Moonlight Avoidance Behavior in Leach's Storm-Petrels As a Defense Against Slaty-Backed Gulls

MOONLIGHT AVOIDANCE BEHAVIOR IN LEACH'S STORM-PETRELS AS A DEFENSE AGAINST SLATY-BACKED GULLS

YUTAKA WATANUKI Instituteof AppliedZoology, Faculty of Agriculture,Hokkaido University, Kitaku kita 9 nishi9, Sapporo060, Japan

ABSTRACT.--Diurnalactivity patternsof Leach'sStorm-Petrels (Oceanodroma leucorhoa) and Slaty-backedGulls (Larusschistisagus) were investigated.The petrels reducedactivity in moonlightin May and Junewhen the predationrate by gulls was relativelyhigh. Petrel activitylevels were inverselycorrelated with light intensitiesand the correspondingrisk of predationby the gull. Thissuggests that nocturnalactivity and moonlightavoidance by the petrel in its colonyare an effectivedefense against diurnal predators.Activity synchroni- zationof the petrelwas most marked during the full moon,further supportingthe predator- avoidancehypothesis. Received 15 October1984, accepted 27 April 1985.

MANX Shearwater (Puffinuspuffinus; Harris which are coveredwith Calamagrostislangsdorffii, iso- 1966), Leach's Storm-Petrel (Oceanodromaleu- lated rockstacks, and cliff ledges.A few pairsof Black- corhoa;Harris 1974), Fork-tailed Storm-Petrel (O. tailed Gulls (L. crassirostris)nested on these sites. furcata;Harris 1974,Boersma et al. 1980),and The Slaty-backedGull is an important predator of Cassin'sAuklet (Ptychoramphusaleuticus; Tho- adult petrels on the island. Although Jungle Crows (Corvusmacrorhynchos) excavated petrel burrows and resen 1964, Manuwal 1974) are strictly noctur- ate adults, eggs, and chicks,predation by the small nal in their colonies and are less active on crow population (13 pairs) was not significant. moonlit nightsthan on dark nights.Cody (1973) Activitypatterns.--Observations of flying birdswere discussedthe nocturnal activity of alcids as a made from a blind set on top of a headland about 25 defenseagainst diurnal predators.Gross (1935), m abovesea level. For 5 rain every 30 rain, I counted M.P. Harris (1966), and S. W. Harris (1974) also all birds passingan imaginary 20 x 30-m plane ori- suggestedthat nocturnal procellariiformsare ented vertically with reference to the cliff face op- vulnerableto diurnalpredatory gulls on moon- posite the headland. Two 6-volt electric lights, one lit nights. However, the relationship between set horizontally and the other about 45ø upward at daily activities of procellariiformsand preda- the blind, lit the plane facing toward the sea from tion risk has not been studied. the lower corner to the oppositeside. This reduced the effectof the light on landing birds approaching I studied Leach's Storm-Petrels and Slaty- from the sea.If all the landing petrels were attracted backedGulls (Larusschistisagus) on Daikoku Is- instantaneouslyto the lights, the number of petrels land. Their activity patternsare describedand flying through the lights would increaseduring the the nocturnalbehavior of the petrels in the col- observations. However, the number was rather con- ony is discussedas predator avoidance. stant during 5-rain observations(Fig. 1). A few pe- trelsand gullsflew circularlyin the lights;these were excluded from the data. Observations on moonlit

STUDY AREA AND METHODS nights showedthat Slaty-backedGulls did not avoid the lights. No differential responseto the lights by Thecolony.--The study was conducted on Daikoku gulls and petrelswas assumed.Observations started Island (42ø52'N, 144ø52'E),Akkeshi, Hokkaido, be- in daylight and lasted24 h (n = 19 days).Data taken tween late April and early October1982. The island during a day with dense fog (13-14 May) were ex- is 6.1 km in circumference and treeless, with the excluded from the analysis becauseof low visibility. ception of birch (Betulaermanii) groves in the ravines. Data of 27-28 April were included in those of May. Leach'sStorm-Petrel (the only petrel breedingon the Light intensitywas measured by a lux-meter(+ 10%) island) nestsin the interior partsof the island, which set horizontally and was divided into four classes: is covered with Artemisamontana and Urtica platy- dark (0 lux), moonlight with no cloud cover(<1 lux), philla. Abe et al. (1972) estimatedthat there were twilight (1-5 x 10ß lux), and daylight (>5 x 10ß lux). 1,070,000breeding pairs of petrels,but a more recent Time of sunrise, sunset, moonrise, and moonset were estimate is 415,000 (Watanuki 1985b). About 3,500 from the astronomical tables for Kushiro, about 40 pairsof Slaty-backedGulls nested on maritimeslopes, km west of Daikoku Island. 14 The Auk 103: •4-22. January 1986 January1986] MoonlightAvoidance in Leach's Petrel 15

TABLEI. Breedingschedule of the petrel and its nest duties shown by egg-daysand chick-daysin the study plots.

May June July Aug Sept No. of eggslaid 0 335 16 0 0 No. of chicks hatched 0 0 199 64 0 Egg-days 0 5,785 6,305 390 0 Chick-days 0 0 1,965 7,220 6,935

marked by individually numbered rings. Their nests were checkedevery day between4 and 9 July to determine the length of incubation stints. I estimated I I I I i I I I I chick feeding frequency by weighing 15 chicks at 1 2 3 4 5 0900 and 2100 on 9 August, and at 0300 and 1500 on 10 August. A feeding was assumedto have occurred Time after lighting (min.) when a chickgained weight during this interval.This is a minimum estimatesince more than one feeding Fig. I. Number of flying Leach'sStorm-Petrels per may occurin the interval. Night nest attendancein 30 s in relation to time after lighting by two 6-volt the prelaying period was estimatedby checking if lights, one set horizontally and the other about 45ø toothpicksplaced at the entranceof the active bur- upward. The 5-rain experimentwas repeatedthree rows the previousday were moved. times. The significanceof the correlation coefficient(r)' and the differencebetween values was examinedby ANOVA (Sokal and Rohlf 1969). Overlap of petrel and gull activity was assessedby Pianka'soverlap index (Pianka 1973): RESULTS O• = ß xiyi/(g x? ß y?)% Breedingof petrels.--Petrels arrived in late where x, = the number of petrels flying per 5 rain and y• = the number of gulls flying per 5 min. The April and laid eggs between early June and time dispersionof petrel activity between 1930 and early July. Chicks hatched between mid-July 0230 was determined by Morishita's If index (Mor- and mid-Augustand fledged between late Sep- ishita 1959): tember and mid-October.The incubation duty 15 = K[• x,(xi - I)]/2 x,(• x, - 1), as shown by egg-dayswas high in June and July, and the chick feeding duty shown by where xi = the number of petrels flying per 5 rain chick-dayswas high in August and September and K = the number of samplings(K = 15 in this (Table 1). case). Breeding Leach'sStorm-Petrels visited their Predationby gulls.--I estimated the number of nests0.6 times/night (a total of 63 nestsvisited Leach's Storm-Petrelseaten by Slaty-backedGulls from the energyrequirements of gulls and their food out of 105 nestschecked) in the prelaying pe- composition.The food habits of the gulls were de- riod, fed a chick at least 1.2 times/night (n = termined by collecting pellets and food remains at 15),and exchangedincubation duties at 2.9 days their nestingsites. During the study,collections were (n = 17) on the average.These figures agree made every 5 days in 4 study quadratswith a total with those of other observers (Gross 1935, Wil- of 114 nests,yielding 1,143samples. The energyre- bur 1969). quirementsof Slaty-backedGulls were estimatedby Light intensityand activities.--Leach'sStorm- usinga simple modificationof the model of Furhess Petrels arrived at Daikoku Island about 1 h af- (1978).Calculations of energyrequirements and food ter sunsetand left 1-2 h before sunrise(Fig. 2). compositionof the gulls are shown in Appendices1 and 2. The number of petrels killed was calculated The mean number of flying petrels and gulls for 5-day periodsand was combinedmonthly. was related to light intensity (Fig. 3). Petrels Petrelbreeding.--Ten 4 x 10-m study quadratscon- were nocturnal and rarely flew in twilight or taining a total of 351active petrel burrowswere used. daylight on the island. More petrels flew in I checkedthe nestsevery 5 days and recordedthe darknessthan in moonlightin May (F•,• = 6.50, contents.Twenty-five pairs incubatingeggs were P < 0.05) and June (F•,42= 7.79, P < 0.01); re- 16 YUTAKAWATANUKI [Auk,Vol. 103

-10

Sep i!i -2O -10

-20 •

-10

-lO

18 O0 06 18 oo 06 18 oo 06

Time of day (hrs) Fig.2. Daily changesin the numberof flyingpetrels (dark lines) and gulls (light lines)per 5 min. Heavily shadedareas represent dark times,and lightly shadedareas represent moonlight times.Vertical lines indicate sunrise and sunset. duced activity in moonlight also occurredin killed immediately by Slaty-backedGulls and July, although the difference was not signifi- 1 by Jungle Crows.This suggeststhat there is cant (F,,56= 3.26, NS). Petrels did not avoid considerablepotential predation risk in twi- moonlight in August (F,,46= 0.30, NS) and Sep- light and daylight hours. tember (F•,?2= 3.51, NS). The mean number of The time overlap of petrel and gull activity, petrels flying at each light intensity was neg- shownby lnOxy,was positivelycorrelated with atively correlatedwith that of the gulls (r = the length of the period of moonlight (h; r = -0.662, n = 20, F•,•8= 14.4, P < 0.01; Fig. 3). 0.812, n = 15, F•,• = 21.29, P < 0.01; Fig. 4) Slaty-backedGulls rarely flew in the dark when the data of the dayswith no overlap (i.e. (Figs.2, 3). More gulls flew in moonlight than Oxy = 0) were excluded.Encounter-rate indices in darknessin all the months(May: F•,, = 56.13, (number of petrels x number of gulls) were P < 0.01; June: F•,• = 12.33, P < 0.01; July: calculateddirectly from Fig. 3 and were in the F•,s6= 25.3, P < 0.01; August: F1,46= 5.13, P < order moonlight > dark > twilight > daylight. 0.05; September:F•,?2 = 4.09, P < 0.05). Gull I do not have direct evidence of predation un- activity levels were higher in twilight than in der moonlight, although the indirect evidence moonlight in all months (May: F•,7s= 26.96, above corroboratesGross's (1935) finding that P < 0.01; June: Fi,s• = 36.15, P < 0.01; July: mostencounters between gulls and petrels oc- F•,• = 81.96, P < 0.01; August: Fl,•5 = 79.12, cur during moonlight. P < 0.01; September:Fl,7O = 168.71, P < 0.01) Activitypatterns of petrels.•Most petrel activ- and higher in twilight than in daylight in May ity occurredat about2400 or between2200 and (F,,m --- 34.62, P < 0.01) and June (F•,•s= 11.96, 0100on nightswith a new moon,0130 on nights P < 0.01). with a full moon, 2200 on nights with a half- I observedLeach's Storm-Petrels flying in moon rising at midnight, and 0100 on nights daylight 11 times. Six of these petrels were with a half-moon setting at midnight (Fig. 2), January1986] MoonlightAvoidance inLeach's Petrel 17

o o

o o o o o

Jun.

lOO

<• Jul. 43 lOO

5o 1 2 3 4 5 6 7 8 9 10 Moonlight time (hrs)

Aug. 33 Fig. 4. The correlationbetween length of moonlight and activity overlap of petrels and gulls (log•0Oxy).

ratherthan reactingdirectly to the presentlight 1001Sep. 26 intensity. On 13-14 May, a half-moon night 28 46 (moonriseat 2300) with densefog, the peak of ,.r'--• I '•"•:'• I J petrel activity occurredat 0030 rather than 2200, D M T as would be expectedif there had been no fog Light intensity (the data of this day were excluded in the nu- merical analysis).It seems,therefore, that pe- Fig. 3. Mean number of flying petrels (shaded bars)and gulls (open bars) per 5 rain under dark (D), trel activity is not controlledonly by endoge- moonlight (M), twilight (T), and daylight (DL) con- nousrhythms keyed to lunar cycles,as reported ditions. Samplesizes of gulls and petrelsare the same in fruit bats (Morrison 1978). Because of the and are shown above the bars. low visibility on 13-14 May, petrelsmight have found it more difficult to locate the island and thus delayed their arrival. This suggeststhat as has been reported in Cassin'sAuklet (Ma- petrel activity at the colony was influenced by nuwal 1974) and in Leach's Storm-Petrel in light intensity, anticipation of the lunar cycle, California (Harris 1974). The petrels on Dai- and weather conditions. koku Island arrived later on full-moon nights On nights with a full moon, petrel activity (5-6 May, 5-6 June, and 5-6 August) and ear- was highly synchronized in May, June, and lier on nights when the half-moon roseat mid- August but was dispersedin September(Fig. night (10-11 June,8-9 July, and 11-12 August) 2). The synchronyof petrel activity at night than they did on nights without a moon (27- was positively correlated to the length of 28 April, 23-24 May, 19-20 July, and 15-16 Au- moonlighttime in May, June,July, and August gust; Fig. 2). (r = 0.843, n = 14, F•,•2= 29.47, P < 0.01) but On nights in May, June, and July with quar- not in September(Fig. 5). ter or half-moons,the mean number of petrels Predationby gulls.--The number of flying gulls flying per 5 min during darkness(œ = 155.6, remained high in May, June, and August, but n = 42) was significantlyhigher than that (œ= decreasedin September(F4,•3 = 5.45, P < 0.01; 100.0,n = 30) on moonlessnights in the same Table 2) because nonbreeders and unsuccessful months (F•,3o= 37.47, P < 0.01). The petrels, breedersleft the colonyin Augustand Septem- therefore, seemedto anticipate the lunar cycle ber. The total number of petrels was large in and synchronizetheir activity to the hours of May, June,and July, decreasedin August, and darknesson clear nights with a half-moon, was small in September(F4,•3 = 8.59, P < 0.01; 18 YUTAKA¾VATANUKI [Auk, Vol. 103

around the colony is crucial in avoiding gull 2.0 predation. Petrel adults,eggs, and chicksin the nest burrows are protected from gull predation, and petrel foraging areasdo not overlap those of gulls (Pearson 1968, Wiens and Scott 1975). Therefore, only adult petrels flying on or near coloniesare subjectto gull predation. The negative correlationbetween petrel and 1.5 gull activity levels (Fig. 3) indicatesthat predator avoidancemay be the leading causeof petrel nocturnal behavior at Daikoku. Petrels avoided moonlight in May and June when the predationrate was relatively high, but they did not avoid it in Septemberwhen the predation 1.0 I • I I I I I rate was minimal (Fig. 3, Table 3). The partial 1 2 3 4 5 6 7 8 9 10 correspondencebetween seasonal change in predation rates and seasonal variation in Moonlight time (hr•) moonlight effects on petrel activity further Fig. 5. The correlationbetween length of moon- supports this hypothesis. light and synchronyof petrel activity shown by the The medium synchronyof petrel activity on If index. ß = May, ß = June,ß = July, ß = August, nights with half-moons (Fig. 5) probably was and O = September.Regression line a is for the data the result of short periods of darknessin which of May, June,July, and August;b is for thoseof Sep- petrels were free from predation. The greatest tember. activity synchrony occurredon nights with the highest predation risk (full-moon nights in May, June, and August) and the least syn- Table 2). The number of petrels flying at night chrony on the night without predation risk was positively correlated with the dry-weight (September;Fig. 5). Numbers of flying gulls percentageof petrel remains (Appendix 2) in were relatively constant on the full-moon the gull diets sampledin each5-day period (r = nights comparedwith those of petrels (Fig. 2). 0.886, n = 18, Fz,z6= 58.42, P < 0.01). Therefore, highly synchronouspetrel activity Monthly changesin petrel numbers, num- on these nights may dilute the predator's ef- bers killed by gulls, and predation rates are fects by minimizing the risk that a particular shown in Table 3. Gulls ate many petrels in petrel encountersa gull (Wilson 1975: 41-42). May and June, more in July, fewer in August, Nest duties.--Although I could not distin- and few in September,a pattern matching the guish nonbreeders from breeders during ob- seasonalcycle of estimatedgull energyrequire- servations, nonbreeders were inferred to be ments (Appendix 1) and food composition(Ap- abundant in June and July (Ainley et al. 1974), pendix 2). Thus, the monthly petrel kill ap- and probablyin May, becausethe total number pears to vary with gull energy requirements of flying petrels was high in thesemonths (Ta- and availability of the prey. ble 2). Moonlight avoidance behavior disap- peared in August and September; therefore, DISCUSSION only nonbreeders may avoid moonlight. Be- causenonbreeders do not have as many nest Predatoravoidance.--Slaty-backed Gulls are dutiesas breeders, the threat of predationwould the most important predators of adult petrels be a crucial factor determining their activity on Daikoku Island. I estimatedthat annual pre- pattern. Eight of 12 pairs exchangedtheir in- dation by the gulls approximates13.2% of all cubation duties on a clear, moonlit night (8-9 adult petrels. This appearsto be a potentially July). This implies that moonlight does not re- strong selectionpressure on Leach'sStorm-Pe- strict breeders' activities during the incubation trel, whose adult survival rate is high (Hun- period, although more data are needed to sub- tington and Burtt 1970). stantiatethis. Harris (1966) reported moonlight The daily activity pattern of adult petrels avoidance behavior in nonbreeding Manx January1986] MoonlightAvoidance in Leach'sPetrel 19

TABLE2. Monthly changein the meanof the total numberof flying birdsin 24 h.

May June July Aug Sept No. of daysobserved 4 3 4 3 4 No. of petrels(•) 1,330.3 1,516.7 1,493.8 861.3 187.0 No. of gulls(•) 316.5 340.7 350.5 305.7 152.3

Shearwaters, which were killed by Herring avoidance behavior (Harris 1974). On the other Gulls (L. argentatus)and Great Black-backed hand, Audubon'sShearwaters (Puffinus lhermi- Gulls (L. marinus)(Harris 1965). nieri)and GalapagosStorm-Petrels (Oceanodro- On the other hand, seasonalchanges in the raatethys) are diurnal on their breedingisland, nest duties of breeders may contribute to the where the most important predator is a noctur- seasonal changes in the petrel response to nal or crepuscularowl (Asiogalapagoensis; Har- moonlight. Breeding petrels returned to the ris 1969a, b). Thus, although the predator- colony more frequently in the nestling period avoidancehypothesis generally is supported, than in the incubationperiod. The relative im- further investigationson intercolonial and in- portanceof nest dutiesin the prelaying period terspecificvariation of predationrisk and diur- may be small becauseprelaying exodusis sug- nal activity patterns are needed. gestedin Leach'sStorm-Petrel (Gross 1935, Wa- Foraging.--Grubb (1974) suggested that tanuki 1985b). I assumed,therefore, that nest- abundant food in the ocean'supper waters at duty activity was seasonaland correlatedwith night has been a strong selectiveforce toward the growth of chicks(Table 1). High predation nocturnal activity of Leach's Storm-Petrelsat risk and low nest duties seemed to cause breed- sea.Food of Leach'sStorm-Petrels breeding on ers to avoid moonlight in May and June, but Daikoku Island may be most abundant at night increasingnest duties in July might have taken (Watanuki 1985a). The petrels stay on the is- priority over the high predation risk and con- land for part of the night and thus lose some sequently changed breeders' response to opportunities for feeding at night when prey moonlight. is abundant.A pair exchangesincubation du- B•dard (1976) criticized the predator-avoid- ties at night every 2.9 days, and hence it loses ance hypothesis(Cody 1973) becausethere is one night of feeding opportunitiesabout every intensepredation on somealcid coloniesin the 3 days. Missed feeding opportunities seem to Arctic, where daylight is continuousduring the be potentially crucial becausea petrel must re- summer. On South Orkney Island (60ø43'S, cover 11% of body weight lost during an in- 45ø38'W),where the period of darknessis short cubation stint (Watanuki 1985b). In addition, a in summer, the Black-bellied Storm-Petrel (Fre- pair feeds a chick at least 1.2 times per night gettatropica) is preyed on by the diurnal Great and thus losesmore opportunities. Skua (Catharactaskua) and shows nocturnal ac- Sensoryconstraints probably affectforaging tivity (Beck and Brown 1971). Leach's Storm- time. If Leach'sStorm-Petrels rely heavily on Petrels on Kent Island (44ø02'N, 124ø09'W) and visual cuesto locatefood, they would have the on Little River Rock (41ø02'N, 124ø09'W) also advantage of feeding during the day or on a are killed by diurnal gulls and show nocturnal moonlit night. Leach'sStorm-Petrels can locate activity (Gross 1935) as well as moonlight food by smell at night as well as during the

TABLE3. Monthly changein the numberof petrelspotentially available, number killed by gulls, and predation rate.

May June July Aug Sept No. of petrels' 987,700 908,700 760,500 589,300 547,800 No. killed 36,676 29,376 49,065 15,176 321 Predation rate (%) 3.7 3.2 6.5 2.6 0.1 Includesnonbreeders in May, June,and July.Nonbreeders were assumedto be 16%of the total population (Wiens and Scott 1975). Unsuccessfulbreeders were assumedto have left the colony as soon as they failed. 20 YUTAKAWATANUKI [Auk,Vol. 103 day (Grubb 1972).Gordon (1955) suggestedthat GRUBB,T. C. 1972. Smell and foraging in shear- they are nocturnalfeeders, although they also waters and petrels. Nature 237: 404-405. feed during the day (H. Ogi pers. comm.). Di- 1974. Olfactory navigation to the nesting rect testsof Grubb'shypothesis are lacking. In- burrow in Leach's Petrel (Oceanodromaleucor- hoa). Anita. Behav. 22: 192-202. direct evidencesuggests that the petrelsare not HARRIS,M.P. 1965. The food of some Larusgulls. obligatorynocturnal feeders and that abundant Ibis 107: 43-53. prey at night is not always an important factor --. 1966. Breedingbiology of Manx Shearwater affecting the petrels' nocturnal activity at the Puffinuspuffinus. Ibis 108: 17-33. breedingcolony. In conclusion,predation risk 1969a. Food as a factor controlling the and nest duties seem to be more important fac- breeding of Puffinuslherminieri. Ibis 11I: 432-456. tors determining petrel activity at the colony ß 1969b. The biology of storm petrels in the than food availability. GalfipagosIslands. Proc. California Acad. Sci. 37: 95-166.

ACKNOWLEDGMENTS HARRIS,S.W. 1974. Status,chronology and ecology of nesting storm-petrelsin northern California. I thank the Akkeshi Marine BiologicalStation for Condor 76: 249-261. the use of its facilities and H. Uno for his excellent HUNT, G. L., JR. 1972. Influence of food distribution logisticalsupport in the field. H. Abe and A. C. Tho- and human disturbanceon the reproductivesuc- resen made valuable commentson early drafts of the cessof Herring Gulls. Ecology53: 1051-1061. manuscript,and the Boardof Educationof Akkeshi- HUNTINGTON,C. E., •t E. H. BURTT,JR. 1970. Breed- cho gave permissionto work on the island. ing age and longevity in Leach'sPetrel Ocean- odroma leucorhoa. Abstr., 15th Intern. Ornithol. LITERATURE CITED Congr.: 663. MANUWAL,D. A. 1974. The natural history of Cas- ABE, M., N. ICHIDA, H. SHIMIZU, M. HASHIMOTO, O. sin'sAuklet (Ptychoramphusaleuticus.). Condor 76: YUNOKI,N. OZAWA,• S. OZAWA. 1972. [A new 421-431. attempt to estimatethe number of nestingpetrel, MORISHITA,M. 1959. Measuring of dispersionof Oceanodromaleucorhoa leucorhoa,and wildlife of individual and analysis of distribution pat- the Daikoku Island, Hokkaido.] Tori 21: 346-365. terns. Mere. Fac.Sci., Kyushu Univ. Set. E, Biol. AINLEY, D. G., S. MORRELL,& T. J. LEWIS. 1974. Pat- 2: 215-235. tern in the life historiesof storm petrelson the MORRISON,D.W. 1978. Lunar phobia in a neotrop- Farallon Islands. Living Bird 13: 295-312. ical fruit bat Artibeusjamaicensis (Chiroptera: BECK,J. R., & D. W. BROWN. 1971. The breeding Phyllostomidae).Anita. Behav.26: 852-855. biology of the Black-bellied Storm-Petrel Freget- PEARSON,T.H. 1968. The feeding biology of seabird ta tropica.Ibis 113: 73-90. speciesbreeding on the Fame Islands, North- B•DARD,J. 1976. Coexistence, coevolution and con- umberland. J. Anita. Ecol. 37: 521-552. vergent evolution in seabird communities: a PIANKA, E. R. 1973. The structure of lizard com- comment.Ecology 57: 177-184. munities. Ann. Rev. Ecol. Syst. 4: 53-74. BOERSMA,P. D., N. T. WHEELWRIGHT,M. K. NERINI, •t SIBLY, R. M., & R. H. McCLEERY. 1983. Increase in E. S. WHEELWRIGHT.1980. The breeding biology weight of Herring Gulls while feeding. J. Anita. of the Fork-tailedStorm-Petrel (Oceanodroma fur- Ecol. 52: 35-50. cata). Auk 97: 268-282. SOKAL,R. R., & F. J. ROHLF. 1969. Biometry. San BRISBIN,I. L., JR. 1968. A determination of the ca- Francisco, W. H. Freeman and Co. loric densityand majorbody componentsof large THORESEN,A.C. 1964. The breeding behavior of the birds. Ecology49: 792-794. Cassin Auklet. Condor 66: 456-476. COPY, M. L. 1973. Coexistence, coevolution and convergent evolution in seabird communities. WATANUKI,Y. 1984. Food intake and pellet of Black- Ecology 54: 31-44. tailed Gull, Larus crassirostris.J. Yamashina Inst. FURNESS,R. W. 1978. Energy requirementsof sea- Ornithol. 16: 168-169. bird communities:a bioenergeticmodel. J. Anita. ß 1985a. Food of breeding Leach'sStorm-Pe- Ecol. 47: 39-53. trels (Oceanodromaleucorhoa). Auk 102: 884-886. GORDON,M. S. 1955. Summer ecology of oceanic ß 1985b. Breedingbiology of Leach'sStorm- birds off southern New England. Auk 72: 138- Petrels Oceanodroma leucorhoaon Daikoku Island, 147. Hokkaido,Japan. J. YamashinaInst. Ornithol. 17: GROSS,W. A. O. 1935. The life history cycle of 9-22. Leach's Petrel (Oceanodromaleucorhoa leucorhoa) WIENS,J. A., •t J. M. SCOTT. 1975. Model estimation on outer sea islands of the Bay of Fundy. Auk of energy flow in Oregon coastalseabird popu- 52: 328-399. lations. Condor 77: 439-452. January1986] MoonlightAvoidance inLeach's Petrel 21

WILBUR,H.M. 1969. The breedingbiology of Leach's ments were estimatedfrom foraging time, percent- Storm-Petrel Oceanodroma leucorhoa. Auk 86: 433- age of flapping flight, and energy costsof flapping 442. and gliding flight. Foraging times of nonbreeders WILSON,E. O. 1975. Sociobiology,a new synthesis. were not measured, but were assumed to be similar Cambridge,Massachussetts, Belknap Press. to that of breeders.Existence energy requirementsof adults and chicks were estimated from their body weight and the mean daily ambient temperature. APPENDIX 1 I counted the number of adults on breeding sites and surrounding areas about every 3 weeks. About Calculationof energyrequirements of the Slaty-backed 5,000 birds were on the breeding sites in June and Gull.--A model similar to Furness's (1978) was used July. Nest-sitetenacity of parentsduring daytime in to estimate the energy requirement of Slaty-backed these months was about 70%;accordingly, the num- Gulls.Egg-laying, hatching, chick survival, and chick ber of breeders was estimated to be about 7,000. About growth were monitored at 5-day intervals in the 4 2,000 birds were in surrounding areasand were as- study quadrats(Fig. 6). Mean daily energy require- sumed to be nonbreeders at their maximum number meritsfor egg production,chick existence,and chick in late May. growth at 5-day intervals were then estimated.The Other species-specificparameters are: egg weight averagepercentage of time spent foraging away from at laying, 107 g (n = 25); adult weight, 1,100 g (n = the nest site by adultswas determinedby monitoring 1); arrival of last breeder, 1 May; departure of first 10-20 pairs at nest sitesevery 30 rain at night using breeder, 30 July; departure of last breeder, 25 Sep- a starlight scope.Birds away from the nestswere as- tember;date of peak number of nonbreeders,26 May. sumedto be foraging in flight. Seasonalchanges in The following dates were assumed:arrival of first the foraging time of breedersare shown in Fig. 6b. breeder, 1 March; arrival of first nonbreeder, 1 April; Percentageof flapping flight, measuredby scanning departureof last nonbreeder,30 July. Model-specific flying birds for 1 rain every 5 rain in various wind parametersare the sameas in Furhess(1978). conditions,averaged 50%. Breeders'activity require- APPENDIX 2

Foodof Slaty-backedGulls.--Pellets and food re- Laying mains were dried to constant weight at about 20øC • 100t JkA Hatching, • Fledging g50[ /" 90 80

30 /

• •o 6O

Apr, May Jun, Jul, Aug. Sep. Fig. 6. Breeding phenology and energy requirementsof Slaty-backedGulls. (a) Seasonalchanges in egg-laying, hatching, and fledging; (b) number of May Jun Jul Aug Sep hoursof foraging activity per day;(c) energy require- Fig. 7. Seasonalchanges in diet of Slaty-backed ments.The curvesin (c) representthe increasingen- Gulls shown by dry-weight percentages.(a) Adult ergy requirementsof nonbreeders,chicks, breeders, Leach's Storm-Petrel; (b) fish (dark line) and other and the total population. items (broken line). 22 YUTAKAWATANUKI [Auk, VoL 103 for at least30 days.Adult Leach'sStorm-Petrels, fish, fish, 1,500; crab with shell, 700; shellfish with shell, shellfish, crabs, and chickens were the main food 500; chicken, 3,000 (Brisbin 1968, Hunt 1972, Sibly items.The following equationwas used to determine and McCleery 1983).R•s were assumedto be the same the total wet weight of the ith food: W• = (D• x R• x as those of Black-tailed Gulls (Larus crassirostris)and TER)/(• D•R.C,),where D, = dry weight of the ith food, the following: adult Leach'sStorm-Petrel, 5.9; fish, Ri = regurgitation coefficient of the ith food (show- 317.3; crab, 6.7; shellfish, 2.2; chicken, 30.0 (Watanuki ing the ratio betweenwet weight of intake and dry 1984). The number of petrels killed by gulls was weight of pellets), TER = total energy requirement calculatedby dividing the total petrelwet weight by of the gulls for 5 days,and C• = caloricvalue of the the averageindividual adult weight (48 g, Watanuki ith food. Cis (kcal/kg wet weight) were assumedto 1985b).Seasonal changes in the gulls' diet composi- be the following: adult Leach'sStorm-Petrel, 2,600; tion are shown in Fig. 7.

(continuedfrom p. 13) in male Black-headedGrosbeaks; Adan (Hussein)Isack, biology of the GreaterHoneyguide (Indicator indicator); Dr. Pedro Jordano,pattern of fruit use by wintering frugivorous birds and their implications for bird- dispersedplants in Mediterranean habitats;Frank J. Joyce,nest site selectionby three passerineassociates of Hymenoptera; Donald M. Kent, foraging strategiesof Snowy Egrets (Egrettathula) in the salt marshesof Massachusetts;Roni King, winter territoriality in migratory European Robins (Erithacusrubecula)--habitat selectionand winter survivorship;Francis R. Lambert,co-adaptation between frugivorous birds and fig trees in Malaysianlowland forest;Jeanette L. Lebell, microgeographicvariation in the flight whistle of the Brown- headed Cowbird; David E. Manry, a preliminary study of blood and feather pulp proteins in two South Americanibis (Threskiornithidae)species; Jean-Louis Martin, the population structureof Paruscaeruleus L. (Aves):geographical variation and speciationin the Mediterranean region; Jon Miller, breeding distribution and origin of Water Pipits in the Sierra Nevada;David Morimoto, effectsof forest fragmentationon avian communitystructure and species-habitatrelationships in the southeasternMassachusetts Pine Barrens;Jay Pitocchelli,speciation in the genus Oporornis;Richard O. Prum, courtshipbehavior and ecologyof Masius chrysopterusin Ecuador;Dr. Michael R. W. Rands,the breedingbehavior and habitatof the Arabian Bustard, Ardeotisarabs; Pamela C. Rasmussen,relationships of Fuego-PatagonianBlue-eyed Shags; Mark D. Reynolds, socialbehavior of Yellow-billed Magpies;Dr. Gary Ritchison,the significanceof song repertoiresin the Northern Cardinal; Jeffrey A. Schwartz, vocal similarity in mated Ring-billed Gulls as an aid in parental recognitionby young; Dr. Ron Scogin, floral color and hummingbird vision; Peter E. Scott, the nesting ecologyof desert hummingbirdsin relation to the pollination ecologyof certain nectar plants; Patricia Serrentino,the breedingecology and behaviorof the Northern Harrier (Circuscyaneus) in CoosCounty, New Hampshire;Laurie J. Stuart-Simons,food limitation in birds;Richard R. Snell, hybridization,isolation and speciesrecognition in arcticgulls; Carol Spaw, thick-shelled eggs and their evolutionaryimplications; Charles Sullivan, nest behaviorand developmentof young in the JabiruStork; Dr. Kimberly A. Sullivan, energetics and the development of time-budgeting;Richard John Watling, investigation of statusof Ogea Flycatcher and Blue-crownedLory; Dr. G. CauseyWhittow, water lossfrom eggsof Great Frigatebird(Fregata minor); Dr. David Winkler, a general model of parental care with experimentaltests on the Tree Swallow; John L. Zimmerman, Ortstreuein Henslow's Sparrows (Ammodramushenslowii) and movementsin responseto spring burning of tallgrassprairie.