Multifamily development is hot as rent demand continues to allow prices to go up.

You've probably heard the one about the new national bird? The crane.

After six years of rising apartment rents in U.S. cities, investors from all corners of the real-estate industry are piling into new projects in a bet the boom still has a long way to run.

What's more, global economic uncertainty, if anything, will further juice the liquidity flow into capital driving for more multifamily development. Wall Street Journal staffer Laura Kusisto takes on the story, looks at the pockets of concern, the bubbles of concentrated development on the luxury end of the spectrum and the windows and niches of opportunity and risk.

Kusisto writes:

Some signs of strain are beginning to emerge. The vacancy rate for downtown apartment buildings in U.S. cities jumped to 6.1% in the third quarter of 2015 from 4.4% in the first quarter of 2013, according to Reis. During the same period, the suburban vacancy rate fell to 4.1% from 4.4%—suggesting some renters are leaving high-price inner-city areas and moving farther out.

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