovers that we throw in the bin at the park should encourage us to reflect on ourselves: more nuts and fruit and fewer chips and cookies would be better for humans as well as for birds.
"During this project, I observed that the breeding season is particularly challenging for adult house sparrows but unfortunately, I do not have information on how their offspring are coping.
"I hope to investigate how the stresses of urban life at an early age influence the condition of these birds as they reach adulthood."
A study was published in the journal Frontiers in Ecology and Evolution.