Reproductionand Survivalin a Declining Population of the Southern Dunlin Calidris alpina schinzii Paul Eric J6nsson
J6nsson,P.E. 1991.Reproduction and Survival in a DecliningPopulation of theSouthern Dunlin Calidrisalpina schinzii. Wader Study Group Bulletin 61, Supplement: 56-68. Reproductivesuccess and survival was studied in a declining population of theSouthern Dunlin in SW Sk,Sne,SSweden, in 1981-1986.Hatching success was low; on average only 30% of clutches hatched, dueto intense nest-predation from mainly Crows, Foxes and mustelids. Some nests (8%) were also lostby tramplingfrom grazing animals. Within a season,there was no significantdifference in hatchingsuccess between early and late clutches, but the overall annual hatching success was higher in yearswith an early onset of breeding.On average,the Dunlins yearly produced 0.3 - 0.4 hatched young/adult.Fledging success was estimated at36% and survival from fledging tothe age of one year at56%. Adult survival, estimated from rerum rates, averaged 89% in males and 77% in females (83% of thesexes combined). The sex-relateddifference in return-ratesbelieved to reflecta loweractuai survivalrate in females, assuggested bythe male-skewed sex-ratio inthe population. The average life expectancyfor an adult Dunlin is estimated at5.4 years and the mean longevity at7.4 years. Survival tendedto decrease with increasing age (up to 5 yearsafter ringing). The oldest bird recorded in the studywas 17 yearsold. The observed reproductive rate was found to be insufficientto maintaina stablepopulation and unless nest-predation issignificantly reduced, the Dunlin will become extinct in SW Sl•ne within20 to 30 years. PaulE. J6nsson,Dept. of Ecology, Ecology Building, University of Lund, 223 62 Lund, Sweden.
Introduction Study area and methods
On the south-easternedge of its distribution, Thestudy was performed in theyears 1981-86, alongthe coast of theBaltic andthe North Sea, on the coastalwet meadowsof SW Skfine, the SouthernDunlin Calidrisalpina schinzii SouthSweden (Figure 1). Duringthe study hasdecreased markediy in numbersduring the period, these meadows held the bulk of the past50 years(Gromadzka 1983, Cramp& Sim- Swedishschinzii-population; 60-80 pairsout mons 1983, Piersma 1986). The main mason of anestimated total of about250 pairs (Tjern- for thisdecline is believedto bethe rapidly di- berg1985). Due to alteredmanagement (abo- minishingareas of suitablebreeding habitat, lishedgrazing and hay-cutting) and exploita- i.e. wet meadowsand pastures, grazed by do- tion (expansionof urbanand industrial areas) mesticanimals, or otherwiseharvested by man of wetmeadows, suitable breeding habitats for (Larsson1969, Dybbro & JOrgensen1971, Dunlins and other waders have become scarce Soikkeli&Salo 1979,Emanuelsson & Kjel16n andfragmented in the whole of SouthSweden 1981, Hansen 1985, Tjernberg 1985, Kr61 (Emanuelsson& Kjell•n 1981, Tjernberg 1986). 1985).The totalarea of typicalDunlin-habitat in SW Skfineis, at thetime of writing,less than In thispaper I presentthe results of a six-year 300 ha, most of which is situated around the studyon a decliningpopulation of theSouthern shallow Foteviken Bay (about 55ø17'N, Dunlin in SW Skfine, South Sweden. Data on 12ø58'E). reproductiverate and survivalare presented, andthe possible factors controlling the popula- Themain study area, Vellinge •ingar (South), is tion parametersand maintenance of thepopu- locatedon theeastern shore of FotevikenBay lation are discussed. (Figures1 and2) andis comprisedof approx. 60 haof wet,intensively grazed shore meadow.
WSG Bulletin61, Suppl.: 56-68 (April 1991) J6nsson: Dunlln 57
Table 1. Mean population size and density of breedingDunlins in SW Sk•ne, 1981-1986. Number Maximum Area of breeding (ha) pairs density (pairs/ha) Main study area Vellingelingar (S) 60 14-24 0.40 Control areas VellingeItngar (N) 58 15-20 0.35 Eskilstorps1tngar 55 10-25 0.46 Kungstorp 9 1-2 0.22 Hammars Nlts 33 3-5 0.15 Ar16vs1tngar 44 3-4 0.09 Jtmgsniiset 10 1-2 0.20
the visits to the control areas were to estimate the numberof breedingbirds and to checkfor andidentify marked individuals, however, I al- so ringed a few adults and chicks. In 1983- 1985, the main studyarea was expandedto in- cludealso Vellinge gngar (North) and.•ngsng- set.
i THESAœTI• SEA Nests were searchedfor by careful investiga- tions of suitableparts of meadows,and by Figure 1. Locationof the studyareas (in black) on the coast of SW SloPe, South Sweden.
Fields
N This locality still holdsone of the largestcon- centrationsof breedingDunlins in Sweden(15- 25 pairs during the studyperiod). The areais FOTEVIKEN BAY separatedfrom thenorthern part of Vellinge'•'a- gat by a ditchand a small(140 x 80 m) treeplan- 0 50 tO0 1SOrn I Fields tation. However, grazing animals can move freelyover the whole area, totalling almost 120 ha. The meadowsare grazed annually, from late May to well into October,by 150-250 cattle • FeslucaLollurn perenne-veg.rubraI and 50-80 riding horses.A furtherdescription !!i}::i[!:'Juncus gerardii-veg. of the areais givenin J6nsson(1985). .:'!•?' ! Reedbeds :.'.?. Fields The control areas consistedof the remaining Dunlin localities in SW Skfine(Table 1), most L• Baresand of which are located within 4 km of the main studyarea; although two localities, •ngsngset and Ar16vs, are more distant (8 and 22 km, re- spectively)(Figure 1). The Danish island of Saltholm,which in 1980 held up to 15 pairsof Dunlin (Hansen1985), is situated25 km NW of Vellinge th'agar,but was for practicalreasons not includedin this study.
In the main studyarea, my aim was to capture andmark asmany birds as possible, and to find Figure2. Map of the mainstudy area, Vellinge •tngar all nestsin everyseason. The main objectives of (S), showing the distributionof some important vegetationtypes.
WSG Bulletin 61, Suppl.: 56-68 (April 1991) 58 J6nsson: Dunlln watchingthe behaviour of thebirds. During the tions (see Soikkeli 1973, Ferns & Green 1979 firstperiod of egg-laying,from mid-April until and J/Snsson1987). mid-May,nests were relatively easy to findjust by searchingon the dry spotswith low vegeta- On everyvisit to thestudy areas, colour-ringed tion;i.e. the only placeswhere it waspossible birdswere lookedfor and, if possible,identi- for the birdsto lay their eggsat that time. Later fied usingbinoculars (10x40) and a telescope in the season,with virtually the whole area (20-40 x 60). With favourablelight conditions, driedup andwith highervegetation, nests were colour-ringscould be read accuratelyat a di- more difficult to find (see JOnsson1985). On stanceof 150to 200meters, using the 40x mag- averageI estimatethat 75-95 % of all nestsin nif'lcationocular on the telescope. the main studyarea, were foundannually.
All nestsand broodswere plottedon a field Results map,and their fate subsequentlyrecorded. The hatching date of incubated broods was Nesting success estimatedby immersingthe eggsin water(see Westerskov (1950) and van Raasen et al. The frequencyof clutchesthat surviveduntil (1984). Nest losseswere recorded as well asthe hatching,varied betweenyears from 16% in probable causesfor these losses(predation, 1983, to 38% in 1984, averaging30% for the tramplingor desertion).In predatednests, signs whole studyperiod (Table 3). of the predatorwere looked for. Thus,a largemajority of the clutcheslaid each Adult birds were caughton the nestduring in- year weredestroyed or deserted.The mostim- cubation,using walk-in-traps (see Bub 1974), portantfactor affecting nest survivalis preda- while chickswere caughtby hand,as soonas tion. In total, 60% of all clutches were taken, possibleafter hatching. completelyor partially,by predators.In most casesno signof the predatorwas found, but All birds caughtwere ringed with one metal- sometimesegg-shell fragments showed the ring and 2-5 colouredplastic rings. A unique characteristic marks of Crows Corvus corone combinationof colour-ringswas usedfor each or small Mustelids(most probablyStoat Mu- single individual. Table 2 gives the annual stelaerminea). Another important predator on numbersof ringedbirds in the main studyarea Dunlin eggswas probably the Red Fox Vulpes and the control areas. vulpes,which was seen regularly patrolling the meadows for waders' nests (see also J/Snsson Sex, in adultbirds, was determinedfrom plu- 1985). magecharacters and measurements, in combi- nation with subsequentbehavioural observa- Partial predation,i.e., where just one or two
Table 2. The annual numbers of Dunlins trapped and ringed in the main study area and the control areas.Left figure = adults,right figure= young.
1981 1982 1983 1984 1985 1986
Vellingeartgat (S) 43/31 14/18 9/3 1/6 5/6 1/- Vellinge•tngar (N) -/- 1/- 15/7 23/28 10/25 3/3 Eskilstorps -/- -/- 2/7 -/- -/- -/- Kungstorp -/- -/- -/- -/- -/- 3/- Hammarsnlts -/- -/- -/- -/- -/- -/- ArlOvs•tngar -/- 1/3 1/- -/- 2/2 1/- •ngsn'aset -/- 2/4 1/4 -/- -/- -/- Total 43/31 18/25 28/21 24/34 17/33 8/3
WSG Bulletin61, Suppl.: 56-68 (April 1991) J6nsson: Dunlln 59
Table 3. Nestingsuccess in a Dunlin populationin SW Sk•ne in 1981-1986.Figures refer to clutches. Hatched Predated Trampled Deserted Total n (%) n (%) n (%) n (%) n (%) 1981 13 (36.1) 20 (55.5) 2 (5.6) 1 (2.8) 36 (100) 1982 8 (28.5) 18 (64.3) 1 (3.6) 1 (3.6) 28 (100) 1983 6 (16.2) 26 (70.3) 4 (10.8) 1 (2.7) 37 (100) 1984 12 (37.5) 18 (56.2) 2 (6.3) 0 (0) 32 (100) 1985 12 (29.3) 21 (51.2) 7 (17.1) 1 (2.4) 41 (100) 1986 9 (31.0) 19 (65.5) 0 (0) 1 (3.5) 29 (100) Total 60 (29.6) 122 (60.0) 16 (7.9) 5 (2.5) 203 (100)
,_ eggswere taken from the clutch,accounted for = 11.3,0. l>p>0.05, df = 6) differencebetween nearly7% of thenest losses. Generally the birds theobserved survival rates and those expected abandonedthe remainingeggs after a partial with an equal survival rate over the whole predationhad occured, but in two casesincuba- period.This periodof increasednest predation tion continued and the reduced clutches coincidedwith theannual release of thegrazing hatchednormally. herdsof cattle and horses,in the last days of May (J6nssen1985). The seasonalchanges in nestsurvival, with re- gard to predation,are expressedas daily sur- Lossesdue to tramplingby grazing animals vival ratesaccording to themethod developed representeda minor part, only 11%, of thetotal by Mayfield (1961, 1975) andJohnson (1979). nestlosses (Table 3). However,in singleyears, Daily survivalrate may be definedas the prob- e.g. 1985,trampling may destroymany clutch- abilty,P, thata nestpresent one day would sur- es.I haveno data to separatetrampling by cattle vive to the next day.It is estimatedfrom thefor- from thatby horses,but experiencefrom other mula: Daily survival rate P = A/(A+B) 1.00 ß whereA = totalnumber of'nest days' (the sum of all dailytotals of nestspresent during the ob- servationperiod), and B is the total numberof nestslost. The day on which a nestwas lost is p 0.95 not countedas a nestday and nestswere as- sumedto havebeen lost half way betweenthe lasttwo checks. Data were grouped weekly du- -700 ring the nestingperiod and P-values given for - 600 0.90 - 500 Nest- eachweek are presentedin Figure3. The pat- days tern emergingshowed a high survivalrate du- - 400 ringweek 1 (15-20April), with virtually no los- ses,then a markeddecrease during week 2 and - 200 3, with P falling to 0.94. During weeks4-6 (6- iß • --1003000 26 May), the daily survivalrate was rather stab- 1 2 3 4 5 6 7 8 le, around0.97, but droppedto a clearlylower f Week Grazing animals level (0.92) in week7 (27 May-2 June).The va- released riationfound in daily survivalrate (P), is signi- Figure3. Daily survivalrates of Dunlins'nests and ficantly differentfrom that expectedwith an eggs in different weeks, during egg laying and equalrateover the whole nesting period (Chi2= incubation.P-values (dots) are calculated for lossesby 26.9, p<0.001, df = 7). Excludingdata from predationonly and given within 95% confidence intervals.Hatched columns = numberof nestdays per week7, thereis still an almostsignificant (Chi 2 week.
WSG Bulletin 61, Suppl.: 56-68 (April ] 091) 60 J6nsson: Dunlin meadows in SW Skgtne indicates that horses, breedingarea very soonafter fledging,making due to their more lively behaviour(especially a directcontrol of fledgingsuccess impossible. when in large groups),cause more trampling To getan estimate of fledgingsuccess, I used an lossesthan cattle (Andell & J/Snsson1986). indirect method, on the basis of the return rate of birdstinged as chicks and later controlled in In the rather few cases where whole clutches the studyarea. During the years1981-1985, a have been deserted,it is likely that one of the totalof 144 chickswere tinged. Out of these,24 parentbirds have died. (16.7 %) werecontrolled 1-5 yearslater (Table 5). The numberof birds alive one year after hatching,N 2,may be calculatedaccording to Hatching success the formula of Soikkeli (1970a):
Hatchingsuccess measured as the frequency of =2-•n (100/S)x-1 (1) hatchedclutches, was on average29.6% (Table N1x= 1 x 3). Comparingfirst clutcheswith replacement where m is the number of birds found for the and second clutches, the frequency of first time when x yearsold, and S is the mean successfulclutches was 26.9% (n = 1982) and annualsurvival rate after the ageof oneyear. 32.4 (n = 34), respectively.The differencewas Using the data in the tight-hand column of not statisticallysignificant (Chi2 = 0.43, p<0.5, Table 5, andassuming that S duringthe second df = 1). Neitherwas there any significant differ- yearof life is the sameas during later years, i.e. encein hatchingsuccess between early and late 83 %, then N• = 29, or 20.1% of the tinged clutches;hatching frequency being 26.0% for chicks. clutchescommenced before the 5th of May (median date for start of egg-laying in all Each year, a smallernumber of independent clutches),and 27.6% for clutcheslaid after this fledglingsare observed before departure. In the date (Chi2= 0.0.2, p<0.8, df = 1). years1981-1985, 18, or 12.5 %, of thetinged chicks were seen as fledglings, whereas 8 The proportionof eggslaid thathatched, varied (44.4%)were recovered in lateryears. The total between15.5 % in 1983 (n= 103)and28.6 % in numberof tingedfledglings, Nf, may be calcu- 1985 (n =91), with an overall totalof 23.7 % for lated from the following equations: the years 1981-1985 (n = 514). In clutches ,nfo , whichwere closelyinspected during hatching, Nf: NiI•----} (2) 116eggs(94 %) hatchednormally, while 8 eggs lfo (6%) were either infertile (n = 6) or desertedbe- wherenfo is the numberof observedfledglings, fore the chick was hatched(n = 2). and N•fo the number of observedfledglings
Measuringhatching success as the numberof Table 4. Hatching successin a Dunlin population chicks hatchedper adult bird showsthat the in SW Sk/tnein 1981-1986.1 = main studyarea, 2 populationin the main studyarea produced on = main studyarea + control areasVellinge iingar average0.34 youngper adultand year. Includ- (N) and•ngsn•iset (cf. Fig.ure 1) ing datafrom two of thecontrol areas (Vellinge Adults Successful Young Young •ingarN and'Angsn•iset), which were thor- present nests hatched per adult oughlyinvestigated in 1983-1985,the overall hatching successwas 0.40 young per adult 1 2 1 2 1 2 1 2 (Table 4). 1981 50 14 - 40 0.80
1982 47 6 - 18 0.38
1983 47 86 2 6 4 17 0.09 0.20 F!edgingsuccess 1984 37 77 3 13 10 35 0.27 0.46 Due to the crypticcolouration and elusive be- 1985 35 75 3 11 9 34 0.26 0.45 haviour,Dunlin chicks are very difficult to find 1986 38 2 - 6 0.16 andobserve after they have left thenest (see al- so Heldt 1966, Holmes 1966, Soikkeli 1967). Furthermore, the young normally left the Total 254 373 30 52 87 150 0.34 0.40
WSG Bulletin61, Suppl.: 56-68 (April 1991) Jbnsson: Dunlin 61 alive after one year. We know from abovethat The reasonsfor chick mortality during the N1 -- 29 and nfo= 18. As with N1, we get from fledgingperiod are not known in detail,but I am equation1 thatNifo = 10. Thus,Nf= 52.2, which surethat many youngare takenby predators, gives us an estimated fledging successof primarily Crowsand Red Foxes.Small chicks 36.3 % (= 52.2/144). (less than a week old) are also vulnerable to longerperiods of cold and wet weather,when To use this method for calculatingfledging food availability is reduced and permanent success I also assume that survival in the first broodingby the parentsis neededto maintain year is the samefor thosefledglings actually the bodytemperature. A few chicksprobably observed,as for thosenot observed. At present, alsodie eachyear due to tramplingby grazing there is no reason to believe that this should not animals. be the case.
A possiblesource of error,which may resultin First-year survival a toohigh estimate of fledglingsuccess, is that somebroods may succumbbefore the chicks An estimateof thepost-fledging survival of the are ringed. However, during the years 1981- first year of life was obtainedfrom the return 1986, I have only registeredthree suchcases rateof birdsringed as chicks. Following the cal- wherethe newly-hatchedbroods were killed by culationsmade above (see under 'Fledging predatorsbefore ringing and if these chicks success'),assuming that the birds observedas were includedin the analysis,estimated fledg- fledglingsare representativefor all young, ing successwould be modifiedto a smallextent first-year survival was estimated at 55.6 % only, to 33.5 %. A more serioussource of error (Nlfc•fo x 100). Most birds were recovered may affect my estimateof fledging successif whentwo yearsold (Table 5), whichis the age thereturn rate of first-breedingbirds is signifi- at which they usuallybreed for the first time cantlylower thanthe actualrate of survival.It is (Heldt 1966, Soikkeli 1967).Breeding by first- reasonableto believe that someof the young year birds was only confirmed in three cases disperseto breeding sites outside the study (all females), although some young males area. However, since nearly all the suitable establishedterritories each year andtried to at- breedingareas in the regionwere checkedfor tract females. ringedbirds annually, the numberof live birds missedmust be limited. This is supportedby Among the 24 birdsringed as chicksand later the fact that Soikkeli (1970b) in Finland found recoveredin the studyarea, 17 were malesand no caseof first breedingDunlins havingdis- only 7 females.The differenceis closeto stat- persedoutside a radius of 5.0 km from thebirth- isticallysignificant (p = 0.06, two-tailedbino- place. mial test).
Table 5. Age when recoveredfor the first time and survivalup to the age of one year in Dunlins ringed as chicks in SW Sk•ne in 1981-1985. M=males, F=females.
Year of ringing
Age when first 1981 1982 1983 1984 1985 Total recovered M+F M+F M+F M+F M+F M+F
1 2+1 0+0 0+0 0+0 4+0 6+1 2 4+0 2+ 1 1+ 1 2+2 - - 9+4 3 0+0 1+0 1+0 .... 2+0 4 0+1 0+0 ...... 0+1 5 0+1 ...... 0+1
Tot. recov. 6+3 3+ 1 2+ 1 2+2 4+0 17+7 No. ringed 31 25 21 34 33 144 Surv. 1 year (N1) 11 1 4 5 4 29 Surv. 1 year (%) 35.5 20 19.1 14,7 12,1 20.1
WSG Bulletin 61, Suppl.: 56-68 (April 1991) 62 Jbnsson: Dunlin
Table 6. Annual return rates of adult Dunlins in $W Sk•ne 1981-1986.
Males Females
Individuals Individuals Individuals Individuals presentin returning Return presentin returning Return Total return previous following rate previous following rate rate year year year year
1981-1982 24 22 0.92 24 18 0.75 0.83 1982-1983 35 31 0.89 29 26 0.90 0.89 1983-1984 44 38 0.86 42 34 0.81 0.84 1984-1985 52 45 0.87 46 33 0.72 0.80 1985-1986 56 52 0.83 44 31 0.71 0.83
Total 211 188 0.89 185 142 0.77 0.83
First year survival of early-born chicks (1982/83) wasthe returnrate higherin females (hatchedbefore 31 May) did not differ from than in males: 90 % vs. 89 %. thatof late-bornchicks (hatched after 31 May); thereturn rate of younghatched early in thesea- In both males and females, the annual return son was 16.7 % (n ringed = 84), as compared rate tends to decreasewith increasingage with 17 % for late-bornones (n ringed= 59). (Table 7). When expressedas a linearregres- sion(Figure 4), thisrelationship was statistical- ly significantin bothmales (r =-0.86, p<0.02,n Adult survival = 5) and females(r = -0.95, p<0.01, n = 5). However,when using a Chi2-testand, compar- Adult survival was estimated from the return ing theobserved proportions of returningbirds rate to the studyarea of ringedbreeding birds. with those expectedfrom a null-hypothesis As in all migratorybirds, the reliabilityof sur- (predictingno differencein returnrate with in- vival ratesderived from returnrates depends on creasingage), no significantdifferences were the site tenacityof the birds.In the Southern found:Chi 2 = 4.15, p>0.20, df = 3 (males)and Dunlin, malesare known to be very faithful to Chi2= 4.20, p>0.20, df = 3 (females). their breedingarea and rerum year after year to the same territory (Soikkeli 1967, 1970b). Most of theadult mortality is believedto occur Thus,non-returning adult males can with great duringthe non-breeding season, i.e., on migra- certainitybe regarded as dead. Female survival tion, or on the winteringgrounds (see Soikkeli is moredifficult to estimatesince they disperse between breeding seasonto a higher degree than males (Soikkeli 1970b, own observati- lOO ons). However, since this studycovered most of the suitable Dunlin localities within a radius of 100 km, the numberof femalesdispersing outsidethe area, are probably very few. Annual returnrates may thereforebe regardedas good 50, ß c• r=- 0.86 minimum estimates of survival in adult Dun- y= 97,9- 3.9 X lins. z z &• r=- 0.95 y=892-6.0x The overall annual return rate of adult Dunlins in SW Skfine,was 83 %, varyingbetween 80 % in 1984/85 and 89 % in 1982/83 (Table 6). The 1 2 3 4 5 averagemale return rate (89 %) was signifi- YEARS AFTER RINGING canflyhigher than that for females(77 %) (Chi2 Figure4. Returnrates of adultDunlins in relationto = 12.1, p<0.001, df = 1). Only in one year increasingage. See text for discussionabout statistical significance.
WSC•Bulletin 61, Suppl.: 56-68 (Apdl 1991) J6nsson: Dunlln 63
Table 7. Return rate of adult Dunlinsin relation to increasingage in the populationof SW Sk•ine1981- 1986. M = males, F=fcmal½s.
individualspresent individualsreturning % returning in previousyear followingyear Years after M F M+F M F M+F M F M+F ringing 1 75 74 149 71 60 131 95 81 88 2 60 52 112 52 41 93 87 79 83 3 40 36 76 35 27 62 88 75 82 4 24 18 42 21 11 32 88 61 76 5 12 5 17 9 3 12 75 60 71
1967,Evans & Pienkowski1984). In thisstudy, for females5.9 years.2. Mean longevitycan be onlytwo birdswere definitively known to have derivedalso from calculationsusing only birds died in the breeding area. These were both of known age,i.e., birdsringed as chicksand malestaken by avianpmdators (a Merlin Falco later controlledin the study area. During the columbariusand a SparrowhawkAccipiter ni- studyperiod 24 suchDunlins were controlled. sus,respectively) during the prelayingperiod, Sevenof thesehave sincedisappeared and are earlyin the season.However, at leasttwo more considereddead. The totalage of thedead birds birds(both females) disappeared during the in- amountedto 13.5 years,assuming equal mor- cubationperiod, probably taken by somemam- tallity rate in the fin:stand secondhalves of the malian predatorat night while incubatingthe year.The remaining 17 birds still alivein 1986, eggs. had a total ageof 49 yearsand their additional totallife expectancywas 17 x 5.4, or 91.8 years. The meanlogevity of these24 Dunlinswould Life expectancyand longevity thus be (13.5 + 49 + 91.8)/24, or 6.4. years. Similar calculationsfor males (n = 17) and fe- With a constantannual mortality rate M (in %, males(n = 7), givemean longevity values of 9.1 = 100 x annual survivalrate), the averageex- yearsand 5.1 years,respectively. pectationof life is 100/M-0.5 years.With an averageannual mortality rate for adultsof 17% Mean longevityof youngDunlins, hatched or (Table 6), life expectancywas calculatedas fledged,is of coursemuch shorterthan that of 100/17-0.5, or 5.4 years.Separating the sexes, breedingbirds. meanlife expectancyfor maleswas 8.6. years (M = 11%), and for females 3.9 years (M = In the studypopulation there were at leastfour 23 %). birdsattaining an ageof 14 years,and another thatreached 7 to 11 years.These birds were all, Thesevalues of life expectancy,were usedto with oneexception, ringed as newly fledged ju- calculatean estimated mean longevity for adult veniles near Falsterbo in southernmost Skfine Dunlinsin two ways: 1. Assumingthat most of (cf Figure1). The two oldestringed birds, both the adultsare breedingfor the first time when males,aged 17 and 16 years,respectively, were caughtand ringed, we needonly to know the still alive in 1987 (pers.obs.). meanage of first-breedersand then add life ex- pectancyto obtainmean longevity. The mean age of 9 first-breeders,ringed as chicksand Discussion controlledbreeding in later years,was 2.0 + 0.36 years. Thus, mean longevity for adult Reproductive success birdswill be 7.4 years(sexes combined). As- sumingfurther that them is no difference in the Fromthe results presented above, it is clearthat meanage of first-breedingmales and females, reproductivesuccess for Dunlinsin SW Sk•ine meanlongevity for maleswill be 10.6years and is severelylimited by nestpredation. The over-
WSG Bulletin 61, Suppl.: 56-68 (April 1991) 64 J6nsson: Dunlln
foodresources now beingavailable for thepre- dators.
• 40 As mentioned earlier, there was no difference '• _ in hatchingsuccess between early and late clutches.In other wader specieshatching .• 30 r=-0.92'•,• successwas found to be low earlyin theseason, • 20 y--65.7-1.17x due to a high predationrate (Byrkjedal 1980, • p
WSG Bulletin61, Suppl.' 56-68 (April 1991) J6nsson: Dunlln 65 ferentselective agents, such as predation, feed- were basedon recordsfrom only two succes- ing efficiencyand extreme environmental con- sive years. ditions (Goss-Custard 1980, Evans & Pien- kowski 1984). Generally,sexual differences in returnrate of wadersare small, although in mostcases, males The data collectedin this studydoes not allow tendto showthe highestvalues (Oring & Lank any estimate of second-year survival, but 1984). The obviousquestion to askis of course accordingto whatis knownfrom other species, if thesesexual differences actually refer to a it is not significantlydifferent from thatof older higher survivalin males,or if they reflect the birds (Evans & Pienkowski 1984). higherdispersal rate of the females,which is well documentedin many species(Soikkeli In adultbirds, survival is generallyconsidered 1970b,Oring & Lank 1984,pets. obs.). to be constantand independent of age (Perrins and Birkhead 1983). This was also the con- In thepresent case, I havereason to believe that clusionby Soikkeli (1970a) aboutthe Dunlin the observed difference in return rate between populationin Finland.However, in this study the sexesis not solely the resultof differential return rates indicate a decreased survival with dispersal.Besides the relatively high efficiency increasingage (Table 7, Figure 4), and Hild6n in searchingfor tinged birds, with nearly all (1978) founda similartendency in hispopula- suitableDunlin localitiesin thesurrounding re- tion of Temminck's Stint Calidris temrninckii. gion controlledeach year, there are two facts Using the methodof Hild6n (1978) and calcu- supporting a genuinedifference in survival lating a linearregression of returnrates in rela- between the sexes: tionto ageafter tinging, revealed significant in- verse correlationsfor both sexes(Figure 4). I. Amongthe tinged birds recorded as missing However, testingthe differencesin returnrate (not observed)in one year, some are re- betweendifferent yearsafter tinging, with a coveredalive in a later year. The frequency Chi'--test,these were not statisticallysignifi- of such 'missed'birds is not higher for fe- cant. A similar test made on Hild•n's (1978) mmes(18.9 %,n - 53) than for males (17.9 data,also failed to establishany significantde- %, n - 28), whichwould be expectedif dis- creasein returnrate with increasingage (Chi'-= persalwas the majorreason for the lower fe- 8.20, 0.1.. Table 8. The operational sex-ratio in a Dunlin-popu- The pronounceddifference in return rate be- lation in SW Skfine in 1981-1986. Data from the tweenmales and females, as found in thisstudy, main study area. (see Table 6) has not been reported for any other monogamouswader speciesor popula- No. of No. of Ratio tion. Soikkeli (1970a) in hisstudy of Dunlinsin males females males:females Finland, found no significant sexual differ- encesin returnrate (77 % in malesand 72 % in 1981 26 24 1.08:1 females).The averagereturn rate for malesin 1982 25 22 1.14:1 Finland was also significantlylower than that 1983 25 22 1.14:1 foundin thisstudy (Chi2= 10.0,p<0.01, df= 1). 1984 22 15 1.47:1 Miller (1983), in a studyof theLeast Sandpiper 1985 21 14 1.50:1 Calidris minutilla on Sable Island, Nova 1986 22 16 1.38:1 Scotia,reported a returnrate for malesof 65 % and for females 38 %. However, these values mean 24 19 1.29:1 WSC• Bulletin61, Suppl.: 56-68 (April 1991) 66 J6nsson: Dunlln There was a clear tendencythat more males sameas in thisstudy (36.3 %). As a furthercom- than females return as first-breeders to their parison,Hild6n (1978) reported an average birth-place,as seenfrom Table 5. Soikkeli fledgingrate of about0.7 youngper adultin a (1970b) found no significantsex-difference populationof Temminck'sStint. (30 males,27 females)among returning young ringedas chicks, and similarly equal sex-ratio Is thenthe recorded reproductive rate sufficient amongfirst-breeders are foundin mostother to balancethe annualmortality rate? Is the waderspecies studied (Oring & Lank 1984). populationself-supporting? As about 17 % of The only caseindicating a moredistinct sex- the adultsdie annually,the proportionof first- biasin natalphilopatry or juvenile survivalis breedersshould also be 17 %, which meansthat thatof theLong-billed Curlew Numenius ame- the population must produce 0.17 first- ricanusin SW Idaho,reported by Redmond& breedersper year and adult. With an annual Jenni (1982), where 9 males and two females fledgingrate of about0.14 youngand a post- were recovered on their natal breeding fledgingsurvival rate of about56 %. then0.14 x grounds. 0.56 = 0.08 youngreach the age of oneyear. Using the data in Table 6, we can estimatethe In adults, the lower annual survival rate of fe- proportionsof birds breedingwhen one and males may be related to the costof reproduc- two yearsold, at 35 % and65 %, respectively.If tion. The laying of one or two four-egg thesurvival rate during the secondyear of life is clutches,each one weighing 80-90 % of thefe- the same as for adults, i.e. 83 %, then the male'sown weight, is probablya greatener- number of farst-breedersproduced per adult= getic strainand it has beendemonstrated that 0.08 (0.35 + 0.65 x 0.83) = 0.07. Thus, the re- females of different Calidris-speciesloose productiveoutput of thepopulation is farbelow weightrapidly during the whole incubation pe- the 0.17 first-breedersneeded to keep it self- riod and the early brood-rearing period supported.The low hatching successis the (Ashkenazie& Safriel 1979, Erckmann 1981, main factorresponsible for the insufficientre- J0nsson& Alerstam 1990). This indicatesthat productiverate. To balancethe adult mortality, femalesare exposed to a moreserious energetic hatchingsuccess in the studyarea mustbe in- stress,just after the breedingseason, which creasedfrom the prevailing 0.34-0.40, to about might leadto an increasedmortality during the 1.0 youngper adultand year. followingmigration to the moultingareas (e.g. the SouthernDunlin). No. breedinõ pairs 188_ Anotherexplanation for therelatively low sur- vival of femalescould be that they, due to their largersize, are more vulnerable to aerialpreda- 68_ ters like Merlins and Sparrowhawks.The se- lectiveadvantage of smallsize for flightagility, 48. avoidinga predatorand makingquick tums, 28. has been emphasizedby many authors(e.g. •Control a Andersson& Norberg 1981, Jehl & Murray [•l]• Naln area 1986). 1981 1982 1983 1984 1985 1986 Year Maintenance of the population Figure6. Developmentof theDunlin population in SW Sk,Snein the years1981 to 1986. As already indicated by the high nest-preda- tion, the yearly reproductiveoutput in the Provided that immigration and emigration studiedpopulation is low;on average 0.13-0.15 balances,the recordedvalues of mortality and fledgedyoung per adult. In comparison,the reproductiveoutput shouldbring, as a con- Dunlin populationstudied by Soikkeli(1967, sequence,an annual decrease of thepopulation 1970) in SW Finland, producedon average in theorder of 10 %. As seenfrom Figure6, the 0.26 fledged young per adult and year, as- actualdecrease during the six-yearperiod, has sumingthat the fledging rate in Finlandwas the beenaround 5 % per year. This indicatesthat WSG Bulletin61, Suppl.: 56-68 (Apdl 1991) Jbnsson: Dunlln 67 there is an annual immigration rate of about 1985&s arbete.VdrFdgelvfiorld 45: 85-92. 5 %, whichpartly compensatesfor the low in- Andersson, M. & Norberg, A. 1981. Evolution of ternalreproduction. However, since the South- reversed sexual size dimorphism and role partitioning among predatory birds, with a size ern Dunlin is reportedto be decreasingin all scalingof flight performance.Biol. J. Linn. Soc.15: countries around the Baltic Sea (Gromadzka 105-130. 1983,Hansen 1985, Tjernberg 1985, Klafs and Ashkenazie, S. & Safriel, U.N. 1979. Breedingcycle StQbs1987), the external support to thepopula- and behaviourof the SemipalmatedSandpiper at tion in SW Skgtneis not likely to persistfor any Barrow, Alaska. Auk 96: 56-67. Bainbridge, I.P. & Minton, C.D.T. 1978. The extendedperiod of time. With no radical im- migrationand mortality of the Curlew in Britain and provementsin the reproductivesuccess, the Ireland.Bird Study25: 39-50. Dunlin populationof SW Skgtnewill slowly Botkin, D.B. & Miller, R.S. 1974. Mortality ratesand meet the fate of extinction a decade or so, into survival of birds. Am. Nat. 108: 181-192. the next century. Boyd, H. 1962. Mortality and fertility of European Charadrii. Ib/s 104: 368-387. Bub, H. 1974. Vogelfang und Vogelberingungzur The roleof nest-predationas a regulatingfactor Brutzeit.Ziemsen, Wittenberg. for birds, like the Dunlin, breeding in wet Byrkjedal, I. 1980. Nest predafionin relationto snow meadows has become evident in South Sweden cover - A possiblefactor influencingthe start of in recent years, especially following the breedingin shorebirds.Ornis Scan& 11: 249-252. outbreakof sarcopticmange in Red Foxes Coulson, J.C. & Woolet, R.D. 1976. Differential survivalrates among breeding kittiwake gullsRissa (Lindstrbm 1987). Significantly increased tridactyla(L.). J. Anita.Ecol. 45: 205-213. hatching successhas been reported for e.g. Cramp, S. & Simmons,K.E.L. (eds.)1983. TheBirds Dunlins(J0nsson 1990) andLapwings (Flodin of the WesternPalearctic. Vol. III. Oxford Univ. et al. 1990) in areas where the foxes have Press,Oxford. disappeared.Continuing area decrease and the Dybbro, T. & J6rgensen,O.H. 1971. Udbredelsenaf Stor Kobbersneppe(Limosa limosa) Alm. Ryle fragmentationof well-managedwet meadows (Calidris alpina), Brushane(Philomachus pugnax) may alsoincrease the impactof nest-predators og Klyde (Recurvirostraavosetta) i Danmark1970. on waderpopulations breeding in thesehabitats Dansk. Orn. Foren. Tidsskr. 65:116-128. (J/3nsson1990). Emanuelsson,U. & Kjell6n,N. 1981.Kltrrsnappan som hltclcf•tgeli Skfine 1930-1981. Anser 20: 233-240. Erckmann, W.J. 1981. The evolution of sex-role reversaland monogamy in shorebirds.Ph.D. Thesis, Acknowledgements Univ. of Washington. Etheridge, B. 1982. Distribution of Dunlin Calidris Bo W. Svenssoninspired me andgave me valu- alpina nestson an areaof SouthUist machair.Bird able adviceduring the first yearof field-work. Study29: 239-243. Assistancein thefield wasalso provided by Per Evans, P.R. & Pienkowski, M.W. 1984. Population Andell and Anne-Marie Thelin. Figureswere dynamicsin shorebirds.In: Burger,J. & Olla, B.L. (eds.). Shorebirds: Breeding Behavior and drawn by Viktoria Andersson,Eva Karlsson Populations.Plenum Press, New York. and Steffi Douwes. Thomas Alerstam and Sam Ferns, P.N. & Green, G.H. 1979. Observationson the Erlinge gave valuablecomments to the first breedingplumage and prenuptialmoult of Dunlins draft, while David Fleet and Hermann H6tker Calidris alpina capturedin Britain. Gerfaut 69: 285- 303. providedfurther comments and suggestions to o FIodin, L.-A., Nor6n, L.-G. & Hirsim•iki, H. 1990. thefinal manuscript. The studywas financially (Nestsite selection and hatching success of Lapwing supportedby theDepartment of AnimalEcol- Vanellusvanellus at Getteron.)Vdr Fdgelvfirld49: ogy, University of Lund, the Royal Physio- 221-229. (Swedishwith EnglishSummary) graphicalSociety in Lund, the Royal Swedish Goss-Custard,J.D. 1980. Competitionfor food and Academy of Sciences (Hierta-Retzius- interferenceamong waders. Ardea 68:31-52. Foundation) and the National SwedishEnvi- Gromadzka,J. 1983.Distribution of breedingsites and numbersof southernDunlin Calidrisalpina schinzii ronment Protection Board. on southern Baltic coast. Not. Orn. 24: 31-36. Hale, W.G. 1980. Waders.Collins, London. Hansen, M. 1985. Bestandenaf Stor Kobbersneppe References Limosa limosa, Almindelig Ryle Calidris alpina, Brushane Philomachus pugnax og Klyde Andell,P. & j6nsson,P.E. 1986.Projekt svartbent Recurvirostra avosetta i Danmark i 1980. Dansk strandpipare- en presentationsaint redovising av Orn. Foren. Tidskrift79:11-18. WSG Bulletin61, Suppl.: 56-68 (Apdl 1991) 68 J6nsson: Dunlln meadows in southern Sweden. Oikos 20: 136-155. Harris, M.P. 1967. The biology of Oystercatchers o Haematopusostralegus, on Skokholm Island, S. Lindstr0m, A. 1987. Rfivskabbeni Sverige.SNV Wales. Ibis 109: 180-193. Rapportno. 3274. Heldt, R. 1966. Zur Brutbiologie des Mayfield, H. 1961. Nesting successcalculated from AlpenstrandlitufersCalidris alpina schinzii.Corax exposure.WilsonBull. 73: 255-261. 1: 173-188. Mayfield, H. 1975. Suggestionsfor calculatingnest Hilden, O. 1978. Populationdynamics in Temminck's success. Wilson Bull. 87: 456-466. Stint Calidris temmincldi. Oikos 30: 17-28. Meltofte, H., Elander, M. & Hjort, C. 1981. Holmes,R.T. 1966.Breeding ecology and annual cycle Ornithologicalobservations in northeastGreenland adaptationsof the Red-backedSandpiper (Calidris between74ø30 ' and76ø003I lat., 1976.Medd. GrOnl. alpina) in northernAlaska. Condor68: 3-46. Biosci. 3: 1-53. Holmes, R.T. 1970. Differencesin populationdensity, Miller, E.H. 1983. Habitatand breedingcycle of the territorialityand food supply of Dunlinon arcticand Least Sandpiper (Calidris minutilla) on Sable subarctic tundra. In Watson, A. (ed.). Animal Island, Nova Scotia. Can. J. Zool. 61: 2880-2898. populations in relation to their food resources. Oring,L.W. & Lank, D.B. 1984.Breeding area fidelity, Blackwell, Oxford. natal philopatty, and the social systems of Jehl, J.R., Jr. 1971. Patternsof hatchingsuccess in sandpipers.In: Burger, J. & Olla, B.L. (eds.). subarcticbirds. Ecology 52. 169-173. Shorebirds:Breeding Behavior and Populations. Jehl, J.R., Jr. & Murray, B.G., Jr. 1986. The evolution Plenum Press,New York. of normal and reversesexual size dimorphismin van Paassen,A.G., Veloman, O.H. & Beintema, A.J. shorebirdsand other birds. In Johnston,R.F. (ed.). 1984. A simple device for determinationof Current Ornithology. Vol.3. Plenum Press,New incubationstages in eggs.Wildfowl 35: 173-178. York. Perrins,C.M. & Birkhead,T.R. 1983.Avian Ecology. Johnson, D.H. 1979. Estimating nest success:the Blackie,Glasgow. Mayfield methodand an alternative.Auk 96: 651- Pienkowski, M.W. 1984. Breeding biology and 661. populationdynamics of RingedPlovers Charadrius J6nsson,P.E. 1985. Locationof nest,habitat choice and hiaticula in Britain and Greenland. J. Zool. 202: 83- breeding success of Dunfins in Sk,•tne. In: 114. M.Tjernberg (ed.). The SouthernDunlin Calidris Piersma, T. 1986 (compiler). Breeding waders in alpina schinzii in Sweden.Statens Naturvgtrdsverk. Europe.Wader Study Group Bull. 48, Suppl.:1-116. Rapport.SNV PM 1928. Redmond,R.L. & Jenni, D.A. 1982.Natal philopatty J•nsson, P.E. 1987. Sexual size dimorphism and and breedingarea fidelity of Long-billedCurlews disassortativemating in the Dunlin Calidrisalpina (Numeniusamericanus): patterns and evolutionary schinzii in southern Sweden. Ornis Scand. 18: 257- consequences.Behav. Ecol. Sociobiol.10: 277-279. 264. Soikkeli, M. 1967. Breeding cycle and population J6nsson, P.E. 1990. KarrsnappanCalidris alpina dynamicsin the Dunlin Cah'drisalpina. Ann. Zool. schinzii som hackfAgeli Sk•e 1990 - numerar, Fennici 4: 158-198. klitckningsfraging och populationsutveclding. Soikkeli,M. 1970a.Mortality and reproductive rates in a Anser 29: 261-272. Finnishpopulation of Dunlin Calidrisalpina. Omis J6nsson,P.E. & A!erstam, T. (1990). The adaptive Fennica 47: 149-158. significanceof parentalrole division and sexual size Soikkeli,M. 1970b.Dispersal of DunlinCalidris alpint dimorphismin breedingshorebirds. Biol. J. Linn. in relation to sites of birth and breeding.Ornis Soc. 41: 301-314. Fennica 47: 1-9. K!afs, G. & •;tabs, J. (eds.) 1987. Die Vogelwelt Soikkeli,M. 1973.Size variations of breedingDunlins in Mecklenburgs.3rd ed.Fischer, Jena. Finland.Bird Study20: 151-154. Kr61, E. 1985. Numbers, reproductionand breeding Soikkeli, M. & Salo, J. 1979. The bird fauna of behaviourof Dunlin Calidris alpina schinziiat the abandonedshore pastures. Ornis Fennica 56: 124- Reda mouth, Poland. Acta Orn. 21: 70-94. 132. Kr61, E. 1986. Coastal meadows in Poland - the Tjernberg, M. (ed.) 1985. The Southern Dunlin vanishing habitat of breeding waders. Vdr Calidris alpina schinzii in Sweden. Statens VdgelvfirldSuppl. 11: 93-98. Naturvgtrdsverk.Rapport. SNV PM 1928. Kus, B.E., Ashman, P., Page, G.W. & Stenzel, L.E. Westerskov,K. 1950. Methodsfor determiningthe age 1984. Age-related mortality in a wintering of gamebird eggs.J. WildlifeManagement 14:56- populationof Dunfin.Auk 101:69-73. 57. Larsson, T. 1969. Land use and bird fauna on shore WSG Bulletin61, Suppl,: 56-68 (April 1991)