Feeding Behavior in Laughing Gulls: Compensatory Site Selection by Young

Condor85:467-l73 G The CooperOmthological Society 1983

FEEDING BEHAVIOR IN LAUGHING GULLS: COMPENSATORY SITE SELECTION BY YOUNG

JOANNA BURGER

AND MICHAEL GOCHFELD

ABSTRACT.-Feeding behavior of different age classesof Laughing Gulls (Larus atricillu)was studied in different habitats in Texas and Mexico; feeding methods were compared. Adults generally had higher capture successrates and shorter intervals between obtaining food items than young, while subadultswere intermediate in both measures.Where food was most readily available (i.e., garbage dumps and fish offal), differencesamong ageclasses were negligible.Age differences increased when intervals between obtaining food increased, reflecting the greater difficulty of the feeding task. Proportionately more young fed in those situations where their interfood intervals and successrates most closely approached those of the adults. Young gulls thus appeared to compensate for their generally lower feeding successby feeding at certain sites.

Age differencesin foraging behavior and feed- coast of the Gulf of Mexico. We wanted to ing successhave been found for several avian learn whether age differences in ability varied species,including Little Blue Herons (Egretta with the difficulty of the task, and whether caerulea;Recher and Recher 1969) Brown habitat useand method of feeding differed with Pelicans(Pe1ecanu.s occidentalis; Orians 1969), age. We selectedLaughing Gulls becausethey Olivaceous Cormorants (Phalacrocorax oli- are abundant, feed in a variety of habitats, and vaceus;Morrison et al. 1978) Sandwich Terns use different feeding methods; we chose the (Sternasandvicensis; Dunn 1972) Royal Terns Gulf of Mexico region becauseboth adults and (S. maxima; Buckley and Buckley 1974) young are common there. Glaucous-winged Gulls (Larus glaucescens; We predictedthat (1) for difficult tasksyoung Barash et al. 1975, Searcy 1978) and Herring should take longer and/or be successfulless Gulls (L. argentatus;Verbeek 1977a, b, c, In- often than adults, (2) young should congregate golfsson and Estrella 1978). In general, birds in those areas where the task is easiest, and feed more effectively as they gain experience. where their feeding successapproaches that of One measure of feeding effectiveness is the adults, and (3) subadults should be interme- “capture successrate, ” defined as the number diate in ability, approachingadult rates in eas- of items obtained per number of capture at- ier tasksand young ratesin more difficult tasks. tempts. Age differencesin capture successhave been usually attributed to the difficulty of the STUDY SPECIES AND SITES task (such as plunge-diving which requires a The Laughing Gull, a monotypic, hooded gull, period of learning), or the difficulty in learning breeds locally on the Atlantic coast from New to recognizesuitable food items, or both. Many England to Florida and the West Indies, on the aspects of foraging behavior may vary with Gulf of Mexico to Yucatan, and on rocky islets age, such as the time required to search for off the coastof Venezuela and northern Mexico prey, pursue the prey once it is found, and (A.O.U. 1957). The main breedingarea is from capture,handle, and eat the prey. “Search time” the Carolinas through the Caribbean. The may be defined as either the total time a bird specieswinters from the Middle Atlantic states searchesfor food each day or the time required to Peru and Brazil. to find individual prey. Search time varies in We identified three age classes:adults (with relation to the ability to find the prey, or the an all-white tail), subadults(resembling adults percentageof successfulcaptures, since an un- in body plumage but with a partial narrow sub- successfulattempt lengthensthe time required terminal tail band), and young(dark-plumaged to locate another prey item. birds with a broad tail band, seeDwight 1925). Variations in age differences in foraging In our field work on the Yucatan Peninsula we within a speciesunder different conditionshave readily encountered enough young and adult not been hitherto studied. We examined the plumaged birds for sampling, but only a small foraging behavior of Laughing Gulls (Larus proportion of gulls in subadult plumage. atricillu)under different conditions along the We studiedLaughing Gulls in Galveston Bay

14671 468 JOANNA BURGER AND MICHAEL GOCHFELD and at the Texas City landfill in Texas, near interfood interval (using a stopwatch) for 20 Progresso, 32 km north of MCrida (Yucatan, randomly chosenindividuals of each ageclass. Mexico), and near Campeche, Champoton, When the gulls were feeding by more than one Seybaplaya, and Isla Aguada (Campeche, method, we recorded the interfood interval for Mexico). We remained at each study site for 20 randomly chosen individuals of each age at least one day. class for each foraging method. After timing the interfood interval of one individual we METHODS switchedto another, alternating ageclasses. To We examined the foraging behavior of Laugh- avoid timing the same individuals repeatedly, ing Gulls along the Gulf of Mexico near Gal- we sampled birds in all areas of the flock. We veston, Texas and in Campeche and Yucatan, compared the size of food items with the size Mexico in January 1979. To minimize con- of the bill and found that at each site, gulls fed founding variables suchas seasonand weather, on relatively the same size item; agedifferences we limited our study to three weeks, sufficient within any foraging site thus were not attrib- to study a variety of feeding situations. For utable to differencesin food particle size. How- four days we observed Laughing Gulls feeding ever, since volume and energy values differ in Galveston Bay and on garbageat the Texas among foods, we did not compare foods among City landfill (where a bulldozer intermittently study sites or feeding methods. distributed the garbage over the dump). For In addition to interfood intervals, we also the remainder of the study period we studied recorded capture successrates where possible the gulls on the Mexican Coast. All observa- (successrate = number of items obtained per tions were made on calm, clear, sunny days in number of attempted captures). At all study order to minimize differences due to weather sites, we recorded the number of Laughing (see Dunn 1973). We made observations from Gulls and the age composition every 10 min about 07:OOto 19:OOand did not find age dif- throughout the observation period, for each ferences in time of feeding. Since we did not method being studied. We made two transects mark individuals, our study does not provide along the Yucatan coastfrom Progress0to Car- information on age differences in the propor- men to count all Laughing Gulls to determine tion of each day that individuals fed (but see age distribution. Cooke and Ross 1972, Buckley and Buckley Most analyses were performed using non- 1974). parametric contingency tables or correlation We distinguishedseveral methods of feeding procedures (Siegel 1956). Where appropriate, (after Ashmole and Ashmole 1967, Ashmole we performed other analyseson interfood in- 197 1, Simmons 1972): surface-feeding on a tervals (using log-transformed data) and on garbagedump, aerial snatching,plunge-diving, percentages(using arcsine transformation; So- aerial dipping, and contact-dipping. “Plunge- kal and Rohlf 1969). diving” (where the gulls descendquickly to the water head first, and submergethemselves only RESULTS partially while capturing fish) was observed at In Texas, adults comprised 95% of the foraging a stream 3 km south of Progresso,at a river Laughing Gulls, whereas along the entire Yu- mouth 30 km west of Champoton, in the Gulf catan coast the ratio of age classeswas: 60% of Mexico 16 km southwestof Champoton, at adult, 5% subadult, and 35% young. The gulls’ Seybaplaya, and behind ferry boats at Galves- choice of feeding habitats ranged from natural ton, Texas and Isla Aguada (35 km east of (harbors, streams,rivers, ocean) to man-influ- Carmen, in Campeche). “Dipping” refers to enced(plunging for fish behind ferries, and over seabirdsdipping only their bills into the water a streamwhere a culvert had concentratedfish), to pick up small food items from the surface. and to man-made situations (garbage dump, We observed aerial dipping (dipping while offal). At Seybaplaya, Laughing Gulls fed by a flying) at Seybaplaya and Champoton, and variety of methods within sight of one another: contact dipping (dipping while swimming) at aerial dipping, contact dipping and plunge- Seybaplaya and Progress0Harbor. At Cham- diving. Although young comprised only 30% poton we observed gulls feeding by aerial dip- of the gulls present at Seybaplaya, a higher ping at a 2-m long dead fish partly exposed in percentagethan expected fed by aerial snatch- shallow water. At Seybaplaya, a fishing village ing of offal and dipping for invertebrates, and 55 km south of Campeche City, fishermen a lower percentageplunge-dived for fish (Table butchering sharks tossed offal to waiting 1). Successrate varied according to feeding Laughing Gulls. method and age (Table 1). Young were as suc- We defined the “interfood interval” as the cessful as adults when snatching offal in the period between obtaining successive food air, lesssuccessful than adults when aerial dip- items. At each feeding location we timed the ping, and much lesssuccessful than adultswhen FEEDING BEHAVIOR IN LAUGHING GULLS 469

TABLE 1. Effect of age on location of feeding and successrate (df = 2 for x2 tests).

n Adult Subadult Y0Wlg x ’ P Percent occurrence

AerialOverall snatching (offal) 200 6635 45 2: 89.6 0.001 Aerial dipping (invertebrates) 160 55 5 40 7.6 0.02 Plunge-diving (fish) 100 II 3 20 10.8 0.005 Successrate Aerial snatching 150 62 60 52 0.4 NS Aerial dipping 150 96 75 60 6.8 0.05 Plunge-diving 100 83 60 24 12.9 0.005 plunge-diving for fish. Considering occurrence that the proportion of young was negatively and successrates, young fed mostly where their correlated with the index (Spearman’s rank r, successrate was closestto that of adults, and = -0.68, yt = 14, P < 0.007). avoided feeding where their rate was a third We usedstepwise multiple regressionon arc- of that of the adults (Table 1). Similarly, while sine transformation of the percentageof young plunge-diving for fish at Progress0 Harbor, at each of the 14 feeding situations. Of the young were less successfulthan adults (15% independent variables (ageefficiency index, in- vs. 77%, x2 = 7.17, df = 1, P < 0.01, y2= 46). terfood interval, capture successrate), the age The interfood interval varied among for- efficiency index accounted for 54% of the vari- aging methods and age classes(Table 2). In- ability in the percentageof young in particular tervals were shortest in man-made habitats feedingsituations (F = 12.7, P < 0.005) while (dumps, offal), and longestwhile plunge-diving the youngs’ interfood interval accounted for for fish (except when a school of fish was dis- only 25% of the variability in the percentage covered, Table 2). Comparing all foraging of young (F = 4.14, P -c 0.05). Thus, merely methods, except snatching offal in flight and having low interfood intervals did not cause surface feeding at the dump in the absenceof the young to forage in a particular place. These bulldozers,we found statisticallysignificant age results indicate that young gulls concentrated differences, with adults obtaining food more their foraging in places where their interfood quickly than young. Subadultsusually had in- interval approached the adult interval, further termediate interfood intervals. Figure 1 shows confirming prediction 2. the relationship between adult interfood in- We likewise examined the feeding of sub- tervals and those of young and subadults(data adult gulls. In five of eight situations where in Table 2). These measuresare correlated for there were enough subadultsto sample, their adults and subadults(Spearman ’s r, = 0.72, y1 interfood intervals were intermediate to those = 9, P < 0.05; Siegel 1956) and for adults and of adults and young (Table 2). To test whether young r, = 0.97, y1= 14, P -c 0.001). In gen- subadultsfared better when feeding was rela- eral, the young took relatively longer in feeding tively easy, we computed the age efficiency in- situations where adults had the longest inter- dex for subadultsversus adults (from Table 2) food intervals, and they approached adult in- and examined this in relation to the adult intervals where adults had low interfood inter- terfood intervals. The two values were posi- vals (Table 2). These resultsare consistentwith tively correlated (Y, = 0.72, y1= 8 cases, P < prediction 1. 0.05). Thus it was in the feeding situations We predicted that young should concentrate where adult interfood intervals were shortest their foraging where their interfood intervals that the subadult intervals were closestto the were closestto those of adults. Assuming that adult values. In all caseswhere adult intervals the adult interfood interval would be related exceeded 4 s, the subadults’ intervals were to the difficulty of the task (or the availability closer to those of the young. While this is con- of prey), we computed an age efficiency index sistent with prediction 3, the eight casesrep- for the young by dividing the mean interfood resent a small sample, the study having been interval for the young by the mean interfood limited by the paucity of birds in subadult interval for adults in each foraging situation plumage. (the 14 samplesreported in Table 2). A value of 4 indicates that young take four times as DISCUSSION long as adults to find food. Plotting the per- Age differencesin feeding behavior have been centage of young in each feeding situation found in all gulls and terns examined thus far, against the age efficiency index (Fig. 2) shows although the differences are not consistent

FEEDING BEHAVIOR IN LAUGHING GULLS 471

140 607 0 0 I 50-

: =5 4om 2 100 0 k X X 8 0 -5 O 30- _ 80 5 0

S k 2c- 0 a 0 0 1 o- 0 0 0 a I 00 i i s 4 5 z 20 AGE EFFICIENCY INDEX FIGURE 2. Percent of young LaughingGulls present in each feeding situation in relation to the ratio of mean interfood interval of young divided by mean interfood 1’0 2’0 3’0 4b interval of adults (the age efficiency index). ADULT INTERFOOD INTERVAL (set)

FIGURE 1. Interfood intervals (in seconds)of young (x) and subadult (dots) Laughing Gulls plotted against adult clams or scallopson hard surfaces,and to dig interfood intervals for all foragingsituations. The diagonal for garbage. Evidence that successimproves shows where their rates would be equivalent. with age was found in young Herring Gulls: their ability to open shellsimproved from Oc- tober to April (Ingolfsson and Estrella 1978). across species. Verbeek (1977a) found that Similarly, Burger and Gochfeld (198 1) report- adult Herring Gulls feeding on starfish were ed that Herring Gull young feeding on a dump more successfulon the first dive, were chased improved from Septemberto February in their by food pirates less often, and fed on the star- ability to pirate food from conspecificsand to fish beds for lesstime than did immature gulls. avoid being victims of other pirates. One must Ingolfsson and Estrella (1978) reported that consider, of course, that less successfulyoung first-year Herring Gulls opened scallops less may simply starve, leaving the more successful successfullythan did adults.Young had to drop young to be studied. scallops more often, and frequently dropped The distribution and availability of prey are them on inappropriate (i.e., soft) surfaces. other factorsto which birds must respond(Zach Similarly, Glaucous-winged Gulls differ ac- and Smith 1981). Age-related differences in cording to age in ability to open clam shells habitat use have been reported. Moyle (1966) (Barash et al. 1975) and capture fish (Searcy found that gulls in immature plumage were 1978). The successrates (captures/attempts) forced by aggressiveadults to feed in subop- for several other speciesshow similar trends, timal foraging areas on the edgesof a salmon with adults generally being more successful stream in Alaska. Several authors have noted (Table 3). In one of the most extensive studies that young gulls often congregate on dumps of the factors affecting foraging success,Buck- (Schreiber 1968, Spaans197 1, Cooke and Ross ley and Buckley (1974) found that young and 1972). Presumably food is easier to find or adult Royal Terns had similar capture success “capture” at a dump than at more traditional rates (captures/unit time), but adults dived feeding habitats or methods (i.e., plunge-div- twice as often and so obtained more food per ing for fish). Similarly, learning might play less unit time than did young. of a role in feeding ability at a dump, compared Differences in successrates may reflect the to other habitats, becausefood is more abun- difficulty of the task and the time required to dant, stationary, and not under water or sand. learn the techniques. It takes time to learn to Davis (1975) color-marked Herring Gull fam- dive for fish or other invertebrates, to drop ily groups, and ascertainedthat young did not 472 JOANNA BURGER AND MICHAEL GOCHFELD

TABLE 3. Successrates as a function of age for selected species.Shown are sample sizes for the samples (percent successin parentheses).

Feeding success rates Signiticanceb Species Adults YOUlg x’ P Method source Brown Pelican 487 (69) 1,482 (49) 59 .OOl Plunge-diving Orians (1969) Olivaceous Cormorant 773 (18) 1,287 (11) 20 .OOl Pursuit-diving Morrison et al. (1978) Little Blue Heron 846 (75) 493 (68) 9.9 .Ol Walking Recher and Recher (1969) Herring Gull 28 (64) 19 (16) 10.8 .OOl Diving Verbeek (1977a) Glaucous-wingedGull 291 (69) 523 (50) 28.0 ,001 Plunge-diving Searcy(1978) Laughing Gull Fishing-no frenzyc 42 (52) 36 (8) 17.1 ,001 Plunge-diving This study Fishing-frenzy< 45 (88) 35 (51) 10.7 .Ol Plunge-diving This study Swim dipping 40 (94) 40 (80) 4.1 .05 Dipping This study Carrion 20 (98) 20 (50) 10.2 .Ol Dipping This studv Offal 120 (65) 50 (52) 4.6 .05 Dipping This study Garbage 40 (100) 40 (98) 1.0 .50 Walking This study Sandwich Tern 579 (17) 778 (13) 24.0 ,001 Plunge-diving Dunn (1972) Royal Tern 85 (38) 77 (38) 1.0 .50 Plunge-diving Buckley and Buckley (1974)

= Percent of all attempts that are successful. b Computed bv Burner and Gochfeld. ‘ ‘ Feedmg frenzies are rapid bursts of diwng actiwty localized in a small area. go to feeding areas with their parents;they fed time a gull searchesfor a given food item. Krebs in a wide variety of locations, and tended to (1978) discussedforaging models that include feed farther from their nestingcolony than did the cost of searchingfor habitats, yet he also adults. noted that there were no data estimating the Young seabirds can compensate for lower costs(time, energy) of searchingfor prey items, feeding successrates by foraging for a longer particularly when prey are located in patches. part of the day. Sucha tendency hasbeen noted We have also found that most studieson feed- for Herring Gull young feeding at dumps and ing in seabirdsdo not usually include the costs on starfish (Cooke and Ross 1972, Verbeek associatedwith searchingfor specificprey items 1977c, Burger 198 1). Young Olivaceous Cor- once feeding habitats have been located. We morants foraged longer each day to compen- therefore suggestthat two “search times” need sate for their longer feeding intervals (Morri- to be measured:the time required to find feed- son et al. 1978). Buckley and Buckley (1974) ing habitats, and the time required to locate reported that Royal Tern young roosted for prey. Our measure, the interfood interval, in- less time and at different hours each day than cludes the latter. did adults. Another problem with our study is that we Several factors contribute to the age differ- selectedsuccessful birds, since we did not re- encesin foragingand feeding ability in seabirds cord an interval until a bird found an item. including the following: (1) ability to locate This biased our data in favor of the most suc- feeding areas, and to find and recognize prey, cessfulbirds. It may not bias our relative dif- (2) frequency of attempts, (3) ability to capture ferences among feeding methods and age prey (successrate), (4) appropriate dive heights, classes,but is neverthelessan important vari- and (5) ability to distinguish appropriate sur- able. facesfor dropping prey. We believe that a basic We found that successvaried among feeding problem with our study and others is that methods, with adult and young successrates another factor, “real search time,” was not generally increasing together. In this study computed. Investigators have not followed in- adults had significantly shorter interfood individual birds all day to determine how much tervals than young, except at the Texas City time they required to search for feeding hab- garbage dump and while feeding on offal at itats (or patchesof food). Instead, investigators Seybaplaya (Table 2). The interfood interval have observed birds actually feeding, individ- of subadults, however, was sometimes closer uals who have already found the feeding hab- to that of adults (dipping for invertebrates), itat. We believe that a “habitat search time,” sometimes intermediate (plunge-diving in a needsto be added to Schoener’s (197 1) “search stream), and sometimescloser to that of young time” in order to reflect more accurately the gulls (dipping for carrion). We suggestthat the FEEDING BEHAVIOR IN LAUGHING GULLS 413 interfood intervals of subadultsmay reflect the BURGER,J., AND M. GOCHFELD.198 1. Age-related dif- difficulty of the task, where previous experi- ferencesin piracy behaviour of four speciesof gulls, ence may be the determining factor. Larus. Behaviour 771242-267. COOKE, F., AND R. K. Ross. 1972. Diurnal and seasonal Interfood intervals varied as a function of activities of a post-breeding population of gulls in foraging method, feeding site, and age. For ex- southeasternOntario. Wilson Bull. 84: 164-l 72. ample, the interfood intervals for adults who DAVIS,J. W. F. 1975. Specialization in feeding location were plunge-diving for fish varied from means by Herring Gulls. J. knim. Ecol. 44:795-804. DUNN. E. K. 1972. Effect of age on the fishing abilitv of of 2 to 120 s, and these same values ranged Sandwich Terns Sterna sandvicensis.Ibis 114:3kO- from 6 to 140 s for young (Table 2). Further, 366. the relationship between the interfood inter- DUNN, E. K. 1973. Changesin fishing ability of terns vals of adults and young was not constantfrom associatedwith windspeedand seasurface conditions. one feeding situation to another. That is, Nature 244:520-52 1. DWIGHT,J. 1925. The gulls (Laridae) of the world. Bull. sometimes young required four to five times Am. Mus. Nat. Hist. 52:63-408. aslong to captureprey (Fig. 2). We found higher INGOLFSSON,A., AND J. J. ESTRELLA.1978. The devel- percentagesof young Laughing Gulls feeding opment of shellcrackingbehavior in Herring Gulls. in those habitats and by those methods where Auk 95:577-579. KREBS, J. R. 1978. Optimal foraging:decision rules for their interfood intervals more closely ap- predators,p. 23-63. In J. R. Krebs and N. B. Davies proached the interfood intervals of adults (Fig. [eds.], Behavioural ecology, an evolutionary ap- 2). Even at one site(Seybaplaya, Table l), young proach. Blackwell Scientific Publications, Oxford. made up a different percentageof each feeding MORRISON,M. L., R. D. SLACK,AND E. SHANLEY,JR. flock we observed. Although young comprised 1978. Age and foraging ability relationships of Oli- vaceousCormorants. Wilson Bull. 90:414-422. 30% of the resting flocks, they made up 20% MOYLE,P. 1966. Feeding behavior of Glaucous-winged of the flock feeding on fish, 40% of the flock Gull on an Alaskan salmon stream. Wilson Bull. 78: feeding by dipping, and over 50% of the flock 175-190. feeding on offal. Although birds could seeeach ORIANS,G. H. 1969. Age and hunting successin the Brown Pelican (Pelecanusoccidentalis). Anim. Be- other within this area, the young elected to feed hav. 17:316-319. by methods and on foods where their interfood RECHER,H. F., AND J. A. RECHER. 1969. Comparative intervals came closest to that of adults. Our foraging efficiency of adult and immature Little Blue findings demonstrate one behavioral choice Her&s(Florida caerulea).Anim. Behav. 17:32C-322. available to young to maximize their overall SCHOENER.T. W. 1971. Theorv of feeding strategies. Annu: Rev. Ecol. Syst. 212691404. - - foraging efficiency. Young cannot exercisethis SCHREIBER,R. W. 1968. Seasonal population fluctua- option universally becausesuch situations are tions of Herring Gulls in central Maine. Bird-Banding neither always available nor always predict- 39:81-106. able, and becausethe food (e.g., garbage)may SEARCY,W. A. 1978. Foragingsuccess in three ageclasses not be sufficiently nutritious. of Glaucous-wingedGulls. Auk 95:587-588. SIEGEL,S. 1956. Nonparametric statisticsfor the behavioral sciences.McGraw-Hill, New York. ACKNOWLEDGMENTS SIMMONS,K. E. L. 1972. Some adaptive featuresof sea- We wish to thank B. G. Murray, Jr. for valuable discus- bird plumage types. Br. Birds 65:465-521. sions, and R. Trout for statistical advice. SOKAL, R. R., AND F. J. ROHLF. 1969. Biometry. W. H. Freeman and Co., San Francisco. LITERATURE CITED SPAANS,A. L. 1971. On the feeding ecologyof the Her- ring Gull Larus argentatusPont. in the northern part AMERICANORNITHOLOGISTS UNION.’ 1957. Check-list of of the Netherlands. Ardea 59: l-l 88. North American birds. 5th ed. American Ornithol- VERBEEK,N. A. M. 1977a. Comparative feeding behav- ogists’ Union, Baltimore. ior ofimmature and adult Herring Gulls. Wilson Bull. ASHMOLE,N. P. 1971. Sea bird ecologyand the marine 89:415-421. environment, p. 224-286. In D. S. Famer and J. R. VERBEEK,N. A. M. 1977b. Age differencesin the digging King [eds.], Avian biology. Vol. 1. Academic Press, frequency of Herring Gulls on a dump. Condor 79: New York. 123-125. ASHMOLE,N. P., AND M. J. ASHMOLE. 1967. Compara- VERBEEK,N. A. M. 1977~. Interactions between Herring tive feeding ecologyof sea birds of a tropical oceanic and LesserBlack-backed gulls feeding on refuse. Auk island. Peabody Mus. Nat. Hist. Yale Univ. Bull. 24: 94~726-735. 1-131. ZACH, R., AND J. N. M. SMITH. 1981. Optimal foraging BARASH,D. P., P. DONOVAN,AND R. MYRICK. 1975. Clam in wild birds?, p. 95-l 10. In A. C. Kamil and T. D. droppingbehavior ofthe Glaucous-wingedGull (Lar- Sargent [eds.], Foraging behavior. Garland STPM us giaucescens).Wilson Bull. 87:60-64. Press,New York. BUCKLE;,F. G., AND P. A. BUCKLEY. 1974. Comparative feeding.ecologv of wintering adult and iuvenile Royal Department of Biology, Rutgers University,New Bruns- Tems(Aves:-iaridae, Sterkinae). E&logy 55: lOi3- wick, New Jersey08903. Addressof secondauthor.. Envi- 1063. ronmental and Community Medicine, UMDNJ-Rutgers BURGER,J. 1981. Feeding competition between Laugh- Medical School,Piscataway, New Jersey08854. Received ing Gulls and Herring Gulls at a sanitarylandfill. Con- 29 October 1982. Final acceptance19 February 1983. dor 83:328-335.