Renters are on the rise in America's biggest cities, but many tenants are scrambling to keep up with growing rent bills and shrinking vacancies, according to a study being released Thursday.

From Boston to Miami, New York to Los Angeles, more than half of tenants are paying what experts consider unaffordable rents, says a report by New York University's Furman Center, which studies real estate and urban policy, and bank Capital One, which is a leading affordable-housing lender and financed the research.

While various housing experts have noted such trends, the study zooms in on 11 of the nation's most populous cities. Overall, it's a portrait of increasing competition and often slipping affordability, but the picture isn't universally bleak and looks noticeably different from city to city.

"The study brings into light the limited options there are for renters," Capital One community finance chief Laura Bailey says.

A look at the findings:

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THE CITIES

The study analyzed U.S. Census Bureau data from 2006 to 2013 on the central cities of the 11 most populous U.S. metropolitan areas: Atlanta, Boston, Chicago, Dallas, Houston, Los Angeles, Miami, New York, Philadelphia, San Francisco and Washington, D.C.

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RENTERS ON THE RISE

As of 2013, most residents were renters in nine of the 11 cities, all except for Atlanta and Philadelphia, compared with five in 2006. At least 60 percent of residents are now tenants, rather than owners, in Boston, L.A., New York and Miami. Nationwide, about 35 percent of people rented in 2013, up from 31 percent in 2006, the Census Bureau says.

Experts trace much of the rise in renting to the 2008 mortgage and financial crisis, which left some people unable and others reluctant to own homes. And when rent becomes a stretch, leaving less income to save toward homeownership, "it's a reinforcing cycle," Furman Center faculty director Ingrid Gould Ellen says.

But other factors may include home-downsizing within the giant and aging baby boom generation and hefty college debt that slows some young people's saving for a home purchase.

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MORE RENTALS BUT LESS AVAILABILITY

In each city, the amount of rental housing grew faster than any rise in owner-occupied homes. In fact, the data suggest some homes were converted to rentals.

Nonetheless, the vacancy rate declined everywhere except Miami and Washington, where increases were slight. San Francisco surpassed New York for the title of tightest rental market: New York's 3.8 percent vacancy rate was the lowest in 2006, but by 2013 San Francisco had the floor with a mere 2.5 percent. New York, L.A. and Boston were hovering around 3.5 percent. Atlanta, meanwhile, had the highest vacancy rate of the cities in the survey, at nearly 10 percent.

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CLIMBING RENTS, UNEVEN BURDENS

Amid growing demand and tight supply, median rents rose faster than inflation in all the cities but Dallas and Houston, where they were nearly flat. Washington's median rent shot up by 21 percent over the seven years, to $1,307 a month. New York's rose by 12 percent, to $1,228. The calculation is in inflation-adjusted for 2013 dollars, includes utilities and encompasses market-rate, rent-regulated and subsidized housing.

New York has about 1 million rent-regulated apartments, perhaps helping explain why it has a lower median rent than Washington, San Francisco ($1,491) and Boston ($1,263). Meanwhile, median rents were under $1,000 everywhere else except Los Angeles ($1,182).

But rents don't tell the whole story of affordability: Renters' median household incomes varied widely over the years. Housing experts like to gauge affordability by the percentage of income that goes to housing costs, with anything over 29 percent being rent-burdened. Over 49 percent is considered severely burdened.

On that scale, the landscape is uneven. The percentage of rent-burdened tenants grew in six cities while dropping in the rest, and the findings were full of seeming contradictions. San Francisco had the highest median rent but the lowest percentage of rent-burdened tenants, 45 percent; Miami had a far lower median rent, but 68 percent of tenants were burdened.

One reason: San Francisco renters' median household income was $61,200 a year, nearly 1.5 times what their Miami counterparts made.

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TO SAVE ON RENT, STAY PUT

In each city, apartments that had come open within the last five years were less likely to be affordable to low- and middle-income tenants than apartments that hadn't.

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Reach Jennifer Peltz on Twitter @jennpeltz.