Of interest to North Atlantic Pterodromaphiles is this paper: Zino et al. (2008) attached. They have this to say on the field identification of this pair:
"The separation of Zino's and Fea's Petrels in flight has been much debated amongst experienced ornithologists and birdwatchers (Fisher 1989, Gantlett 1995, Tove 1997, 2001, Steele 2006). From the biometrics analysed, the bill is by far the easiest form of differentiating the two species, but for an observer in the field this is difficult. Using digital photography and image analysis, Tove (2001) demonstrated that there are differences in the shape of the wings of these species. The relative primary lengths measured (Appendix 1) confirm this, the tip of the wing of P. madeira being rounder, but such technology is available to few people. Wing feather pattern in the sample studied showed a large individual variation with very similar patterns in both species (Fig. 6). Steele (2006) discusses the problem at length and is in agreement that it is still nearly impossible to differentiate between P. madeira and P. feae in flight."