Stomach oil and reproductive energetics in Antarctic prions,Pachyptila desolata

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Abstract
We studied energy-provisioning rates and energy requirements of nestlings, as well as energy-expenditure rates of adults in Antarctic prions (Pachyptila desolata) on sub-Antarctic Bird Island, South Georgia. Special emphasis was placed on investigating the role of stomach oil as an energy source for nestlings. Nestlings were periodically weighed overnight (n = 91 nestling-nights, 5 nights), and the mass of food consumed by nestlings was estimated from the sum of positive mass increments recorded over 3-h intervals. Prion nestlings consumed, on average, 36.6 g food per day at the age of peak body mass. The probability of each parent feeding its nestling overnight was 0.57, but feeding frequency varied significantly among nights. The median volume of stomach oil in proventriculi of nestlings was 0.9 mL (n = 44), and was negatively correlated with nestling age. Prion nestlings had a relatively high conversion efficiency of food mass to body mass (0.615), indicating a high energy density of nestling meals. Nestling energy budgets revealed that nestling energy requirements could not be met unless a portion of the food mass delivered by the parents consisted of stomach oil. We conclude that Antarctic prions represent an intermediate position in the spectrum of procellariiform dependency on stomach oil during nestling rearing, with diving petrels (which do not produce stomach oil) at one extreme and storm-petrels at the other. Field metabolic rates of adults feeding nestlings, measured by means of the doubly labeled water technique, averaged 4.16 mL CO2/(g∙h), or 391 kJ/day. We suggest that this relatively low metabolic rate while foraging at sea is important for stomach oil formation, as it permits adults to allocate a greater proportion of energy from ingested food to their young