Nesting ecology of Magenta Petrel (Taiko) - It's a man's world

A recent study by Hayley Lawrence and colleagues published in the Journal of Avian Biology (on-line ahead of print) describes the male-female ratio of the endangered Magenta Petrel Pterodroma magentae (aka Chatham Island Taiko). The world population is estimated at around 150 individuals with only 8-15 pairs known to have nested in recent years (Imber et al. 2005). Considerable effort is going into protecting and expanding the one known nesting colony located in dense scrub on the main Chatham Island (Rekohu/Wharekauri, decimal degrees -44.0705°, -176.634°). The new study summarizes genetic analysis of blood samples taken from fledglings and adults returning to the colony. As expected the sex ratio of chicks is close to parity, as of course are the pairs occupying burrows but the researchers find a striking dearth of females when they sample visiting adults. The males dig the nesting burrows and then attract females to them. The paucity of visiting females may help to explain the number of unpaired males. For examples, one male was at least 16 years old when he first bred. Conversely, most females are new breeders in the first season they are detected at burrows. The article does not specifically address the whereabouts of the 'missing females'. Have they perished before reaching breeding age (probably 7 years or longer) or is the density of territorial males too low to attract them to the colony where they can be sampled by researchers? A new colony at Sweetwaters south of the main Chathams settlement of Waitangi, that is being established within a fenced enclosure (to keep out introduced predators) will include an increased density of burrows and a slightly higher number of male chicks. Work on related petrel species such as Gray-faced Petrel (Pterodroma macroptera gouldi) has shown that males exhibit a stronger philopatric tendancy and will thus provide a stronger draw to passing females.

Hayley A. Lawrence, Craig D. Millar, Graeme A. Taylor, Linda D. Macdonald and David M. Lambert (2008) Excess of unpaired males in one of the World's most endangered seabirds, the Chatham Island taiko Pterodroma magentae. Journal of Avian Biology. On-line ahead of print.

M. J. Imber, G. A. Taylor, A. J. D. Tennyson, H. A. Aikman R. P. Scofield, J. Ballantyne, D. E. Crockett (2005) Non-breeding behaviour of Magenta Petrels Pterodroma magentae at Chatham Island, New Zealand. Ibis 147(4):758–763.