Wing Ecomorphology of Seabirds from Johnston

The Condor 101:549-556 ? The Cooper Ornithological Society 1999

WING ECOMORPHOLOGYOF SEABIRDS FROM JOHNSTONATOLL'

FRITzHERTEL Departmentof Biology, Universityof California,Los Angeles, CA 90095-1606, e-mail:[email protected] LISA T. BALLANCE Ecology Program,NOAA, NMFS, SouthwestFisheries Science Center,8604 La Jolla Shores Drive, La Jolla, CA 92037

Abstract. Wing morphologyof nine species of seabirdsfrom JohnstonAtoll in the central tropical Pacific was analyzed to determinehow wing size and shape correlatedwith observed foraging behavior and, in some species, the energetic cost of flight. Red-tailed Tropicbirds(Phaethon rubricauda)and ChristmasShearwaters (Puffinus nativitatis) had lower wing areas, shorter wing spans, and higher relative wing loading than would be predictedfrom mass alone. Brown Noddies (Anousstolidus) and Red-footedBoobies (Sula sula) had lower wing loading, Brown Boobies (Sula leucogaster)and Sooty Terns(Sterna fuscata) had higher aspect ratios,and Brown Noddies had lower aspect ratio than would be predictedfrom mass alone. Aspect ratio showed greaterintraspecific variation than the other variables. In most cases, predicteddifferences in wing morphologycorrelated well with observedforaging differences among species, and species that did not differ significantlyin body mass differed with respect to wing size and shape; these morphologicaldifferences reflectedvarying flight and foraging behaviors.Sooty Ternshad a higher aspect ratio and higherwing loading than Brown Noddies reflectingtheir more pelagic lifestyle, and Christ- mas Shearwatershad a lower aspectratio and higherwing loadingthan Wedge-tailed Shear- waters (Puffinuspacificus) reflectingtheir pursuitplunging behavior. Key words: ecomorphology,flight, Laridae,Phaethontidae, Procellariidae, seabirds, Su- lidae, wings.

INTRODUCTION Walker 1993), and some pelagic seabird species traverse hundreds or thousands of kilometers Morphological studies usually provide important per on a basis in search of food insights into the relationship between organisms day regular (Stri- kwerda et al. Jouventin and Weimerskirch and their ecology, because morphological struc- 1986, This is true in the tures are intimately linked to performance, and 1990). particularly tropics, where is low relative to lati- performance determines the suite of ecological productivity high tudes and Ballance strategies available to a species. Interspecific (Ainley Boekelheide 1993, and Pitman and thus there should be variation in morphology is indicative of the dif- 1999), selection for that allow ferent roles played by each species within a strong flight capabilities seabirds to locate and secure in the face of community, however the success of character- prey diminished Some izing these roles depends on choosing the ap- feeding opportunities. tropical seabirds such as Terns propriate morphological features that correspond Sooty (Sterna fuscata) and Red-footed Boobies have meta- with the performance character of interest. (Sula sula) bolic costs of much lower than would be Flight is a performance parameter about flight based on mass alone and which much can be learned through the study of predicted body (Flint Ballance and exhibit be- ecomorphology. For almost all birds, flight is in- Nagy 1984, 1995), they havioral that increase timately linked with a suite of morphological adaptations flight profi- the use of wind as an traits, all of which are interdependent and rela- ciency, including energy source Ballance tively highly constrained. (Pennycuick 1982, 1995, Spear and Given these and Pelagic seabirds provide an intriguing system Ainley 1997). physiological behavioral it is that mor- in which to study ecomorphological aspects of adaptations, expected also exist for flight. The spatial scale of pelagic systems is phological adaptations proficient large relative to terrestrial ones (McGowan and flight. Warham (1977, 1996) presented data for several procellariiform species that revealed how ' Received 2 July 1998. Accepted 22 March 1999. wing morphology reflected ecology. For exam-

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TABLE 1. Species sampledfrom JohnstonAtoll. Valuesrepresent mean ? SE. Wing loadingin Newtons m-2.

Mass Wing loading Wing span Wing area Species n (kg) Aspect ratio (N m-2) (m) (m2) Procellariiformes Wedge-tailedShearwater 40 0.38 ? 0.01 10.41 ? 0.06 37.94 + 0.63 1.01 ? 0.01 0.10 ? 0.001 ChristmasShearwater 12 0.34 ? 0.01 10.34 ? 0.78 51.51 ? 1.48 0.82 ? 0.02 0.07 ? 0.001 Pelecaniformes Brown Booby 14 1.26 ? 0.06 12.14 + 0.13 64.06 + 2.38 1.52 ? 0.06 0.19 ? 0.004 Masked Booby 7 1.90 ? 0.06 12.16 + 0.29 84.76 3.13 1.64 ? 0.03 0.22 ? 0.005 Red-footedBooby 42 1.10 ? 0.02 11.43 + 0.09 54.98 ?_ 0.78 1.50 ? 0.01 0.20 ? 0.002 Red-tailedTropicbird 15 0.65 ? 0.02 10.92 + 0.08 56.67 ? 1.49 1.11 ? 0.01 0.11 ? 0.001 Charadriiformes Sooty Tern 27 0.20 ? 0.01 11.23 + 0.10 26.92 ? 0.39 0.90 ? 0.004 0.07 ? 0.001 Brown Noddy 30 0.19 ? 0.003 9.67 + 0.06 23.38 ? 1.89 0.85 ? 0.004 0.08 ? 0.001 White Tern 2 0.11 ? 0.003 11.12 ? 0.12 23.54 ? 0.76 0.70 ? 0.001 0.04 ? 0.001 ple, diving-petrels (Pelecanoididae), which pur- of seabirds currently breed on the atoll and 9 of sue prey underwater using wing propulsion, these, representing three orders and four families showed convergent morphological adaptations of birds, were sampled for this study (Table 1). with auks (Alcidae), which also pursue prey un- Birds were live-captured, measured, and re- derwater by using their wings. Other studies leased unharmed within a period of 8 min. All have suggested that certain wing traits are as- species were sampled during the breeding sea- sociated with low energetic cost of flight in sea- son (incubating, brooding, and/or provisioning birds; Southern Giant Petrels (Macronectes gi- young). The feeding behavior of each species ganteus) are similar in mass to some albatrosses was categorized based on their most typical but have differently shaped wings and greater feeding behavior as determined by Ashmole flight costs (Pennycuick 1982, Obst and Nagy (1971) and from our own field observations. 1992). Tropical seabirds must be selected for profi- DATA COLLECTION cient in with flight, especially comparison high Measurements were collected following the latitude seabirds. also exhibit a of They diversity methods outlined by Pennycuick (1989). Body behaviors that reflect differenc- flight ecological mass (kg), wing length (m), and wing area (m2) es in for and searching acquiring prey (Ashmole were recorded for each bird. Wing length was 1971). In this paper we examine the wing mor- calculated as the rectilinear distance from the of nine seabird in the phology species tropical sternum, a homologous landmark, to the tip of Pacific with the that premise wing morphology the outstretched wing along the ventral surface reflects these differences. ecological Morpholog- of the bird; wing span was thus twice the wing ical differences are used to make predictions length (Fig. 1). The fully outstretched wing was about which are based on results from ecology, traced with the wing positioned ventral side studies previous (Norberg 1985, 1989, Rayner down. This tracing (partial wing area) was later and are tested in- 1988, Pennycuick 1989) by digitized using the program SIGMASCAN (Jan- what is known about the distribution vestigating del Corp., San Rafael, California) to determine and of these seabirds. feeding ecology its area. The tracing was shorter than the wing because of the area between the METHODS length body proximal edge of the wing and sternum, known STUDY AREA AND SPECIES as the root box (Pennycuick 1989). This rect- Johnston Atoll, a group of four small islands angular area was quantified by multiplying the (each - 2.5 km2), is located approximately width of the wing where it meets the body, and 1,200 km southwest of Hawaii in the eastern the difference between the wing length and the tropical Pacific (16045'N, 16931 'W). Vegetation length of the wing tracing (Fig. 1). The root box consists of 3 native and 124 introduced species was added to the partial wing area, thus wing (Amerson and Shelton 1976). Thirteen species area was twice this value (Fig. 1).

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results are conductedwithin the context of seabirds. WingLength IN DATAANALYSIS No sexual distinctionwithin species was made i? in these analyses either because there was no PartialWing Area apparentsexual dimorphismor sample sizes between known sexes were too small for statistical Root Box analyses. To determinewhether there was a bi- ased effect from species with larger sample sizes, a species mean was generatedfor all variables and used in all analyses;these resultswere compared with analyses using individual data. Results were similarin all but one case (see be- FIGURE1. Measurementstaken of the wing.Wing low), therefore only results for individuals are lengthmeasured from the sternumto the tip of the wing,thus wing span = 2 X winglength. Partial wing presented. areacalculated from a tracingof the birdwing. Root A Pearsoncorrelation matrix was generatedto box quantifiedby multiplyingthe widthof the wing show relationships among all variables. Body whereit meetsthe body by the differencebetween size is an importantecological variable, there- and thus area= winglength partialwing length, wing fore regressions of 2 X (rootbox + partialwing area). least-squares logi0-trans- formed variables (wing area, wing span, aspect ratio, and relative wing loading) were used to investigate allometric relationships between These data were used to calculate load- wing each of these variables and log body mass; the ing and aspectratio, the lattera size-independent mean absolute value of residuals from the re- variablethat can contain information significant gression indicated which species deviated most and 1988, As- (Norberg Norberg Rayner 1988). for each variable. Species similar in body mass a dimensionless number that de- pect ratio, were examinedfor significantdifferences in oth- scribes the shape of the wing, was calculatedas: er variables. relative and wing span2/wingarea Body mass, wing loading, aspect ratio were used to constructa multidimensional Higher values for this index indicate a compar- morphospace,which provided a graphicalrep- atively narrowerwing, and lower values a com- resentationof the relationshipsamong all spe- parativelybroader wing. cies. Axes were representedas standarddeviates Wing loading describesthe weight (force) that of the mean value for each variable.All analyses is borne per unit area of the wings, or the were done using SYSTAT(Wilkinson 1988). amount of weight carried by the bird's wings; the units are in Newtons (force) m-2. It was cal- RESULTS culated as: CORRELATIONSAMONG VARIABLES statistics for mass and body mass X gravitational constant (a)/wing Summary body wing pa- area rametersare given in Table 1. Ourresults for the two shearwaters agree closely with those of Largerbirds typically have a higher wing load- Warham (1977, 1996). Analysis of individual ing than smaller birds because of area-volume specimens showed a significant correlation scaling relationships (Hildebrand 1988). Nor- among body mass, wing loading, aspect ratio, berg and Norberg (1988) used relative wing wing span, and wing area (r > 0.59, P < 0.001 loading (wing loading divided by the cube root for all variables).The lowest correlationswere of body mass) for comparisonswith geometric between aspect ratio and each of the other var- similarityand to help account for the effects of iables (0.59 < r < 0.68). Analysis of species body size; thus relative wing loading also was means indicateda similartrend with one excep- used in this analysis. As a group, seabirdstypi- tion, aspect ratio. Body mass, wing loading, cally have long, narrow wings compared with wing span, and wing area were all significantly most other birds, and so our comparisonsand correlated (r > 0.84, P < 0.001 for all vari-

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0.53X - 0.77; Fig. 2b), and with relative wing loading (R2 = 0.35, P < 0.001; Y = 0.14X + z 1.77; Fig. 2c). The exception was aspect ratio; c S.1 w this relationship was not significant (R2 = 0.36, P = Y = 0.06 X + and z_ 0 sT T 0.06; 1.06; Fig. 2d), o intraspecific variation tended to be greater for . this variable than the other measures of wing morphology. -.1 -5 b Mean regression residuals were generally greatest for two species, which also were far- -.7 . w B thest from the 95% confidence intervals, Red- tailed Tropicbirds (Phaethon rubricauda) and o z N T Christmas Shearwaters (Puffinus nativitatis). 3. 0 -1.1': Both had shorter wing spans, lower wing areas, 0 s x and higher relative wing loading than would be from mass alone 2a-c, -1.5 predicted (Figs. respec- Brown Noddies and 2 C tively). (Anous stolidus) Red-footed Boobies (Sula sula) had greater neg- z ative residual values for wing loading, suggest- S1.8 ing they have a relatively lower than predicted s 0 wing loading (Fig. 2c). Mean residual values of j aspect ratios were most positive for Brown Boo- S1.6 bies (Sula leucogaster) and Sooty Terns (Sterna 0 N fuscata), suggesting higher aspect ratios (nar- 1.5 rower wings); mean residual values were most 1.16 dM negative for Christmas Shearwaters and Brown S1.12 S Noddies, suggesting lower aspect ratios (wider S1.08 -.8 - -A -2 2 A wings) (Fig. 2d).

1.04O D c, SPECIESSIMILAR IN BODY MASS oR ANOVA for all species indicated two species " .96 X pairs that did not differ significantly in body .92 mass. Sooty Terns and Brown Noddies (hereaf- -12 -1 -.8 -.6 -A -2 0 2 A .6 ter, did not show mass LOG BODYMASS (kg) tern-noddy pair) body differences, but were significantly different with on FIGURE 2. Least-squaresregressions log body to all (P < 0.001 for mass for seabird from JohnstonAtoll for respect wing parameters species (a) all The small size = for log wing span, (b) log wing area, (c) log relativewing values). sample (n 2) loading, and (d) log aspect ratio. Letter codes: B = White Terns (Gygis alba) precluded their use for Brown Booby, F = White Tern, M = Masked Booby, statistical comparisons. Wedge-tailed Shearwa- = N Brown Noddy, R = Red-footed Booby, S = Sooty ters pacificus) and Christmas Shear- = = (Puffinus Tern, T Red-tailed Tropicbird, W Wedge-tailed waters also did not differ in mass but Shearwater,X = ChristmasShearwater. body showed significant differences for all wing parameters (P < 0.001) with the exception of as- ables), however there was no significant corre- pect ratio (P > 0.5). lation between aspect ratio and any other variable (r < 0.78, P > 0.1). GROUPDIVERSITY A three-dimensional plot of body mass, aspect REGRESSIONSWITH LOG BODY MASS ratio, and relative wing loading depicted how In general, body mass was a better predictor of species segregated along these axes (Fig. 3). A wing size than wing shape. There was a signif- nested ANOVA showed significant differences icant relationship of body mass with wing span at the family and genus level for each variable; (R2 = 0.90, P < 0.001; Y = 0.30X + 0.14; Fig. boobies and tropicbirds (Pelecaniformes) had 2a), wing area (R2 = 0.89, P < 0.001; Y = greater values for all three variables, terns

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2 aspect ratio and each of the other variables is partly a result of it being a dimensionless number in which the effect of size has been mini- 1B mized. The coefficient of variation for aspect ratio was greater than for any other variable and in all species. As mentioned previously, size-independent variables are important for illustrating adaptive significance (Norberg and Norberg 0 1988, Rayner 1988), thus particular attention is given to aspect ratio. With respect to our study, -2 the intraspecific variation in aspect ratio sug- gests fewer constraints on this variable than other parameters of wing morphology. 212 -1 MORPHOLOGICALPREDICTIONS OF ---1 1Loading wn ECOLOGY tRelat're Results from and theoretical research FIGURE 3. Three-dimensional for empirical morphospace to determine be- body mass, relative wing loading, and aspect ratiofor designed general relationships seabird species from JohnstonAtoll. Axes plotted as tween wing morphology and flight performance standarddeviates of the mean value for each index. can be used in this particular study to predict Lettercodes as in Figure 2. Pelecaniformes(B, M, R, from For had values for all three Charad- ecology morphology. example, aspect T) greater variables, ratio be used as a of distance riiformes (F, N, S) showed the converse, and Procel- might predictor lariiformes(W, X) had intermediatevalues except for traveled when foraging; the more far-ranging the aspect ratio, which was similarto Charadriiformes. species, the higher the aspect ratio (Norberg 1985, Rayner 1988). This relationship is exem- plified by albatrosses, among the most far-rang- showed the and (Charadriiformes) converse, ing of seabirds (Jouventin and Weimerskirch shearwaters had intermediate (Procellariiformes) 1990) and those with the highest aspect ratios for was values except aspect ratio, which similar (Hildebrand 1988, Warham 1996). to terns (Table 1, Fig. 3). Similarly, wing loading might be used as a of distance traveled when be- DISCUSSION predictor foraging cause the energetic cost of flight is likely to be BODY SIZE positively correlated with wing loading. For ex- Not surprisingly, variables with the highest cor- ample, Obst and Nagy (1992) found that South- relations among each other reflected some mea- ern Giant Petrels (Macronectes giganteus) had sure of size: wing span, wing area, body mass, greater flight costs, higher wing loading, and and to a lesser extent wing loading. Body size lower aspect ratio than several albatrosses of is an important determinant of feeding flock similar mass; the former regularly feed on shore composition in the tropical Pacific, particularly or in nearshore waters, whereas the latter rarely in flocks in which boobies were the numerically do so (Harrison 1983, Hoyo et al. 1992). dominant species (Ballance et al. 1997). Larger A third prediction is that wing loading will be body size was important for gaining access to negatively correlated with glide/flight speed prey while potentially excluding others from the (Pennycuick 1989). Thus, we would predict that same. In addition, Brown Boobies and Masked only those species with relatively low wing Boobies (Sula dactylatra) typically feed by loading will be able to utilize foraging strategies plunge diving, where they enter the water, often that require low flight speeds. This would be es- from great heights, to seize prey below the sur- pecially important in tropical regions where face (Ashmole 1971, Hoyo et al. 1992, Ballance wind speeds are typically lower compared with et al. 1997). Masked Boobies are the largest winds at higher latitudes. boobies and their size allows them to plunge Tern-noddy pair. The tern-noddy pair did not deeper than Brown Boobies, providing access to differ significantly in body mass, yet they dif- prey potentially unavailable to a smaller species. fered significantly in all wing parameters mea- The comparatively low correlations between sured. Sooty Terns had a higher aspect ratio,

This content downloaded from 130.166.34.124 on Tue, 05 May 2015 23:04:44 UTC All use subject to JSTOR Terms and Conditions 554 FRITZHERTEL AND LISA T. BALLANCE greaterwing loading, and longer wing span (Ta- flect differences in feeding behavior.For exam- ble 1). Therefore,we expect them to have lower ple, Wedge-tailedShearwaters are known to pur- flight costs, range fartherto forage, and fly at sue prey aerially (Ballance 1993), whereas greaterspeeds than Brown Noddies. ChristmasShearwaters have not been recorded These predictions are largely supported by to do so. Christmas Shearwatersalso pursuit distributionalpatterns. Sooty Terns forage over dive (underwaterflight) more frequentlythan do great distances (80-500 km) and remain at sea Wedge-tailedShearwaters (Ashmole 1971), so a for months to years, rarely resting on the water higher wing loading (smaller wing area) might (Pitman1986, Flint 1991). In the easterntropical reflect this behavior given the greaterforce re- Pacific, they are found in greatestabundance in quiredto move the wings throughwater relative areas of low productivity(Ballance et al. 1997). to air. In supportof this idea, a relatively high Thus we expect that selection for proficientlo- wing loadingwas reportedfor diving-petrelsand comotion would be strong in this species, and alcids, both of which use wing propulsionun- indeed Sooty Ternshave flight costs much lower derwater to pursue their prey (Warham1977, than would be predictedfrom mass alone (Flint 1996). More informationis needed on the en- and Nagy 1984). These differences were be- ergetics and feeding behavior of these shear- lieved to be due largely to wing morphology,in waters. a manner consistent with the results presented Boobies. The threebooby species differedsig- here. In contrast,Brown Noddies forage closer nificantly in body mass, however Red-footed to shore and usually returnto land to roost at and Brown Boobies differed only slightly. Red- night (Ashmole 1968). To our knowledge, flight footed Boobies had a significantlylower aspect costs for this species have not been quantified, ratio and wing loading than the other two spe- but it would seem that selection for low flight cies, wing loading being anomalouslylow (Fig. cost, and associated wing morphology, would 2c). We would thereforepredict that Red-footed not be as strongas for the more widely foraging Boobies would have a lower cost of flight and Sooty Tern. would not forage as widely as Brown Boobies. Shearwaters. The two shearwatersalso did These predictions appear to be only partly not differ significantly in body mass, but they supported. Red-footed Boobies forage over differed significantly in most wing measure- much greaterdistances than Brown Boobies, the ments. Wedge-tailed Shearwaters had greater latterforaging in more nearshorewaters (Pitman wing span, greater wing area, and lower wing 1986, Hoyo et al. 1992). For Red-footed Boo- loading than ChristmasShearwaters, but were bies, Ballance (1995) found their measuredcost similar in aspect ratio; the latterspecies fell no- of flight well below that predicted by aerody- tably below the regression for wing span and namic theory for a bird of their size. Differences wing area, and above that for wing loading in aspectratio may be explained,in part,through (Figs. 2a-c, respectively). Accordingly, one differences in feeding behavior.Whereas most might predict that Wedge-tailed Shearwaters boobies use plunge-diving (Ashmole 1971, would have slower flight speeds, and increased Hoyo et al. 1992, Ballance et al. 1997), Red- maneuverabilityat such speeds, both resulting footed Boobies in the tropical Pacific also feed from low wing loadings (Norberg 1985, Rayner by pursuingand capturingflying fish in the air 1988). Additionally,because the power required (Ballance 1993). This behavior requires quick for flight can be reducedby combininglow wing maneuvering, and presumably, a low cost of loading with high aspectratio (Rayner1988), we flight. Relatively short, narrowwings would be expect flight costs would be lower for Wedge- advantageous for quick maneuvering (Rayner tailed than for ChristmasShearwaters. 1988) but precisely how this relates to aspect These predictions are difficult to test, given ratio requires further investigation. Red-footed our lack of knowledge about the at-sea ecology Booby wings may representa compromisebe- of shearwaters, particularly Christmas Shear- tween selection for a low cost of flight/widefor- waters.Both species forage far from shore (Har- aging range (higher aspect ratio), and an aerial rison 1983, Pitman 1986) and both commonly pursuitpredatory behavior (lower aspect ratio). feed in flocks in association with subsurface More behavioraldata in other regions and more predators(Ballance et al. 1997). We suspect that sulid species are needed to resolve this relation- the morphologicaldifferences between them re- ship.

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Tropicbirds. Red-tailed Tropicbirds had short- any particular trait is likely to be suboptimal er wing span, lower wing area, and greater wing when viewed in only one context; animals prob- loading (Figs. 2a-c, respectively) than predicted ably exhibit some combination of morphological based on body mass; aspect ratio was slightly traits that potentially reflect a compromise for lower than expected (Fig. 2d). This morphology multiple functions. An example is the Red-foot- predicts that flight speed and cost of flight would ed Booby; this species is highly pelagic, a niche be high (Norberg 1985, Rayner 1988). that would select for high aspect ratio, yet its Red-tailed Tropicbirds travel great distances foraging mode of aerial pursuit of flying fish (200-500 km) when foraging (Pitman 1986, might select for a comparatively lower aspect Schreiber and Schreiber 1993), but not much is ratio for maneuverability. known regarding their foraging ecology other Limitations on community structure and the than that they are typically plunge divers (Ash- diversity of functional types will likely change mole 1971, Hoyo et al. 1992, Schreiber and with different species. For seabirds, breeding as- Schreiber 1993). It seems paradoxical that a pe- semblages also are determined by other factors lagic feeder would have morphological features such as available substrates, island size, and sto- resulting in high cost of flight, although feeding chastic events. Many seabird colonies also have areas that are predictable in space and time been affected by human factors within the past might explain this wing morphology and con- few hundred years (Steadman 1995) and so sequent performance traits. Red-tailed Tropic- these historical disturbances on seabird distri- birds rarely feed in flocks in association with bution must also be considered whenever pos- subsurface predators (Schreiber and Schreiber sible. Whether character or competitive dis- 1993, Ballance et al. 1997), despite the fact that placement occurs among seabirds based on wing this is a prevalent foraging strategy for many ecomorphology will require more species in tropical seabird species (Au and Perryman 1985, each functional and ecological category de- Au and Pitman 1986). Flock feeding requires the scribed above, and a comparison among estab- to in order to access to the ability hover gain lished pelagic communities. rapidly moving areas at the water surface where prey is available, and maneuverability in order ACKNOWLEDGMENTS to avoid collisions with the numerous other birds We thankD. Jacobs, W. Koenig, R. Pitman,T. Plum- feeding in close proximity. Low wing area, short mer,J. Warham,and an anonymousreviewer for help- ful comments. for LTB was receivedfrom the wing span, and high wing loading are not fea- Funding tures that the slow for American Ornithologists'Union, the Departmentof permit flight necessary at UCLA, the Hawaii/PacificIslands National Further Biology such maneuverability. investigation of WildlifeRefuge Complexof the U.S. Fish andWildlife tropicbird feeding behavior and energetic con- Service, and the Southwest Fisheries Science Center sequences is needed. of the NationalOceanic and AtmosphericAdministra- tion. GROUPDIVERSITY LITERATURE CITED Before making valid functional comparisons or interpretations across taxa, differences in phy- AINLEY,D. G., ANDR. J. BOEKELHEIDE.1993. An eco- should be accounted and methods logical comparisonof oceanic seabirdcommuni- logeny for, ties of the south Pacific Ocean. Stud. Avian Biol. have been developed to help account for these 8:2-23. differences (Felsenstein 1985, Gittleman and AMERSON,A. B., JR., ANDP. C. SHELTON.1976. The Kot 1990, Hertel and Lehman 1998). Results naturalhistory of JohnstonAtoll, central Pacific from the three-dimensional group analysis (Fig. Ocean. Atoll Res. Bull. No. 192. Smithson.Inst., there is a Washington,DC. 3) suggest phylogenetic component ASHMOLE,N. P 1968. size, size, and eco- that needs to be this will be done in Body prey addressed; logical segregationin five sympatrictropical terns the future by the inclusion of more species and (Aves: Laridae).Syst. Zool. 17:292-304. by comparing seabirds from Johnston Atoll with ASHMOLE,N. P. 1971. Sea bird ecology and the marine In D. other pelagic groups in the eastern tropical Pa- environment,p. 223-286. S. Farnerand J. cific. R. King [eds.], Avian biology. Vol. 1. Academic Press, New York. FUTURECONSIDERATIONS Au, D. W K., ANDW. L. PERRYMAN.1985. Dolphin habitats in the eastern tropical Pacific. Fishery Organisms sometimes are assumed to be opti- Bull. 83:623-643. mally adapted to their environment. However, Au, D. W. K., ANDR. L. PrITAN.1986. Seabirdinter-

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