This is probably the hot-season breeding form: larger, more deeply forked tail, narrower, larger and more pointed wings, larger bill.
Band-rumped [Madeiran] Storm Petrel comprises five or more species worldwide including cryptic sympatric species that breed at different seasons
(Smith et al. 2007; Friesen et al. 2007; Monteiro et al. 2008).
Grant's Storm Petrel (O. sp. nov.), a cool water form, is one of these (Robb & Mullarney 2008; Howell et al. 2010; Howell 2012) www.worldbirdnames.org