Diet and postnatal energetics in convergent taxa of plankton-feeding seabirds

-TO evaluate the influence of diet composition on seabird postnatal development, I examined the relationships between feeding rate, energy intake, energy partitioning, and chick growth in three species of high latitude, plankton-feeding seabirds: Least Auklet (Aethia pusilla), South Georgia Diving Petrel (Pelecanoides georgicus), and Common Diving Petrel (P. urinatrix). Average dietary lipid content varied from 22 to 46% of dry mass among the three species. Least Auklet chicks were fed meals rich in wax esters; on average 63% of total energy intake was in the form of wax. Interspecific comparisons of chick dietary composition and energy budgets indicated that calanoid copepods, a wax ester-rich food, was a balanced source of protein and lipids. However, the lipid content of Antarctic krill (Euphausia superba) was insufficient to meet maintenance energy requirements of Common Diving Petrels. Lipid-rich diets were associated with shorter brooding periods, higher rates of nestling fat deposition, and larger lipid reserves at fledging. Accumulation rates of lipid-free dry matter were similar in the three species despite differences in energy intake related to diet. The energy cost of growth was a relatively minor component of nestling energy budgets; most assimilated energy was allocated toward maintenance and fat deposition. Selection apparently favors high latitude seabirds that meet the high energy requirements of their chicks by providing a lipid-rich diet. Differences in diet composition explained much of the variation in postnatal development among the three species, but there was no evidence that energy limited growth per se. Received 11 December 1989, accepted 30 July 1990. PELAGIC seabirds share a suite of life history traits that includes single chick broods, slow growth, and long development periods (Lack 1968). These reproductive traits have been attributed to limitations in providing food to the chick. Lack (1968) inferred that the reproductive pattern in seabirds serves to reduce energy requirements at the nest to a level that a breeding pair can consistently meet. Researchers assumed that the availability of energy to a breeding pair is constrained by both the scarcity and the unpredictability of potential prey at sea (Ashmole 1963, 1971; Harris 1977; Nelson 1977; Case 1978). Energy limitation is viewed as a pervasive force selecting for observed patterns in seabird reproduction (Drent and Daan 1980). Much of the variation in post-hatching developmental patterns in the seabird family Alcidae is apparently a consequence of differences in feeding ecology (Sealy 1973). Species that forage farther offshore and feed their young less frequently have slower growth rates, de' Present address: Cooperative Wildlife Research Laboratory and Department of Zoology, Southern Illinois University, Carbondale, Illinois 62901 USA. posit larger fat reserves, and fledge before they attain adult mass. Ricklefs and White (1981) showed that growth in the oceanic Sooty Tern (Sterna fuscata) is slower than growth in the nearshore-feeding Common Tern (S. hirundo), and that daily energy requirements of Sooty Tern chicks peak at a lower level than those of the Common Tern. Also, the lipid reserves of Sooty Tern chicks were approximately 4 times greater than those of Common Terns, which is consistent with longer fasting periods for the young of the offshore species. These results suggest that the slower growth and larger fat depots of Sooty Tern chicks are related to low and variable rates of energy provisioning. Ricklefs and White (1981) estimated that doubling the growth rate (accumulation of lipidfree dry matter) of Sooty Tern chicks would result in only a 20% increase in total energy requirements of the chick and only a 5% increase in adult energy requirements. They concluded that energy could not limit growth rate in this species. Similarly, in a study of the energetics of slow growth in Leach's Storm-Petrel (Oceanodroma leucorhoa), Ricklefs et al. (1980a) estimated that the accumulation of lipid-free dry matter (LFDM) amounted to <5% of the 131 The Auk 108: 131-146. January 1991 This content downloaded from 157.55. 39.234 on Sat, 16 Jul 2016 06:10:47 UTC All use subject to http://about.jstor. org/terms 132 DANIEL D. ROBY [Auk, Vol. 108 total daily energy requirement. The energy equivalent of daily lipid deposition was 3-8 times that of LFDM, indicating that variation in growth rate would be expected to have little affect on total energy requirements of the nestling. The regularity in feeding of Leach's StormPetrel chicks implies that the large lipid reserves of nestlings are not an adaptation to survive fasts (Ricklefs et al. 1985), but instead they may represent an "energy sink" for chicks fed lipid-rich but protein-poor diets (Ricklefs 1979). These findings contradict the hypothesis that slow growth in offshore-feeding seabirds is an adaptation for reducing energy requirements at the nest and imply that other factors are responsible for long development periods. Ricklefs (1979, 1983) has made a strong case for constraints at the tissue level as the primary factor limiting growth in semiprecocial seabird