The foraging ecology of non-breeding Wrybills (Anarhynchus frontalis) in the Firth of Thames : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Ecology at Massey University, Palmerston North, New Zealand
The Firth of Thames in the North Island of New Zealand is one of the most important wintering sites
for Wrybills (Anarhynchus frontalis), second only to the Manukau Harbour. Together these two
estuarine areas support approximately 85% of the entire Wrybill population between late summer
and early spring each year. While the breeding biology and ecology on their braided river breeding
grounds in the South Island have been well documented, the foraging ecology of Wrybills in their
non‐breeding habitats has not been deeply studied. Wrybills possess a uniquely shaped bill
considered to be an adaptation to their life on the South Island braided rivers during their breeding
months. However, despite this they use their bill very effectively on the tidal flats of their winter
habitats. In this thesis I studied the foraging ecology of Wrybills in the western Firth of Thames, with
a focus on the factors affecting their low‐tide feeding distribution, and how diet and intake rates
varied with foraging mode.
The distribution of foraging Wrybills was correlated with a number of environmental variables
(sediment type, sediment softness, water content, and polychaete abundance and biomass).
Foraging Wrybills showed a preference for areas of tidal flat close to shore with soft sediment and
high polychaete biomass.
Wrybills exhibited different foraging modes that were used in areas with different environmental
conditions. Birds fed (1) visually, walking slowly and obtaining most of their biomass intakes from
large polychaete worms, (2) by tactile means, capturing mainly small worms or (3) a combination of
the visual and tactile methods. Visual feeding tended to occur in drier, sandier sediments and tactile
in wetter, muddier areas close to shore. Despite proportionately different intakes of large and small
polychaete worms across the different foraging modes, the total biomass intake rates were similar.
In addition to polychaete captures, tactile foragers in particular frequently took mouthfuls of
sediment, an action which raised the possibility that they may be feeding on surficial biofilm. Stable
isotope analysis of Wrybill faeces, blood and feathers revealed some evidence of biofilm feeding in
Wrybills at the Firth of Thames.