Age of First Breeding in Common Murres

January1994] ShortCommunications andCommentaries 207

lich Ethologie und Psychologie der Anatiden. NUECHTERLEIN, G. L., AND R. W. STORER. 1985b. In- Pages598-702 in ProceedingsV International Or- terspecificaggression in steamer-ducks.Condor nithological Congress.Berlin, 1910. 87:568. KEAR,J. 1972. Reproductionand family life. Pages RAVELING,D.G. 1970. Dominancerelationships and 80-124 in The swans (P. Scott, Ed.). Houghton agonistic behavior of Canada Geese in winter. Mifflin, Boston. Behaviour 37:291-319. KILFIAM,L. 1958. Territorial behavior of wintering SAVARD,J.-P. L. 1982. Intra- and interspecific com- Red-headed Woodpeckers.Wilson Bull. 70:347- petition between Barrow's Goldeneye and Buf- 358. fiehead. Can. J. Zool. 60:3439-3446. KRUUK,H. 1967. Competition for food between vul- SAVARD,J.-P. L. 1984. Territorial behavior of Com- tures in East Africa. Ardea 55:171-203. mon Goldeneye,Barrow's Goldeneye and Buffie- LIMPERT,R. J., H. A. ALLEN,AND W. J. L. SLADEN. 1987. head in areasof sympatry. Ornis Scand. 15:211- Weightsand measurementsof wintering Tundra 216. Swans. Wildfowl 38:108-113. SAVARD,J.-P. L., ANDJ. N. SMITH. 1987. Interspecific LIVEZEY, B. B., AND P.S. HUMPHREY. 1985a. Territo- aggressionby Barrow'sGoldeneye: A descriptive riality and interspecific aggression in steamer- and functional analysis. Behaviour 102:168-184. ducks. Condor 87:154-157. SCOTT,D. 1977. Breeding behaviour of wild Whis- LIVF,ZEY, B. B., AND P.S. HUMPHREY. 1985b. Inter- tling Swans. Wildfowl 28:101-106. specificaggression in steamer-ducks.Condor 87: STEPHENS,M. L. 1984. Interspecific aggressivebe- 567-568. haviour of the polyandrousNorthern Jacana.Auk MAcROnERTS, M.H. 1970. Notes on the food habits 101:508-518. and fooddefense of the AcornWoodpecker. Con- VERNER,J. 1975. Interspecific aggression between dor 72:196-204. Yellow-headedBlackbirds and Long-billedMarsh McKINNEY,F. 1965. Spacingand chasingin breed- Wrens. Condor 77:328-331. ing ducks. Wildfowl 16:92-106. WALTERS,J. 1979. Interspecific aggressivebehavior MURRAY,B.G. 1971. The ecologicalconsequences of by Long-toed Lapwings. Anim. Behav. 27:969- interspecificterritorial behavior in birds. Ecology 981. 52:414-423. NUECHTERLEIN,G. L., AND R. W. STORER.1985a. Ag- Received4 December1992, accepted17 November1993. gressivebehavior and interspecifickilling by Fly- ing Steamer-Ducksin Argentina. Condor 87:87- 91.

The Auk 111(1):207-209, 1994

M.P. HARRIS,' D. J. HALLEY,2 AND R. L. SWANN3 •Instituteof TerrestrialEcology, Hill of Brathens, Banchory,Kincardineshire AB31 4BY, UnitedKingdom; 2Departmentof Biologyand Pre-clinicalMedicine, University of St. Andrews, Fife KY16 9TS, United Kingdom;and 314St VincentRoad, Tain, Ross-shireIV19 1JR,United Kingdom

An accurateassessment of the age of first breeding 6ø35'W) on the Atlantic coast. On the former island, is an essentialprerequisite for the formulation of pop- data reported here were collectedfrom 1981 through ulation models.Relevant quantitativedata, however, 1992,a period during which numbersdeclined slight- are difficult to collect in specieshaving long periods ly. On Canna, the population more than doubled be- of delayed maturity. We report on the ages of first- tween 1974 and 1983, but then declined slightly be- recorded breeding of Common Murres (Uria aalge) fore stabilizing from 1987 through 1992. obtained from long-term studiesat two Scottishcol- Methods.--On the Isle of May, murre breeding onies:the Isle of May, Firth of Forth (56ø11'N,2ø33'W) ledges are accessible,and chicks have been banded in the North Sea; and Canna, Inner Hebrides (57ø03'N, in small numbersfor over 30 years.Banding has been 208 ShortCommunications and Commentaries [Auk, Vol. 111

TABLE1. Common Murres of known age and breeding statuson the Isle of May, 1991 and 1992.

Age (years)

4 5 6 7 8

60' 1991 Number seen • 21 55 18 19 7 Definitely breedingb (%) 0 18 55 42 43 40 Definitely had never bred (%) 95 75 44 37 14

1992 20' Number seen '• 68 16 35 14 15 Definitely breedingb (%) 1 13 51 71 80 Definitely had never bred (%) 99 75 34 14 13 0

Breedingstatus of a few individuals was not determined. Age (years) Includes a few individuals that bred in a previous year, Fig. 1. Ages of first-recordedbreeding of 321 known-agedCommon Murres (histogram,left axis), carried out in a regular and systematicmanner since and accumulatedpercentage of 152 individuals from 1981,with chicksbeing markedwith numberedhard- 1974-1980 cohortsfound breeding by any age (filled metal bands (which display the number upright on circles,right axis) on Canna. both sidesof the leg) and most with an additional plain or individually numbered year-specificcolor band. With practicethe numberedbands can be read one-halfthe six-year-oldsseen at the colonybred, or with a telescopeat rangesup to 60 m. However, the had bred in a previousseason, and about one-third birds are relatively tame and, normally, can be ap- definitelydid not breed(Table 1). Three (14%)of 22 proachedmuch more closely. Careful searchesfor eight-year-oldswere not breedingand, as far as we banded birds were made daily during each breeding knew, had never bred. season1981-1992. The breedingstatus of eachbanded Both median and modal agesof first breeding of bird located was recorded. In 1991 and 1992, almost the 321 known-aged individuals on Canna were sev- daily checkswere made of the sites held by most en years.The range was 3 to 15 years(Fig. 1). Con- known-agedbirds (regardlessof whether or not the sideringjust the 1974-1980cohorts, most surviving birds had previously bred there) and where non- membersof which have now probably recruited to breedersgathered. Thus, we havedetails of the birds the population,the medianage of recordedbreeding of eachage seen at the colonyeach season, including was eight years and the mode nine. There was no their breeding status.A few birds could not be so evidencethat the modalage of firstbreeding changed classified.Some murres were sexed based on mating systematicallyduring the study.It was:7 yearsin the behavior. cohort from 1977 (n = 31 individuals), 1978 (26), 1982 On Canna, breeding areaswere only visited once (43), 1983(37) and 1984 (36); 9 in 1975 (21), 1976 (22), eachyear 1974-1992.During thesevisits chicks were and 1980(33); and 10 in 1981(15). For the 1974cohort bandedand attemptswere made to catchbanded birds (13),modal age was either six or eightyears. The 1979 that were incubating or brooding. cohort producedonly four breeding records. The age of a bird is given here as the number of Discussion.--Resultsfrom both study colonies in- calendaryears after banding (e.g. a chick hatchedin dicatea considerablerange in the ageof first breeding 1982and found breedingin 1986is describedas being of Common Murres. Thus, while some birds were fouryears old). The term cohort is applied to all chicks sexuallymature and bred when they were threeyears banded in a given year. We equatethe first record of old, othersdid not breed until at leastnine years.The breedingas the first instanceof breeding,although medianage of first breedingappeared to be one or some birds could have bred undetected one or more two yearsearlier on the Isle of May than on Canna, years before. but this may havebeen an artifactdue to differences Results.--We found 42 known-age individuals in methodology.The mostobvious potential bias in breeding on the Isle of May. Their ages at first re- our estimatesconcerns the degreeto which we failed cordedbreeding were three years(1 bird), four years to detect an individual on the first occasion that it (1), five years(15), six years(16), sevenyears (3), eight bred. Such errors undoubtedly occurred since many years (4) and nine years (2). The median age and murresbreeding for the first time frequentlylose modal age were both six years.The recordsincluded their eggsoon after laying (de Forest1993, pers. obs.). femalesbreeding at five years(five birds)and sixyears This effect could have contributed to the higher age (two), and malesat five years(one) and six (four). The of first breedingrecorded on Canna,where our ob- difference between the sexes was not significant servation schedule was minimal. Both these estimates (Fisher exact test, P = 0.24). In 1991 and 1992, over are higher than thosenormally usedin population January1994] ShortCommunications andCommentaries 209 models for the species(e.g. five years;Birkhead and tality than do adults,but we interpret suchlarge dif- Hudson 1977, Hatchwell and Birkhead 1991). ferencesas being due to some birds residing, and The best method of obtaining an accurate estimate probably breeding, in areas where we could not ob- of the age of first breeding for a speciesis to follow serve them. We see no method of reducing these bi- a series of cohortsuntil all surviving members have ases. bred. In the Common Murre such a study would re- Acknowledgments.--Wethank a great many people quire a minimum of 15 to 20 seasonsof intensive for help with the fieldwork and the ScottishNatural fieldwork. Even then, there will be problems of in- Heritage, J. L. Campbell, and the National Trust for terpretation if there are marked differencesin the Scotlandfor allowing us to work on the two islands. survival of different cohorts.The postfledging sur- D. J. Halley was supportedby a studentshipfrom the vival of chicks reared on the Isle of May does vary JanetThompson Anderson Trust. S. Wanlessand two from year to year, but resightingsof breeding birds referees greatly improved the manuscript with crit- spannedseven cohorts, some of which had high sur- icisms.Work on Canna receivedfinancial supportfrom vival rates, whereas others had low survivorship the ScottishOrnithologists' Club and the Joint Nature (Harris et al. 1992, unpubl. data) Any bias, therefore, Conservation Committee, whereas that on the Isle of should have been reduced. The Canna results should May was part of an integrated seabird monitoring be lessaffected. Ideally, observationsshould be made programme funded by BP Exploration, ScottishNat- both in the colonywhere the chickswere bandedand ural Heritage, the Department of the Environment, elsewhereto determine whether agesof first breeding and the Joint Nature Conservation Committee. differ betweenbirds returning to the natal colonyand those which emigrate. This is rarely possible. LITERATURE CITED On the Isle of May we minimized the bias caused by birds losingeggs soon after laying by determining BIRKHEAD,T. g., ANDP. J. HUDSON. 1977. Population whether or not particular individuals bred by daily parametersfor the Common Guillemot Uria aalge. observations, but this introduced another bias in that Ornis Scand. 8:145-154. it was undoubtedly easier to see a nonbreeder at the DEFOREST, L.N. 1993. The effectsof age, timing of edgeof the colonythan a broodingor incubatingbird breeding, and breeding site characteristicson the in the center. Breeding birds, therefore, may have reproductive successof the Thick-billed Murre, been underrepresented in our samples. Some non- Uria Iotavia. M.Sc. dissertation, Univ. Ottawa, Ot- breeding birds were subsequentlynever seen again. tawa, Canada. A proportion of these may have died, but somepre- HARRIS, M.P., D. J. HALLEY, AND S. WANLESS. 1992. sumablymoved to other coloniesor out of sight. For The post-fledgingsurvival of young Guillemots instance, of the 1986 cohort, 72 individuals were re- Uria aalgein relation to hatching date and growth. corded in 1990, 55 in 1991, and 35 in 1992, figures Ibis 134:335-339. that suggest annual survival rates of 76% between HATCHWELL,B. J., AND T. R. BIRKHEAD.1991. Popu- 1990 and 1991, and 64% between 1991 and 1992. The lation dynamicsof Common Guillemots Uria aalge survivals of breeding adults over these two periods on Skomer Island, Wales. Ornis Scand. 22:55-59. were 92% and 95%, respectively (unpubl. data). Im- matures could well have a greater overwinter mot- Received10 February1993, accepted11 May 1993.