Foraging niche overlap during chick-rearing in the sexually dimorphic Westland petrel
Résumé
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 behaviour 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 (global positioning system and accelerometer data
loggers) and trophic niches (stable isotopes in whole blood) of
chick-rearing individuals (16 males and 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 increasing the
foraging effort or intra-specific competition avoidance during
winter. Additional data are needed to assess the consistency of
foraging niche between the sexes and its reproductive
outcomes in view of anticipated environmental changes.