The roles of morphology, individuality and arrival from migration in the foraging ecology of bar-tailed godwits at the Manawatū River estuary : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology at Massey University, Manawatū, New Zealand
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Date
2018
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Massey University
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Abstract
Bar-tailed Godwits (Limosa lapponica baueri) are a hugely size-variable shorebird exhibiting reverse
sexual dimorphism as well as substantial variation within the sexes. This variation is especially
pronounced in bill lengths, and differences in bill size could result in dietary differences between
individuals. I studied the foraging ecology of individually-marked godwits at the Manawatū River
Estuary in New Zealand, a small site amenable to making repeated observations of birds by
videography. Specifically, I investigated the effects of bill size variation on intake rates and diet
composition of godwits and tested for individual specialisation. Birds were found to use prey-specific
foraging modes, the effect of which far outstripped that of any morphological or environmental factor.
I found that shorter-billed birds (males) had slightly higher and less variable intake rates than their
longer-billed (female) counterparts. The shorter-billed birds focused their efforts on catching small
surface prey such as Potamopyrgus, to the extent where around half of the males specialised on these
small snails. In comparison, longer-billed birds ate a more diverse array of prey items, notably
consuming more worms which were buried deeply in the sediment. In addition to some birds
specialising on certain prey, other prey were taken preferentially by only certain individuals across the
size spectrum. This suggests that the diets of godwits are not only influenced by morphology but also
by individual preferences of the birds themselves.
I also tested for a carry-over effect of prolonged flight on foraging performance of godwits. The 8–10-
day post-breeding migratory flight of godwits direct from Alaska to New Zealand is the longest known
endurance flight of any land-bird. This flight has many expected physiological impacts, including a
reduction in digestive tract mass that could lead to lower functionality of the digestive tract and
therefore limit intake rates after arrival. By monitoring the daily occurrence of marked birds I
determined when birds arrived on migration, and tested whether intake rates and diet choice changed
in the fortnight after birds arrived. I found that intake rates were slightly higher and less variable with
time since arrival. Furthermore, there was an increase in the consumption of the hard-shelled mud
snail Amphibola crenata with time, suggesting a recovery of gizzard mass. Given the scale of the
migration, however, these effects were surprisingly small and imply that diet choice and energy intake
are affected only slightly by a physiological carry-over effect from migration.
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Keywords
Bar-tailed godwit, New Zealand, Morphology, Behavior, Food, Estuarine ecology, Manawatu River