Foraging niche overlap during chick-rearing in the sexually dimorphic Westland petrel

Most Procellariform seabirds are pelagic, breed in summer when prey availability peaks, and migrate for winter. They also display a dual foraging strategy (short and long trips) and sex-specific foraging. The Westland petrel Procellaria westlandica, a New Zealand endemic, is one of the rare seabirds breeding in winter. Preliminary findings on this large and sexually-dimorphic petrel suggest a foraging with no evidence of a dual strategy, within a narrow range and with shared areas between sexes. To investigate further this unusual strategy, the present study determined the fine-scale at-sea behaviours (GPS and accelerometer data loggers) and trophic niches (stable isotopes in whole blood) of chick-rearing individuals (16 males, 13 females). All individuals foraged on the shelf slope of the west coast of New Zealand’s South Island with short, unimodal trips. Both sexes foraged at similar intensity without temporal, spatial or isotopic niche segregation. These findings suggest the presence of a winter prey resource close to the colony, sufficient to satisfy the nutritional needs of breeding without intra-specific competition avoidance or increased foraging effort. Additional data are needed to assess the consistency of foraging niche between the sexes and its reproductive outcomes in view of anticipated environmental changes.
Methods
Individuals were caught in accessible burrow nests (one individual per nest), weighed and equipped with data loggers. Then, the occupancy of these nests was checked until the return of the equipped individuals. At recapture, the data loggers were retrieved, and the individual weighed again, morphometrics and blood sample taken (whole blood stored in 70% ethanol, for stable isotopes analysis and genetic sexing).