Philadelphia's population grew for the eighth year in a row in 2014, and surrounding counties were mostly stable, according to new census data, but the picture was not quite as rosy at it may have seemed in the nation's fifth-largest city.

Philadelphia's population rose an estimated 4,245 to 1,560,297, the census found, aiding a trend that has helped reverse decades of declines.

By the end of 2014, there were 34,291 more people in Philadelphia than when census takers knocked on doors in 2010.

The suburban population rose even more sharply in Montgomery and Chester Counties, while falling in Burlington and Camden Counties and staying relatively stable in Gloucester, Bucks, and Delaware Counties.

While Philadelphia's increase was undeniably good news, the uptick was fueled by an increase in births, rather than an influx of residents, which raised demographers' concerns.

More people left Philadelphia for elsewhere in the country than moved in last year. Had it not been for a high number of births, and an influx of immigrants, the city's population would have fallen, the data showed.

And given people's tendency to flee the city once their children reach school age, Thursday's numbers were all the more sobering, especially in light of ongoing turmoil surrounding the chronically underfunded School District.

"We still have more people leaving the city," said Temple University demographer David Elesh. "It suggests that as people have children, they may be likely to consider a suburban home instead of a city home."

The data underscore the need to repair Philadelphia's public schools. Failure to do so could counteract what has become, in recent years, a younger population.

Told of the numbers, Mayor Nutter said: "I'm just really proud that we've been able to maintain the growth. At the same time, we're a big city and we have our challenges. We just have to stay focused on education and jobs.

"The major challenge," he added, "is our schools."

Philadelphia's one-year tally would have looked better had census analysts not revised their previous-year population estimate.

After taking a second look at birth, death, and tax records from 2013, the census found that the city actually had 2,887 more residents than it had previously reported.

The picture was a bit more straightforward in high-performing Chester and Montgomery Counties, which are among the most affluent in the nation. Population increases in those two counties were not fueled by childbirth, leading Elesh to conclude that an abundance of nearby jobs was likely the cause.

Both counties are near or along the Route 202 corridor, a powerhouse of corporate offices and other white-collar employers along Philadelphia's exurbs.

Jobs played a small role in the story in Philadelphia, but not a negligible one.

Elesh said the city had experienced "a bit" of job growth in recent years. By one count, the number of jobs grew 8,727 from 2011 to 2012, he said. More current data were not available.

"Stability is really what's going on" in the remaining counties, Elesh said.

The population figures for those counties:

Chester County: Up 3,284, for a total of 512,784.

Montgomery County: Up 3,025, to 816,857.

Delaware County: Up 1,116, to 562,960.

Bucks County: Up 229 to 626,685.

Gloucester County: Up 1,011 to 290,951.

Two South Jersey counties continued what appears to be a fairly recent trend of declines: Burlington lost 419 residents, bringing its total to 449,722, while Camden lost 1,087, logging a total of 511,038.

Elesh said it was impossible from the data at hand to draw conclusions about what was causing a few years' worth of drops there.

In Philadelphia, he said, the robust number of births pointed to something heartening. Philadelphia is becoming a younger city. The birth data, he said, may "indicate a better future."

But if those young families ultimately go elsewhere for better schools, it would not bode well for efforts to build sustainable population gains.

For all the talk of empty nesters, young professionals and others moving into the city in recent years, Elesh warned, too many Philadelphians continue to leave.