The superb resource that is NZ Birds Online

Red-footed booby (Photo by Judi Lapsley Miller). Head of subadult (foreground) and adult (background) - both intermediate morph. Darwin Bay Beach, Genovesa, Galapagos Islands.

My go-to site for bird information is no longer the definitive field guide by Heather & Robertson, but instead NZ Birds Online. Essentially a wikipedia for New Zealand's Birds, it gives authoritative information curated by Te Papa's ornithologist, Colin Miskelly, supported by a wealth of crowd-sourced photographs from keen birders and wildlife photographers (like myself!). One of its strengths is the ability to quickly add and modify entries as new information comes to hand, such as the recent sightings of new vagrant birds to NZ like the red-footed booby.