Feeding frequency, meal size and chick growth in the threatened Pycroft's petrel (Pterodroma pycrofti)

Abstract

Gadfly petrels are small to medium size seabirds found mainly in the Pacific Ocean. Many species are threatened or endangered, including four New Zealand species: Chatham petrel (Pterodroma axillaris), Pycroft's petrel (P. pycrojti), Chatham Island Taiko (P. magentae), and Cook's petrel (P. cookii). Conservation actions for these species include the establishment of new breeding colonies on predator-free offshore islands by translocation. Due to the high philopatry of most gadfly petrels, only chicks that have not yet been imprinted with their natal ground (i.e. chicks that have not been outside their burrow yet) can be transferred. Translocation of Chatham petrel and Pycroft's petrel chicks are scheduled in 2002. However, data on the chick stage in these two species, and in small Pterodroma in general, are scant, and techniques to age chicks to determine their transferability are needed. The purpose of this thesis was to examine the feeding frequency, meal size and growth of Pycroft's petrel nestlings and to identify factors that could be used to age a chick of unknown age prior to a translocation.

Fifty Pycroft's petrel chicks were monitored between 17 January and 26 March 2001 on Red Mercury Island, New Zealand. Chicks were weighed everyone or two days to determine their feeding frequency and meal size, and had their bill, tarsus, tail and wing length measured at regular interval until they fledged or until 26 March. Thirty other chicks formed a control group and were weighed and measured on 29 January and 15 March. The age chicks first emerged from their burrow prior to fledging was also determined. Similar data on Chatham petrel were also analysed.

Measurements of the Pycroft's petrel nestlings at fledging were similar to mean adult measurements: mean fledging weight (162 ± 2.6g) was 99% of adult weight, mean wing length (214 ± 1mm) was 98% of adult wing length, mean bill length (24.9 ± 0.2mm) was 102% of mean adult bill length, mean tarsus length (29.2 ± 0.1mm) was the same than mean adult tarsus, and mean tail length (92 ± 1mm) was 94% of mean tail length of adults. Wing and tail length were very well correlated with age expressed in days before fledging (DBF) (r² = 0.95 and r² = 0.86 respectively). Weight was not so closely correlated with age (r² = 0.63), and bill and tarsus length were not correlated to age (r² < 0.2). Meal size was between 35 and 40 g, and the probability of being fed ranged from 0.47 between 23 and 16 DBF, to 0.04 between 7 and 0 DBF.

These results are discussed in the light of other studies on gadfly petrels and recommendations for the selection of transferable chicks and for the feeding of transferred chicks are given.