DIET AND POSTNATAL ENERGETICS IN CONVERGENT TAXA OF PLANKTON-FEEDING SEABIRDS DANIEL D. ROB¾• Departmentof Biology,University of Pennsylvania, Philadelphia,Pennsylvania 19104 USA ABSTRACr.--Toevaluate the influence of diet compositionon seabird postnataldevelop- ment, I examinedthe relationshipsbetween feeding rate,energy intake, energypartitioning, and chick growth in three speciesof high latitude, plankton-feedingseabirds: Least Auklet (Aethiapusilia), South Georgia Diving Petrel (Pelecanoidesgeorgicus), and Common Diving Petrel (P. urinatrix).Average dietary lipid contentvaried from 22 to 46%of dry massamong the three species.Least Auklet chickswere fed meals rich in wax esters;on average63% of total energy intake was in the form of wax. Interspecificcomparisons of chick dietary com- positionand energybudgets indicated that calanoldcopepods, a wax ester-richfood, wasa balancedsource of protein and lipids. However, the lipid contentof Antarctickrill (Euphausia superba)was insufficientto meet maintenanceenergy requirementsof CommonDiving Pe- trels. Lipid-rich diets were associatedwith shorterbrooding periods, higher ratesof nestling fat deposition,and larger lipid reservesat fledging. Accumulationrates of lipid-free dry matter were similar in the three speciesdespite differences in energy intake related to diet. The energy costof growth was a relatively minor componentof nestling energy budgets; most assimilatedenergy was allocatedtoward maintenanceand fat deposition.Selection apparentlyfavors high latitude seabirdsthat meet the high energyrequirements of their chicksby providing a lipid-rich diet. Differencesin diet compositionexplained much of the variation in postnataldevelopment among the three species,but there was no evidencethat energy limited growth per se. Received11 December1989, accepted 30 July1990. PELAGICseabirds share a suite of life history posit larger fat reserves,and fledge before they traits that includes single chick broods, slow attain adult mass. Ricklefs and White (1981) growth, and long development periods (Lack showedthat growth in the oceanicSooty Tern 1968). These reproductive traits have been at- (Sternafuscata) is slower than growth in the tributed to limitations in providing food to the nearshore-feeding Common Tern (S. hirundo), chick. Lack (1968) inferred that the reproduc- and that daily energy requirements of Sooty tive pattern in seabirdsserves to reduceenergy Tern chickspeak at a lower level than those of requirements at the nest to a level that a breed- the Common Tern. Also, the lipid reservesof ing pair can consistentlymeet. Researchersas- Sooty Tern chickswere approximately 4 times sumedthat the availabilityof energyto a breed- greater than those of Common Terns, which is ing pair is constrainedby both the scarcityand consistentwith longer fasting periods for the the unpredictability of potential prey at sea young of the offshorespecies. These results sug- (Ashmole 1963, 1971; Harris 1977; Nelson 1977; gest that the slower growth and larger fat de- Case 1978). Energy limitation is viewed as a pots of SootyTern chicksare related to low and pervasiveforce selectingfor observedpatterns variable rates of energy provisioning. in seabird reproduction (Drent and Daan 1980). Ricklefs and White (1981) estimated that dou- Much of the variation in post-hatchingde- bling the growth rate (accumulationof lipid- velopmental patterns in the seabirdfamily A1- free dry matter) of Sooty Tern chicks would cidaeis apparentlya consequenceof differences result in only a 20% increase in total energy in feeding ecology (Sealy 1973). Speciesthat requirementsof the chick and only a 5% in- forage farther offshore and feed their young creasein adult energy requirements. They con- less frequently have slower growth rates, de- cluded that energy could not limit growth rate in this species.Similarly, in a study of the en- ergeticsof slow growth in Leach'sStorm-Petrel t Present address:Cooperative Wildlife Research (Oceanodromaleucorhoa), Ricklefs et al. (1980a) Laboratoryand Departmentof Zoology,Southern estimated that the accumulation of lipid-free Illinois University, Carbondale,Illinois 62901 USA. dry matter (LFDM) amounted to <5% of the 131 The Auk 108: 131-146.January 1991 132 DANIEL D. ROI•Y [Auk, Vol. 108 total daily energy requirement. The energy I gathered data on chick meal sizes, feeding equivalent of daily lipid deposition was 3-8 frequency,and diet compositionfor three spe- times that of LFDM, indicating that variation ciesof high latitude,plankton-feeding seabirds. in growth rate would be expectedto have little The three speciesare from two convergenttaxa'. affecton total energy requirementsof the nest- auklets (Charadriiformes:Alcidae) and diving ling. The regularityin feedingof Leach'sStorm- petrels(Procellariiformes: Pelecanoididae). All Petrel chicks implies that the large lipid re- three are plankton-feedersthat captureprey by serves of nestlings are not an adaptation to pursuit-diving (sensuAshmole 1971), yet taxo- survive fasts (Ricklefs et al. 1985), but instead nomic compositionof the diet differs consid- they may representan "energysink" for chicks erably (B•dard 1969, Payne and Prince 1979). I fed lipid-rich but protein-poordiets (Ricklefs describethe energy requirementsof chicksand 1979). These findings contradict the hypothesis how thoserequirements are partitioned among that slow growth in offshore-feedingseabirds maintenance, growth, and lipid deposition is an adaptationfor reducingenergy require- functions throughout the nestling period. I ments at the nest and imply that other factors comparedthe compositionof chick diets with are responsiblefor long developmentperiods. the patterns of energy allocation among the Ricklefs (1979, 1983) has made a strong casefor three speciesto test the hypothesisthat differ- constraintsat the tissue level as the primary encesin postnataldevelopment relate to diet. factorlimiting growth in semiprecocialseabird chicks. METHODS AND MATERIALS Studiesof the relationship of diet to seabird developmenthave emphasizedthe quantityof I studiedLeast Auklets on St. GeorgeIsland (56ø35'N, food fed to chicks.Little attention hasbeen paid 169ø35'W) in the Pribilof Islands, Alaska, from June to variation in the quality of chick diets,yet the to August, 1981 and 1982. Nests were located and biochemicalcomposition of seabirdprey varies marked in a large (ca. 129,000breeding individuals; considerably(Clarke and Prince 1980, Ricketts Hickey and Craighead1977) colony on the slopesof and Prince 1981,Prince 1985).Analyses of high- Ulakaia Hill. The study area was describedin detail latitude zooplanktonindicate large variation in by Roby and Brink (1986a,b). Work on diving petrels was conducted on Bird Island (54ø00'S, 38ø02'W), lo- lipid content (Lee et al. 1972, Bensonand Lee cated at the western end of South Georgia, during 1975). Lipids constituteup to 77% of dry mass Januaryand February 1982.Locations of study colo- in somezooplankters (Lee 1974),which implies nies are describedin Roby and Ricklefs (1983). Nest that certain zooplanktondiets may be energy- burrowsand nestinghabitats of the two diving petrel rich but protein-deficient. Considering the speciesare describedby Payneand Prince (1979). variability in composition of high-latitude Nestswere checkeddaily during the hatching pe- zooplankton,prey selectionby adult plank- riod to determine the date of hatching, and known- ton-feedingseabirds may have important con- age chickswere periodicallyweighed and wing chord sequencesfor chick energy intake and the measuredduring the nestling period. A series of partitioning of assimilatedenergy among com- known-age chicks, in approximately equally spaced age classesfrom hatching to fledging, were removed peting functionsof maintenance,growth, and from their nestsaround midday. The actual date of fat deposition. hatching was known for only a few Common Diving If seabird growth is largely energy limited, Petrel chicks,so age was estimatedearly in the nest- and interspecificdietary differencesresult in ling period from the relationship of wing length to significantvariation in diet quality, then much age(Payne and Prince1979). Chicks were transported of the variation in seabird postnatal develop- to field laboratories where metabolic trials were con- ment would be a function of diet composition. ducted. Oxygen consumption was measured volu- Specifically,seabird chicks with relativelyhigh metricallyusing a temperature-controlled,closed sys- lipid diets would grow faster and fledge at a tem (Ricklefsand Roby 1983).Chicks were weighed younger age. Conversely, if chick growth is at the beginning and end of each respirometrytrial on an Ohaus triple beam balance(+0.1 g). Oxygen constrainedby factorsthat operateat the tissue consumptionrates were convertedto cm3.g-• .h • by level, differences in diet composition should dividing by the massof the subjectat the beginning bear little on development. High-lipid diets of the trial. would be reflected in higher rates of lipid de- After respirometrytrials, subjectswere humanely position by chicks,but not higher growth rates sacrificedby thoraciccompression, placed in plastic and shorter nestling periods. bags,and frozen at -20øC for later determination of January1991] SeabirdDiet and Chick Energetics 133 body composition.After partial thawing, eachcarcass sumed diving petrel chicks maintained SMR for 4 h was plucked, skinned, and dissectedinto 15 body each day when adults were normally in attendance parts. Feathers, skin, and body parts