[chris (OP)]

Is it true that dogs howl collectively and, if so, why?

Chris

[MonikaS]

If they belong to the same pack/family, one will start and the others join in, similar to a wolf pack. It translates to 'we belong together and this is our territory, stay away'.
Other dogs will reply, thus marking their own territory.

You can hear it sometimes in rural areas, where the watch dogs on farms will communicate with howls and barks. In between you'll hear 'Shut up, bl**dy dog!'.

[Ethos]

Is it true that dogs howl collectively and, if so, why?

Chris

To stay in rhythm ofcourse,........Hardee,,,har,har.

[RD]

They compete with train horns too... http://www.freesound.org/samplesViewSingle.php?id=69584

(click on the play triangle on the top left of the waveform)