Breeding

The following summary is based on Beck and Brown (1972) on breeding on the South Orkneys, Jouventin et al. (1985) on breeding on the Crozets, Copestake and Croxall (1985) on breeding on South Georgia, and on Brooke (2004).

Wilson's Storm-Petrel comes ashore to breed in the darkest conditions possible, although at the southernmost breeding sites, in the Antarctic, darkness rarely is complete.

This storm-petrel breeds in holes and crevices in cliffs, and rock or scree slopes. The nest may be unlined, or lined with feathers, moss, or other soft objects.

Adults first return to colonies between 1-25 November, with most breeders back by the first week of December. Pre-laying exodus of females, but not males, averages 10.7 days (Beck and Brown 1972). Eggs are laid over the course of 3-6 weeks. Peak laying date varies with locality, trending later at northerly localities. This is reflected in mean dates of 13 December in Terre Adelie, Antarctica, but 25 January on Kerguelen (Copestake and Croxall 1985).

Little variation in egg size throughout range. Mean measurements of 19 eggs from South Georgia are 33.3 x 23.4 mm, and mean weight of 13 fresh eggs is 9.9 g (Copestake and Croxall 1985).

Incubation shifts average 1.8-2.5 day, varying with study (Marchant and Higgins 1990), as storm-petrels may temporarily abandon nests (Marchant and Higgins 1990). Duration of incubation averages 41-48.5 days, including temporary desertions (Copestake and Croxall 1985).

Chicks hatch late January to late March, varying with latitude. Chicks fed on 75% of days (usually at night) at South Georgia and on 93% of days in the South Shetlands. Average number of feeds per day is 1.1. and 1.2 respectively, and the average meal size is 6.5-7.5 and 8.5 g (Croxall et al. 1988, Quillfeldt and Peter 2000). Mean peak weight of chicks reaches 140-192% of adult mass when about 6-7 weeks old, and fledges at 100-145% of adult mass (Beck and Brown, Croxall et al. 1988). Fledging at 35 g on South Georgia and 55.3 g on South Orkneys.

Mean fledging age varies from 48 days in southern colonies to 78 in more northern ones, reflecting the difference of day length at the different latitudes (Marchant and Higgins 1990). In the south, chicks fledge in early March, but into May in the north. It is at these times when inland sightings occur (Marin 2002, Alvaro Jaramillo personal communication).

Breeding success lower than many other tubenoses, with hatching success 35-50% at different locations, and fledging success 28-52%. (Overall success in range 10-30%, Marchant and Higgins 1990). Success lower when sea ice cover is low in the previous winter (Quillfeldt 2001).