Since the New 52 relaunch Nightwing has been coasting along pretty much on a par with its pre-reboot incarnation. I think, all in all, there's been a marvellous consistency to this title for the past 17 years. Some writers have been better than others, but Nightwing has always been a worthwhile, enjoyable read. I'm particularly impressed that this new volume of the series hasn't missed a beat, tonally speaking, despite the first volume actually ending in February 2009 in the aftermath of the Batman RIP saga.
Keep that in mind when we're looking at the stats: there's a 30-odd month gap between the blue issues and the red.
Like Legion of Super-Heroes, I think there's a perennial base audience for Nightwing, and in this case it's about 30,000 people. The two big sales bumps you can see in the volume 1 issues are tie-ins to The Resurrection of R'as al Ghul and Batman RIP.
Jump forward two and a half years, and Nightwing suffers a quite rapid decline in sales following its first two issues. In May 2012 it got a fresh sales bump thanks to a Night of the Owls tie-in issue. It's when you look at the sales figures that it becomes clear why DC are so fond of Bat-Universe crossovers - it's straight-up money in the bank every time.
Now if DC can somehow keep Nightwing's monthly sales hovering around the 45,000 copy mark, I think it will be a major achievement for them. That said, my gut tells me those sales are going to keep declining post-Owls, until we're back to a steady 30,000 readers again. And to be honest, that's no bad thing. We 30,000 readers really like Dick.