J. Field Ornithol., 69(2):235-243
CHRONOLOGY
MATED
OF NESTING
SANDPIPERS
BRETT
EVENTS
IN WESTERN
AND SEMIPALNEAR THE ARCTIC
CIRCLE
K. SANDERCOCK
Departmentof BiologicalSciences
SimonFraser University
Burnaby, British ColumbiaVSA 1S6 Canada
Abstract.--The chronologyof nestingeventswasexamined in WesternSandpipers(Ca/idris
mauri) and SemipalmatedSandpipers(C. pusilia) breeding at Nome, Alaska.The duration
of laying (5 d for a 4-eggclutch) and hatching (53.0 -+ 26.6 SD h, dry chicksto emerge
from star-pippedeggs)wassimilarfor both species.Layingratesof SemipalmatedSandpipers
at Nome were slowerthan other populations,possiblybecauselayingintervalswere greater
than 30 h. Length of incubationwassignificantlylonger in Western(21.5 - 0.5 d) than
SemipalmatedSandpipers(20.3 -+-1.9 d). Early clutcheshad significantlyhigher hatching
success
than late clutchesin Semipalmated
Sandpipers(60.0%, n = 105vs.22.9%, n = 70)
but not WesternSandpipers(68.2%, n = 88 vs.47.3%, n = 55).
CRONOLOGiA DE LOS EVENTOS DE ANIDAMIENTO
ILLA CERCA DEL CJRCULO ARTICO
DE CALIDRIS MAURIY
C. PUS-
Sinopsis.--Seexamin(5la cronclogiade los eventosde anidamientode Calidrismauri y C.
pusillaen Nome, Alaska.La duracitn de la puesta(5 dias para una camadade 4 huevos)y
el eclosionamiento(53.0 -+ 26.6 DE horas,para emergery secarsede un huevopicado) fue
similar para ambasespecies.La tasade puestade C. pusilia en Nome fue menor que para
otraspoblaciones,posiblementedebido a que el int•rvalo de puestafue mayor a 30 horas.
El periodo de incubacitn fue significativamente
menor en C. mauri (21.5 _+0.5 dias) que
en C. pusilia (20.3 -+ 1.9 dias). Las camadasproducidastemprano tuvieronun exito significativamentemayor que las camadasproducidastardes en C. pusilla (60%, n = 105 vs.
22.9%, n = 70), pero no asien la segundaespecie(68.2%, n = 88 vs.47.3%, n = 55).
Time constraintsstronglyinfluence the biologyof migratorybirds that
breed in the arctic. Local conditionsare usuallysuitablefor only a short
period, and early departure may be critical if migrantsare to reach wintering areas successfully.Many shorebirds have compressedbreeding
schedulesat high latitudes(Miller 1983, Pienkowski1984), and northern
populationsmay haveadaptationsthat reducethe length of the breeding
period. The nestingchronologyof shorebirdscan be difficult to investigate becauseit is hard to find nestsbefore laying is complete and predation rates are often high (Evansand Pienkowski1984). Nonetheless,
Dunlin (Calidris alpina) have fasteregg-layingratesin the northern parts
of their breedingrange (Holmes 1966, Soikkeli1967), Red Phalaropes
(Phalaropusfulicaria)have a shorter length of incubation (Schameland
Tracy 1987), and female SemipalmatedSandpipers(C. pusilla) desert
their youngto departearlier (Grattoand Cooke1987).The aimsof this
studywere to determine: the egg-layingrates, length of incubation,duration of hatching, and seasonalrates of nest successin Western Sandpipers (C. mauri) and SemipalmatedSandpipersbreeding at a site near
the Arctic
Circle.
235
236]
B.K. Sandercock
J.Field
Ornithol.
Spring 1998
STUDY
AREA
AND
METHODS
Fieldmethods.--Thereproductivebiologyof Westernand Semipalmated
Sandpiperswasstudiedat a 4 km• studysite,21 km eastof Nome, Alaska
(64ø20'N, 164ø56'W) during May-July in 1993-1995. The two species
breed in the same habitat at this site, and nestsare placed on low, dry
ridgesthat are closeto shallowtundra ponds.
Nestswere locatedby walking the tundra and observingsandpiperbehavior.Incubatingbirdsflushedfrom the nestat shortdistancesand used
distractiondisplaysto lead the observeraway.If the clutchwasnot found
immediatelyafter the parent flushed, the bird wasobservedfrom 20-30
m until
it returned
to the nest. Nests were
marked
with
a line
of small
sticksand a wooden stake 10 m from the nestcup.Date and the time of
daywere recordedon all nestvisits;mosteffort wasin the daylighthours
of 0800-0100
h.
Nestsfound with fewer than four eggswere visited 1-2 times per day
until clutch size remained unchanged for three days.These nestswere
revisited during incubation to float eggs. If a nest contained four eggs
(the maximum clutch size) when it wasfound, 2-4 eggswere immersed
in a small, clear cup of warm water. Eggswere pushed to the bottom
before releaseto prevent egg damagefrom dropping and to ensurethey
were not held by surfacetension.Only undamagedeggswere floated. If
eggstouched the bottom of the cup, the angle betweenthe bottom of
the cup and the center axisof the egg (flat or 20-90ø) wasmeasured.If
eggsfloated freely in the water,it wasrecordedwhether or not the eggshellbroke the surface.Floatingeggsdoesnot affecthatchabilityin shorebirds (Alberico 1995, Van Paassen et al. 1984).
Nestswere visitedevery6-7 d during incubationand then daily close
to estimatedhatchingdate. Observersheld eggscloseto the ear to listen
for the soundof young tapping on the eggshell.If tappingwasheard, we
lightly touched the blunt end of the egg with a fingertip to detect raised
eggshell(or star-pips).In the next stageof hatching,young punctured a
smallopening (hole-pip) in the eggshell;theseholeswerevisiblewithout
handling the eggs. Young were not banded until all viable eggs had
hatched, and preferably when the chick down feathers had dried. The
young usuallydeparted the nest soon after all were dry.
Estimationmethods.--Thenumber of nestswasnot adequateto testfor
annual or seasonalvariation in most cases,so data from the three years
were usuallypooled. Samplesizesdiffer amonganalyses
becausecomplete
information wasnot availablefor every nest.
Egg-layingrateswere calculatedfor four-eggclutchesthat were found
during laying. Three-egg clutcheswere excluded becausea long laying
interval could be causedby eggslost to partial nest depredation or disturbanceduring laying.The few neststhat were not visitedat leastdaily
during layingwere alsoexcluded.The layinginterval betweensuccessive
eggswascalculatedby using the elapsedtime betweentwo different nest
visits.To obtain a minimum estimate, I assumedthat an egg had been
Vol.69,No.9
Nesting
Chronolo•
• in ArcticSandpipers
[237
laid just previousto the first visit, and I only included caseswhere an
additional egg wasknown to have been laid subsequentto the last nest
check.If egg number did not changebetweentwo dailyvisits,the elapsed
time wasa minimum estimateof one laying interval (e.g., 3 eggson day
I at 1000 h to 3 eggson day 2 at 1800 h givesa laying interval of 32 h).
If egg number increasedby one during a 2-3 d period, the elapsedtime
was a minimum estimate of two laying intervals (e.g., 2 eggson day I at
1300 h to 3 eggson day 4 at 1300 h givesa mean laying interval of 36
h). Both estimatesare sensitiveto check frequency,thus minimum intervalsof lessthan 15 h were discarded.The estimatesmaybe biasedtowards
longer laying intervalsbecauseit wasnot possibleto determine the time
of layingwhen an egg appearedbetweentwo daily nestvisits.
The duration of incubationwasestimatedby subtractingthe date the
last eggwaslaid from the date of hatching.For nestsfound during laying,
the date of last laying wasthe day the last egg wasfound. For neststhat
contained complete clutchesof eggsthat floated flat (seebelow), date of
last layingwasassumedto be the day the nestwasfound.
The duration for each stageof hatching wasthe elapsedtime between
the nest visit where some eggsin the nest showedsignsof hatching and
the nest check where the last viable egg had hatched. There wasusually
some hatching asynchronyamong the eggsof a clutch, and nestswere
categorizedon the basisof the most advancedegg.
Determiningnestsuccess.--Nest
fate was defined as one of three outcomes:(1) abandoned:eggswere consistentlycold for a week or more;
(2) depredated:
eggsdisappearedbefore the expected hatching date, or
there were signsof predator activity (broken egg shells,fox urine in the
nestbowl); or (3) hatched: at least one chick was banded and left the nest.
Predatorsoften remove all eggsfrom shorebirdnests(pers.obs.), but the
fate of an empty nestbowl can be ambiguous because parents remove
eggshellsfrom the nest at hatching (Sandercock1996). To clarifyuncertainty in nest fate, I used flotation of eggsto determine stageof incubation (Alberico 1995, Rahn and Ar 1974, Van Passenet al. 1984). If a nest
was found after laying was completed and subsequentlydisappeared,I
compared the date it waslast seenwith the estimatedstageof incubation
and predictedhatchingdate. Egg buoyancywasa good predictorof stage
of incubation,the number of daysof incubationcould be estimatedwith
an error of -+2-3 d if the nest wasfound within the first week after laying
(Fig. 1). This accuracyis comparableto float curvesfor other shorebirds
(Van Paassenet al. 1984).
All testswere two-tailedand consideredsignificantat probabilitylevels
lessthan P = 0.05. Means are presented ___1
SD.
RESULTS
Sandpiper nestswere usuallyfound after laying was completed (Western: 79.3%, n = 164;Semipalmated:85.8%, n = 183). Mostof the clutches found during laying that eventuallycontainedfour eggswere discovered when the clutch wasalmost complete (Western, 1-egg:7.4%, 2-egg:
238]
B. K. Sandercock
J. Field Ornithol.
Spring 1998
•
FLAT
20ø •--•45-64ø •
R•
25-44 ø
•]
WESTERN
27
65-850
90ø
•
AT SURFACE
E•ABOVE
SURFACE
SANDPIPER
15
22
44
25
o
1
2
3 4 5 6 7 8+
14
47
16
19
56
lOO%
75%
50%
25%
u_
0
0%
<o
D_I
SEMIPALMATED
13
17
18
SANDPIPER
27
26
19
33
34
4
1 2
3
4 5
6
7
•- 100%
z
o 75%
5O%
25%
0%
<0 0
DAYS
OF
8+
INCUBATION
FIGURE
1. Changesin the buoyancy
of sandpipereggsduringincubation.Day0 is the
estimateddate of layingfor the last egg; it wasdeterminedfrom either observations
during egg-layingor by back-datingfrom the date of hatch. Observersrecordedthe
anglebetweenthe centreaxisof the eggand the bottomof the float cup.Numbers
aboveeachbar indicatethe samplesizeof eggs,the totalsare basedon 90 visitsto 71
WesternSandpipernestsand 80 visitsto 64 Semipalmated
Sandpiper
nests.The egg
that wasflat on day6 wasprobablyinfertile.
25.9%,3-egg:66.6%,n = 27;Semipalmated,
1-egg:5.5%,2-egg:33.3%,
3-egg:61.1%,n = 18). Nestsfoundearlyin layingweremorelikelyto
havea layingschedule
thatincludedvisitswheretherewasno newegg
(100%of nestsfoundat 1-egg,n = 3; 63.6%of 2-eggnests,n = 11;25.0%
of 3-eggnests,n = 24;Fisher's
exacttest,P < 0.01 [pooling1-and2-egg
clutches]).In the followinglayingschedules,
numbersindicatedays
whereneweggswerefoundduringnestvisits.A dashindicatesa day
where egg number wasunchangedin Western(1-2-34, n = 1; 12-3-4,
n = 1; 2-3-4, n = 1; 2-34, n -- 2; 23-4, n = 1; 234, n = 2; 3-4, n -- 4;
34, n -- 10) and Semipalmated
Sandpipers(1-2-34, n -- 1; 2-34, n = 1;
Vol.69,No.2
Nesting
Chronology
in ArcticSandpipers
[239
23-4, n = 2; 234, n = 2; 3-4, n = 2; 34, n -- 8). The daysthat eggswere
not found did not appearmore frequentlytowardseither the startor end
of laying.
The estimatesof the minimum interval betweentwo sequentialeggsin
Western(mean -- 23.1 h + 3.2, range = 17.7-28.5 h, n = 12) and Semipalmated Sandpipers(mean - 24.1 h + 5.0, range = 17.2-31.4, n = 8)
were sometimesas high as 30 h. The estimatesof the mean minimum
interval between three sequentialeggswere as high as 36 h in Western
(mean = 30.5 h +- 5.8, range = 23.4-36.4 h, n = 6) and Semipalmated
Sandpipers(mean = 28.6 h + 7.2, range = 23.9-36.9 h, n = 3). Both
estimateswere based on a sample of 16 neststhat included visitswhere
no new egg was found.
Length of incubation was significantlyshorter in SemipalmatedSandpipers (mean = 20.3 d ___
1.0, n = 7) than WesternSandpipers(mean =
21.5 d __-0.5, n = 12, Mann-Whitneytest,U = 7.8, P < 0.01). There was
no significantdifferencebetweenthe two speciesin the duration of hatching (all tests,P > 0.05). The elapsedtime between hatching of the last
chick and the different stagesof hatching (both speciespooled) were:
tapping eggs71.5 h --- 42.6 (n = 16), star-pippedeggs53.0 h ___
26.6 (n
= 51), hole-pippedeggs28.1 h _+ 19.9 (n = 18), and wet chicks13.7 h
+ 5.7 (n = 16).
I examined the success
rate (hatchedvs. depredated) of nestslaid before and after the mean annual laying dates. Abandoned clutcheswere
not included in the totals becausefew nestswere deserted each year
(Western, n -- 0-7; Semipalmated,n = 0-2). In SemipalmatedSandpi-
pers, the success
rate of early nests (60.0%, n = 105) was significantly
higher than that of late nests (22.9%, n = 70, Fisher'sexact test, P <
0.001). The successrate of early Western Sandpiper nests (68.2%, n =
88) wasalso higher than late nests(47.3%, n = 55) but the difference
was not significant(Fisher'sexact test, P = 0.15). The pattern was the
same in all years, and similar conclusionswere reached if survival rates
were calculatedwith the Mayfield method.
DISCUSSION
Egg-layingrates.mWesternand SemipalmatedSandpipersat Nome took
5 d to lay a 4-eggclutch, and there were often dayswhen no new eggwas
found. In contrast,Holmes (1972) observedlaying schedulesat seven
WesternSandpipernestsand concludedthat the layingintervalsaveraged
24-25 h. Estimatesof laying intervals between eggs in Semipalmated
Sandpipers
rangefrom lessthan 24 h (n = 1, Norton 1972), 1 egg/d or
24 h (n -- 4, Ashkenazieand Safriel1979), and up to 32 h (Gratto-Trevor
1992). In a studyof SemipalmatedSandpipersin Manitoba, C.L. GrattoTrevor (unpubl. data) found that 72% of nests (n = 25) discoveredat
the 1-eggstagehad a laying schedule (1234) consistentwith a laying interval of 24 h. In the remaining nests(28%), no new eggwasfound after
the first (1-234) or secondegg (12-34) was laid.
Gapsin laying schedulesmay be due to the time of day a nest visit
240]
B.K. Sandercock
J.Field
Ornithol.
Spring 1998
occurs,to a fixed laying interval that is greater than the visitationrate, or
to skipped dayswhere an egg is not laid (Schubert and Cooke 1993,
Wiebe and Martin 1995). An apparent gap would be found if a new egg
waslaid after the observer'sdaily visit.A 24-h visitationrate is an unlikely
explanation for laying gaps in this study becauseI never observed an
increase of two eggsafter a visit where there was no new egg. Furthermore, I used the same methods and visitation rates as C. L. Gratto-Trevor
(unpubl. data), but we found that the laying ratesin two populationsof
SemipalmatedSandpiperswere different.
The layingschedulesand estimatesof layinginterval I observedin Western and Semipalmated Sandpipers are consistentwith a fixed interval
greater than 30 h. In other shorebirds,the time betweenconsecutiveeggs
is usuallygreater than 24 h. Estimatesinclude 23-26 h (phalaropes,Col-
well and Oring 1988, Kagarise1979), 30 h (n = 17, LeastSandpiperC.
minutilla, Miller 1983), 32 h (n = 6, Temminck's Stint C. temminckii,
Breiehagen 1989), 36 h (n = 8, Curlew Numeniusarquata,Mulder and
Swaan 1992), 46 h (n = 15, Greenshank Tringa nebularia,Nethersole-
Thompsonand Nethersole-Thompson
1979) and 47 h (n = 1, Longbilled Curlew N. americanus, Allen 1980). However, Western and Semi-
palmated Sandpipersmay have had gapsin their laying schedulesif the
laying interval was24 h but femalesoccasionallyskippeda day.Indeed,
severalof the abovestudiesnoted that layinggapswere sometimesgreater
than 48 h (Breiehagen 1989, Colwell and Oring 1988, Miller 1983). Daily
visitsare not adequate to test conclusivelybetween the last two explanations, and this questionwarrants further research:
It was surprisingthat the more northerly population of Semipalmated
Sandpipers
at Nome,Alaska(64øN)had sloweregg-laying
ratesthanbirds
breedingat La P•rouseBay,Manitoba (58øN).This is the oppositeof
Dunlin whichhavefasteregg-laying
ratesat Barrow,Alaska(72øN,1 egg/
d or 24 h, Holmes 1966) than in Finland (62øN, 36 h, Soikkeli 1967).
However,slowrates of egg-layingare consistentwith two other aspectsof
SemipalmatedSandpiper reproduction at Nome. This population is unusual in that females often produce clutchesof lessthan four eggsand
rarely renest (B. K. Sandercock,unpubl. data). Interpopulation differences in the fecundity of this speciesmay be related to differences in
timing of breeding (birds at Nome nest 3-4 wk earlier than birds at other
sites,Gratto and Cooke 1987, B. K. Sandercock,unpubl. data), or to sitespecificnutritional constraintson femalesduring egg production.
Lengthofincubation.--Incubationaveraged21 d in WesternSandpipers
and 20 d in SemipalmatedSandpipers,which is consistentwith previous
reports (Ashkenazieand Safriel 1979, Gratto-Trevor1992, Holmes 1972).
Length of incubation in other calidrine sandpipersvariesfrom 20 d (n
= 5, LeastSandpiper,Miller 1983), 21 d (n = 6, Temminck'sStint, Breiehagen 1989), 21-22 d (n -- 23, Dunlin, Soikkeli 1967) to 22 d (n = 1,
White-rumped Sandpiper C. fuscicollis,Parmelee et al. 1968). Interspecific variation in shorebird laying intervals and length of incubation is
Vol.69,No.')
Nesting
Chronology
in ArcticSandpipers
[241
probably related to variation in egg size, all of these factorsare alsopositivelycorrelatedwith body size (Rahn and Ar 1974, Saltheret al. 1986).
Nest attendance (and possiblyincubation) is initiated before laying is
completein Semipalmated
Sandpipers(Ashkenazie
and Safriel1979,Norton 1972) and most other shorebirds(Maxsonand Oring 1980, Miller
1983,Reynolds1987).In thisstudy,the majorityof the nestsfoundduring
laying contained three eggs.If the probability of finding a nest is proportional to the amount of time parents tend the nest (cf. Wiebe and
Martin 1995), my observationsindicate that some Western Sandpipers
also begin attending the nest during laying.
Hatching chronology.--Ifound that sandpiper young usually finished
hatching about 3 d after they had begun tapping on the eggshell.Past
estimatesof hatching duration in Western Sandpipers (24 h, Holmes
1972) and Semipalmated
Sandpipers(24 h, Ashkenazie
and Safriel1979)
appear to have been calculatedfrom the hatching of the first egg to the
last egg. If so, they are comparableto my estimatesof 28 h for the hatching of nestswith hole-pippedeggs.In a comparativestudyof other cali-
drine sandpipers,Norton (1972) also found that hole-pipspreceded
hatching by 12-48 h. The total time required by sandpiperyoung to
emerge from eggs is longer than previouslyrecognized. Future studies
should be explicit in defining how estimatesof different hatching stages
are measured.
Nestingsuccess.--Predation
on the nestsof both sandpiperspeciesincreasedasthe breedingseasons
progressedat Nome. WesternSandpipers
had higher nest successbecausethey tended to breed earlier than Semipalmated Sandpipers(B. K. Sandercock,unpubl. data). In contrast,nest
predation rates decreasedseasonallyin Ringed Plovers (Charadriushiaticula,Pienkowski1984) and Red-neckedPhalaropes(Phalaropuslobams,
Reynolds1987). Ratesof nest predation in arctic shorebirdsare often
linked to numbers of predatorsand their alternate prey (Gratto-Trevor
1994, Pienkowski 1984, Underhill et al. 1993). Microtine rodents were
not observedat Nome, but foxes (Alopexlagopus,Vulpesvulpes),jaegers
(all Stercorarius
spp.), and Sandhill Cranes (Grus canadensis)were present. Nest predation may have increasedseasonallybecausepredator encounter ratesand activitywere affectedby the cumulativenumber of initiated clutches.Pienkowski(1984) argued that nest predation declined
becausethe availabilityof other prey increased. Despite many hours
afield, we rarely observed predator encounterswith nests of any bird.
Alternately, late-nestingsandpipersmay have suffered higher predation
ratesbecausetheywere youngeror more inexperiencedbirds.Late-breeding sandpipersat Nome are usuallynew birds that are unbanded (B. K.
Sandercock,unpubl. data), but whether their nestswere more vulnerable
is unknown.
Shorebirdnumbersare affectedby variationin productivity(Ryan et
al. 1993), but variable nesting successshould not have a major effect on
population size becauseadult survivalrates are high in shorebirds(Evans
1991, Sandercockand Gratto-Trevor1997). Recent population models
242]
B.K. Sandercock
j. Field
Ornithol.
Spring 1998
supportthis notion, and showthat sandpipernumbersare mostsensitive
to changesin the rates of adult mortality (Hitchcock and Gratto-Trevor
1997).
ACKNO•qLEDGMENTS
I am grateful to S. A. Cullen, L. Imbeau, N. Imbeault, R. S. Mulder, P. D. O'Hara, K. M.
O'Reilly, C. A. Schuppli, C. St. Pierre, S. A. Wyshynskiand other colleagueswho worked
hard to find sandpipernestsduring thisproject.R. E. GillJr. (AlaskaScienceCenter,National
Biological Service) and R. Harris (U.S. Parks Service) were helpful in providing logistical
support.SitnasaukNative Corporationkindly permitted accessto their land for this research.
I thank C. L. Gratto-Trevorfor sharingunpublisheddata on sandpiperlayingrates,and F.
Cooke, R. E. Gill Jr., C. L. Gratto-Trevor,D. B. Lank, A.M. MacCharles,G.J. Robertson,J.
A. Robinson,D. Schamel,and T. D. Williamsfor makingcommentson the manuscript.I was
supportedby graduatefellowshipsfrom Queen's Universityand Simon Fraser University.
Fundingfor this researchwasprovidedby the CanadianWildlife Service,the CWS/NSERC
ResearchChair in Wildlife Ecology,a Dean's Grant for Doctoral Field Travel (Queen's University),the Northern StudiesTraining Program, an NSERC Operating Grant (to E Cooke),
and awardsfrom the Frank M. Chapman,John K. Cooper and Jennifer Robinsonmemorial
funds.
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Received23 Jan. 1997;accepted28 Apr. 1997.