The Volume of Stomach Oils Increases during Prefledging Weight Loss in Leach's Storm-Petrel (Oceanodroma leucorhoa) Chicks

Abstract

We have investigated the physiological mechanisms responsible for stomach oil accumulation unique to the Procellariiformes (Duke et al. 1989; Roby et al. 1986, 1989; Place et al. 1986, 1989; Jackson and Place 1990). An earlier study with Leach's Storm-Petrel (Oceanodroma leucorhoa) reported that chicks near fledging (55 days and older) accumulate greater volumes of stomach oil than younger chicks (Place et al. 1989). To better document the age of onset of oil accumulation, we measured the volume of stomach oil in 38 chicks of known hatching date throughout the 1989 breeding season on Little Duck Island, Maine (44?10'N, 68?15'W). The volume of stomach oil in 20 chicks was measured once at ages varying from 14 to 69 days. The volume in 15 chicks was measured at ages 25.3 days (SE = 1.5), 42.0 days (SE = 1.4), and 58.5 days (SE = 1.5). Three additional chicks fledged before a third sampling could be performed. We determined stomach oil volume by dilution of a tritium-labeled glycerol triether ([3H]-GTE), a nonabsorbable, nonmetabolizable lipid-phase marker (Place et al. 1989). Chicks were fed 0.5 cc of olive oil that contained 1.3 MCi/ml of 3H-GTE. We sampled the proventricular lipids, after periods of 1-3 h. Stomach oils were separated from aqueous and particulate phases by centrifugation at 10,000 x g for 10 min before samples were removed for scintillation counting and lipid analysis (Place et al. 1989). A minimum of 2 h was needed to ensure complete equilibration of the tracer (i.e. constant specific activity) with endogenous lipids. To validate the volume estimation technique, we prepared a set of artificial stomach oils by adding increasing quantities of olive oil (0-9 ml) to 1 ml of avian Ringer's in 15 ml conical centrifuge tubes. The oil-Ringer's mixture was vortexed to produce an unstable milky emulsion. Labeled olive oil (0.5 ml) was delivered to each tube by the same procedure as performed in the field. The artificial stomach oils were mixed by hand agitation and sampled 2 h later as described for authentic stomach oils. The volume of the lipid phase can be estimated by this technique with a high degree of precision (Fig. 1). Our study covered >90% of the nestling growth period in Leach's Storm-Petrel chicks (Fig. 2: A and B), and includes the last 15 days before fledging when chick mass decreases (Ricklefs et al. 1980a, 1980b). Stomach oils were found in all chicks sampled, and the volume of stomach oil was highly variable within any age group (Fig. 2C). A repeated-measure ANOVA showed no significant between-bird variation in the 15 chicks sampled repeatedly (F14, , = 731.4, P 0.728), but there was a highly significant (F2,44 = 11.94, P < 0.0002) age effect on the volume of stomach oil. When the data from all 38 chicks were grouped into 5-day age intervals, a one-factor ANOVA revealed a significant effect (F10,78 = 6.64, P < 0.0001) when contrasting the volume of oil in each group. In pairwise comparisons, there was a significant difference (P c 0.05) between the volume of oil in chicks of age groups 60-64 days and 65-69 days, and chicks of all other age groups. After 60 days of age, the mean volume of oil (6.3 ? 2.9, n 25) increased nearly threefold when compared with the mean volume of oil in the previous 54 days (2.3 ? 1.8, n = 52). Earlier stomach oil volume estimates for Leach's Storm-Petrel chicks (1.2 ? 0.9 ml, 39.4 ? 0.5 days, n = 12 [Place and Roby 1986]; 8.3 ? 1.6, n = 12, 65-70 day old chicks [Place et al. 1989]) are consistent with the present estimates. This accumulation of stomach oils occurs when chick body mass decreases, primarily from reductions in