Breeding biology of the fairy prion (Pachyptila turtur) at the Poor Knights Islands, New Zealand

Approximately 40 000 fairy prions breed on Aorangi, one of the larger islands in the Poor Knights group. The activities of the breeding adults and unemployed birds during the period from early egg incubation until the chicks’ departure are discussed. Fairy prions do not return to their breeding colonies until nightfall. Dry rock crevices are favoured nesting sites, and competition for them is keen. Nests are strongly defended against intraspecific intrusions, but confrontations with the Buller's shearwater (Puffinus bulleri) are avoided whenever possible. Eggs are laid in mid October, and are incubated by both adults, which lose weight at the rate of about 8 g/24 h during the average incubation ‘shift’ of 3 days and 2 nights. Chicks are brood.ed continuously for 1–5 days after hatching. The adults feed their young a variable mixture of crustaceans, principally the euphausiid Nyctiphanes australis and the neritic amphipod Parathemisto gracilipes. Large meals sometimes double the chicks’ body weight. The feeding rhythm of adults and chicks is critically examined, and the analysis points out serious inaccuracies in the night and morning weighing techniques often used by field workers. Feeding by both adults continues until the fledgling's last night at the nest. Fledglings leave the island at night during the first week in January weighing an average of 100.5±1.8 g; thirst may be a proximate factor in triggering their departure. Adult prions weigh 96–166 g. The average evening arrival weight is about 134 g and the average departure weight near 111 g, the loss being accounted for by an incubation shift or a chick feeding. Predation and mortality of prions at Aorangi are unimportant factors influencing population numbers. Banding recoveries suggest a life‐span of at least 15 years.