J. Field Ornithol., 63(2):145-154
DETERMINATION
OF
BLACK-CROWNED
HATCHING
DATE
NIGHT-HERONS,
AND
GREAT
THOMAS
FOR
EGGS
OF
SNOWY EGRETS
EGRETS
W.
CUSTER
U.S. Fish and Wildlife Service
Patuxent Wildlife ResearchCenter
P.O. Box 2618
La Crosse,Wisconsin5,1602-2618 USA
GREY W.
PENDLETON
U.S. Fish and Wildlife Service
Patuxent Wildlife ResearchCenter
Laurel, Maryland 20708 USA
R. WILL
ROACH
U.S. Fish and Wildlife Service
Ecological
Services
Field Office
17629 El Camino
Suite 211
Real
Houston, Texas 77058 USA
Abstract.--Flotationof eggsin water and specificgravityof eggsof Black-crownedNightHerons (Nycticoraxnycticorax),SnowyEgrets(Egrettathula) and Great Egrets(Casmerodius
albus)were evaluatedas methodsto determinedate of hatching.Length of incubationand
durationof hatchingperiodwere alsodocumented
for eachspecies.
Althoughspecificgravity
was a better predictorof hatchingdate than egg flotation,both techniqueswere imprecise.
The regressionbetweenspecificgravity and the number of daysbeforehatchingdiffered
amongclutches,but not among eggswithin clutches.Specificgravity of eggspredicted
hatchingdate only to within 3.8 d for SnowyEgrets,and 4.7 d for Black-crownedNightHerons and Great Egrets.The mean incubationperiod was 27.3 d for Great Egrets, 23.7
d for SnowyEgrets and 22.8 d for Black-crownedNight-Herons. For all three species,the
A egg (first egg laid) had a longer incubationperiod than the B or C egg. For all three
species,
the numberof daysbetweenhatchingof A and B eggswassignificantlyless(median
= 1 d) than betweenhatchingof B and C eggs(median = 2 d).
DETERMINACI•)N
NYCTICORAX
DE LA FECHA DE ECLOSIONAMIENTO
NYCTICORAX,
EGRETTA
PARA HUEVOS DE
THULA Y CASMERODIUS
ALBUS
Sinopsis.--Seevalu6el m6todode flotaci6nen agua y gravedadespecificapara determinar
la fechade eclosionamiento
de huevosde Yaboa Real (Nycticoraxnycticorax),Garza Blanca
(Egrettathula) y Garza Real (Casmerodius
albus).E1 largo del perlodode incubaci6ny
duraci6ndel perlododeeclosionamiento
ruetambi6ndocumentado
paracadaespecie.
Aunque
el m6todode gravedadespecificapermiti6 predecirmejor la fechade eclosionamiento
que
el m6todode flotaci6n,ambosresuharonpocoprecisos.La regresi6nentre la gravedad
especificay el nfimerode dias previosal eclosionamiento
difiri6 entre camarias,pero no
entre los huevosde una mismacamada.La gravedadespecificapermiti6 predecirla fecha
de eclosionamiento
en la Garza Biancas61opara un perlodode 3.8 d y de 4.7 d, para las
otrasdosespecies.E1 perlodopromediode incubaci6nresult6set de 27.3 d para la Garza
Real, 23.7 d para la Garza Blanca y de 22.8 d. para la Yaboa. Para todaslas especies,el
primerhuevopuesto(A) resuh6conun perlodode incubaci6nmilslargoque los pr6ximos
dos(B y C). Para todaslas especies
el nfimerode dias entre el eclosionamiento
del huevo
A y B rue significativemente
menor (mediana= 1 d) que el eclosionamiento
entre loshuevos
B y C (mediana= 2 d).
145
146]
T. W. Custer
etal.
J.Field
Ornithol.
Spring 1992
The water immersionmethod(Hays and LeCroy 1971, Nol and Blockpoel 1983, Schreiber1970, Van Paassenet al. 1984), the densityindex
method(Collins and Gaston 1987, Furnessand Furness1981, O'Malley
and Evans1980, Saundersand Smith 1981, Woollerand Dunlop 1980),
or a combination of both methods (Dunn et al. 1979, Westerskov 1950)
have been used to predict the stage of incubation in bird eggs.These
methodsare basedon evaporativewater lossin eggsduring incubation
(Drent 1970, Rahn and Ar 1974). The methodshave not been evaluated
for the Ardeidae (bitterns, herons and allies).
Only qualitativeinformationis availableon the lengthof the incubation
period and the time of hatching between first and last eggsof BlackcrownedNight-Herons (Nycticoraxnycticorax,
Gross1923), SnowyEgrets
(Egretta thula) and Great Egrets (Casmerodius
albus, Harrison 1978,
Palmer 1962). Quantitativestudiesfor otherardeidsindicatethat hatching
of the clutchtakesseveraldaysand that the incubationperiod is longer
in early than in later laid eggsin the clutch (Fujioka 1984, Inoue 1985,
Werschkul 1979).
Ardeidshave recentlybeenthe focusof studiesof siblingrivalry, primarily becauseasynchronous
hatchingis widespreadin the family and
results in chicksof different sizeswithin broods(Fujioka 1985, Inoue
1985, Mock and Parker 1986). In addition,becauseof their high trophic
position, ardeids have been selectedas indicatorsof environmentalcontaminants (Bellward et al. 1990, Custer et al. 1992, Ohlendorf et al.
1979). Therefore,methodsof assessing
datesof hatchingfor thesespecies
have both theoreticaland practical significancefor avian ecologists.
The
objectiveof this study was to evaluatethe use of water immersionand
densityindexmethodsto estimatehatchingdatesof Black-crownedNightHeron, Snowy Egret and Great Egret eggs.We also documentedthe
length of the incubationperiod and the duration of the hatchingperiod
for each species.
STUDY
AREA
AND
METHODS
We studiednestingBlack-crownedNight-Herons, Snowy Egrets and
Great Egretson a dredge-materialislandin LavacaBay near Port Lavaca,
Texas (28ø36'N, 96ø34'W; colony609-121, Texas Colonial Waterbird
Society1982) from 14 Apr. to 8 Jul. 1988. We visitedthe colonyevery
2-3 d during the laying and incubationperiodand everyday during the
hatching period; some nestswe visited daily during the laying and incubationperiod. Nests were flaggedwith ribbons,and eggswere individuallymarkedwith a felt-tippedpermanentink penwhen first observed.
We determinedlaying orderby hatchingorder (Custerand Frederick
1990) or by observation.Date of hatchingwas determinedfrom observation or was basedon eggscracking2 d beforehatchingand pipping 1
d beforehatching(T. W. Custer, pers. obs.). Date of laying was determinedby observationor the assumptionthat oneeggwas laid everyother
day (Fujioka 1984, Inoue 1985, Tremblay and Ellison 1980). The in-
Vol.63,No.2
Determining
Heron
andEgretHatching
Dates
[147
cubationperiod of an individual egg was defined as the time (days)
betweenlaying (day 0) and day of hatching.
Eggs were placed in a beaker of water, and the angle between a
horizontalplane and the long axis of the eggwas measuredto the nearest
5øwith a protractor.If the eggfloated,the diameterof the portion above
the surfaceof the water was measuredwith dial calipers(0.01 mm).
Specificgravitywas determinedby dividingthe mass(g) of the eggby
its volume(cm3).Egg volumewas determinedin the field by subtracting
the massof the suspendedeggin water from its massin air (Custer and
Frederick 1990, Evans 1969, Hoyt 1979). A battery-poweredelectronic
balanceaccurateto 0.1 g was usedto determinemassof eggs.Each egg's
volumewas determinedtwice on separatedays.If the estimateof volume
differedby more than 0.1 ml, a third measurementwas taken and either
one of the three valueswas discardedor an averagevolume calculated
from the three volumes.
We usedmultivariateanalysisof variance(MANOVA) to testwhether
regressions
betweenspecificgravityanddaysbeforehatchingof individual
eggsdiffered among clutchesor by laying order (first, secondor third
eggslaid). As a result of limited data, only three-eggclutches,the most
frequentclutchsize(Custer and Frederick 1990), were usedin thesetests.
Prediction of days before hatching based on angle or diameter were
evaluatedwith linear regression.The significancelevel for all testswas
0.05.
We usedlinear regressionto estimatethe relationshipbetweenspecific
gravityand daysbeforehatching.We usedPRESS methodology
(Draper
and Smith 1981) when evaluatingthe utility of the regressionequations
for predictingdaysbefore hatchingand to reducebias causedby using
the samedata to developand test a methodology.For the PRESS technique, a linear regressionbetweenspecificgravity and daysbeforehatching was calculatedfor each eggwith more than one observation.One egg
was excludedand an averageregressionwas calculatedfrom the remaining
eggs.Predictionof days before hatching for each of the specificgravity
measurements
of the excludedeggwerecalculatedwith theaverageregression.This processwas repeatedexcludingone eggat a time. Ninety-five
percentconfidenceintervalsof the differencebetweenpredictedand observeddays before hatchingwere calculatedas the remaining smallest
and largestdifferencesafter elimination of the smallestand largest2.5%
of the observed differences.
Incubationperiodswere comparedamongspeciesand amongA (first
egglaid), B and C eggsof three-eggclutcheswith two-way analysisof
variance(ANOVA). Tukey's multiple comparisonprocedurewas used
to separatemeans.Differencesin the time for all eggsin a clutchto hatch
(either _<1 d or >-2d) were comparedamongspecies(n = 3) and laying
order (n -- 3) with a three dimensionalcontingencytable. Hypotheses
about the proportion of observationsin each categorywere testedwith
log-linear models(Bishopet al. 1975).
148]
T. W. Custer
etal.
J.Field
Ornithol.
Spring 1992
RESULTS
The relationshipbetweendaysbeforehatchingand angleor diameter
of the eggswas lessprecisethan the relationshipbetweendays before
hatchingand specificgravity(Table 1, Figs. 1, 2). Therefore,angleand
diameterof the eggswere not usedin developingpredictiveequations.
An almostperfectlinear relationshipwas foundbetweenspecificgravity
and days before hatching for individual Black-crownedNight-Heron,
Snowy Egret and Great Egret eggs(R 2 > 0.97 for all but one BlackcrownedNight-Heron, two SnowyEgret andoneGreat Egret eggs).This
relationshipvaried so greatly among eggs,however, that methodsattemptingto exploitdirectlythe individualeggrelationshipsfor prediction
were unsuccessful(unpubl. data). For all three species,the regressions
of specificgravityand daysbeforehatchingdiffered(P < 0.0001) among
clutchesbut not amonglaying order within clutches.Therefore,laying
order was not usedin further predictiveequations.
Ninety-five percentof the predictedhatchingdates,basedon specific
gravity measurements,were within -2.8-3.8 d of the actual date of
hatchingfor SnowyEgrets, -3.2-4.7 d for Great Egretsand -3.8-4.7
d for Black-crownedNight-Herons. These intervalswere shortenedby
about1 d at eachendwhen only specificgravitymeasurements
from early
in the incubationperiod were used(Table 2).
Days beforehatching,basedon the averageof all the specificgravity
vs daysbefore hatchingregressionsfrom the PRESS methodology,can
be predictedfrom specificgravity as follows:
Black-crownedNight-Heron
DBH = 207.5 (SG) - 199.5
Snowy Egret
DBH = 173.5 (SG) - 162.9
Great Egret
DBH = 228.9 (SG) - 218.3,
whereDBH -- daysbeforehatchingand SG -- specificgravity(gm/cm3).
The period of incubationwas longestin Great Egrets and longer in
SnowyEgretsthan in Black-crownedNight-Herons (P < 0.0001, Table
3). In all species,A eggswere incubated1 d longer than B or C eggs.
There was no significant interaction (P = 0.69) between speciesand
laying order for the length of incubation.
The numberof daysbetweenhatchingof sequentialeggs(Table 4) did
not differ amongspeciesfor three-eggclutchesof Black-crownedNightHerons, Snowy Egrets and Great Egrets.The number of daysbetween
hatching of A and B eggswas more often -<1 (range -1-3 d) and
significantlydifferent than the number of days betweenhatchingof B
and C eggs,which was more often _>2d (range 0-3 d, P < 0.0001). The
mediannumber of daysfor the entire broodto hatch for all specieswas
3 d and varied
from 0 to 4 d.
DISCUSSION
Our predictionsof hatchingdate by specificgravity are comparableto
resultsfrom studiesof otherspecies.Ninety-fivepercentof the predicted
hatching dates were within 4.7 d of the observeddate of hatching for
BLACK-CROWNED
NIGHT-HERON
i1•{•
80
24
22
18
•
16
40
14
12
•:
20
0
ß
10
8
i
i
i
i
i
i
i
GREAT
i
i
i
i
i
i
i
6
EGRET
o
26
80
TI
•-
22
60
•
40
•
20
0
0
i
•
i
20
•
18
•
14
•
12
10
8
i
i
I
i
i
SNOWY
i
i
i
i
i
i
i '-6
EGRET
26
24
80
22
•'
20
60
40
•
14
12
oø½
20
10
8
i
i
i
28
i
i
i
i
20
DAYS
BEFORE
i
i
i
i
12
i
i
i
6
4
HATCHING
FIGUI•V.
1. Meanangle(o)+_2 SE(opencircles)
between
a horizontal
planeandthelong
axisof an eggandmeandiameter(ram)_+2 SE (darkened
squares)
abovethewater
surface
of immersed
Black-crowned
Night-Heron,
SnowyEgretandGreatEgreteggs
in relationto daysbeforehatching.
BLACK-CROWNED
NIGHT-HERON
1.10
1.08
1.06
1.04
1.02
1.oo
0.98
0.96
0.94
GREAT
EGRET
SNOWY
EGRET
1.10
1'08
1.
1.04
1.02
0.94
1.08
•
1.04
L9
1.02
1.00
0.94
28
20
DAYS
BEFORE
12
4
HATCHING
FIGURV,
2. Mean specific
gravity(gm/cm3) - 2 SE of Black-crowned
Night-Heron,Snowy
Egret and Great Egret eggsin relationto daysbeforehatching.
Vol.63,No.2
Determining
HeronandEgretHatching
Dates
[ 151
TABLE 1.
Relationships,determinedby linear regression,betweendays before hatching
and angleof eggssubmergedin water, diameterof eggsexposedabovewater and specific
gravity of eggsin Great Egrets, Snowy Egrets and Black-crownedNight-Herons.
Explanatory
Species
Great Egret
variable
Angleb
Black-crowned
Night-Heron
Number
Nests
Eggs
Records
a
0.742
22
54
113
0.279
21
50
115
Specificgravitya
0.872
22
56
244
Angle
0.720
28
73
245
Diameter
0.587
30
79
158
Specificgravity
0.896
30
80
431
Angle
Diameter
Specificgravity
0.646
0.319
0.830
26
27
27
62
61
68
217
112
353
Diameter
Snowy Egret
Sample
size
R2
c
of measurements.
Angle of egg submergedin water.
Diameter of egg exposedabovewater.
Specificgravity (g/cm3) of egg.
Black-crownedNight-Heron eggs,3.8 d for SnowyEgret eggsand 4.7 d
for Great Egreteggs.The hatchingdatesof 90% of Great Skua(Catharacta
skua)eggswereestimatedto within 4 d with specificgravitymeasurements
(Furnessand Furness 1981). Specificgravity was used to age 87% of
Silver Gull (Larus novaehollandiae)
eggsto within 2 d (Wooller and
Dunlop 1980) and 69% of cockatoo(five species
combined)eggsto within
3 d (Saundersand Smith 1981). In contrast,for European Starlings
(Sturnusvulgaris),the 95% predictioninterval for the regressionof egg
age and specificgravity was +2.8 d (Dunn et al. 1979), but, becausethe
incubationperiod of the starling is lessthan one-halfthat of the species
in this study,a narrower confidencelimit is expected.
Data from our studysupportthe suggestion
of Nol and Blokpoel(1983)
that variability of incubationbehavioramongadults may accountfor a
large portion of the variability of specificgravity of eggs.In our study,
the regressionof specificgravityand daysbeforehatchvaried significantly
amongclutchesbut not amongeggswithin clutches.
Our ability to predicthatchingdatesdecreased
asthe incubationperiod
T•a•LE 2.
Ninety-five percentconfidencelimits for differencesbetweenpredictedand
actual days before hatching based on deleting the smallest and largest 2.5% of the
differencesof eggsof Black-crownedNight-Herons, Snowy Egretsand Great Egrets.
Black-crowned
Incubation period
Night-Heron
Snowy Egret
Great Egret
-<29 d before hatch
-3.8-4.7
-2.8-3.8
-3.2-4.7
12-29
-2.6-3.1
-1.5-2.8
-2.1-3.1
d before hatch
152]
7'. w. Custer
etal.
J.Field
Ornithol.
Spring 1992
TAI•LE 3.
Length of incubationby laying order of eggsfor three-eggclutchesof BlackcrownedNight-Herons, Snowy Egretsand Great Egrets.
Laying
order
Species
Great Egret
Number of days
of incubation
Mean + SE
Number
of eggs
A
6
28.3 + 0.4
B
6
27.0
+ 0.3
C
6
26.5
+ 0.6
27.3
+ 0.3 A •
Overall
Snowy Egret
A
6
24.3 + 0.3
B
5
23.2
+ 0.2
C
4
23.3
+ 0.3
23.7
+ 0.2 B
23.5
+ 0.3
Overall
Black-crowned
Night-Heron
A
10
B
9
22.6 + 0.3
C
9
22.3
Overall
+ 0.3
22.8 + 0.2 C
• Interspecificmeans not sharing the same letter are significantlydifferent (two-way
ANOVA: Species,Laying order, Species*Layingorder,a = 0.05). A-eggshad significantly
longerincubationperiodsthan B- or G-eggs,regardlessof species.
increased.For example, for Snowy Egrets, 95% of the predicted days
before hatching were -1.5-2.8 of observeddays before hatchingwhen
specificgravity was measuredmore than 12 d prior to hatching,whereas
the interval was -2.8-3.8 d when specificgravity was measuredon any
day beforehatching.This differenceoccurredbecauseof the large variance
in specificgravity late in the incubationperiod. Westerskov(1950) also
observedthis trend and statedthat the useof specificgravity to age RingneckedPheasant(Phasianuscolchicus)eggswas not recommendedfor the
later part of the incubationperiod.
As a resultof the greaterprecisionof specificgravity,we did not explore
the use of flotation of eggsin water to predict hatching date. Dunn et al.
T^BI•. 4.
Difference (d) in hatchingdate betweenlaying order of eggsin three-egg
clutchesof Black-crownedNight-Herons, Snowy Egrets and Great Egrets.
Difference (d) in hatching date
among laying order
Species
(No. clutches)
Black-crownedNightHeron (n = 13)
SnowyEgret (n = 13)
Great Egret (n = 19)
Overall (n = 45)
B-A
-la
0
2
3
0
C-A
1
2
3
0
2
1
3
5
2
9
2 7
3 11
3
4
1
1
4
8
10 21
12
1
1
5
11
12
29
5
1
1
1
C-B
a In one casethe B-egg hatched before the A-egg.
2
4
1
1
2
3
4
3
6
3
1
3
7
2
2
4
10
9
22
4
3
9
Vol.63,No.2
Determining
HeronandEgretHatching
Dates
[ 153
(1979) includedflotationof eggsin water in their key for agingEuropean
Starling eggs,but placedmore quantitativeemphasison specificgravity.
Likewise,Nol and Blokpoel(1983) discounted
flotationof eggsin water
as an agingtechniquefor Ring-billed Gull (Larusdelawarensis)
eggs.In
contrast,the hatchingdate of 95% of Northern Lapwing (Vanellusvanellus)eggswere agedto within 3 d from the angleof the submergedegg
in water and to within 4 d from the diameterof the eggexposedabove
the surface(Van Paassenet al. 1984). Hays and LeCroy (1971) estimated
age of CommonTern (Sternahirundo)eggsto within 2 d usingflotation
of eggsin water, but their samplesize was small (two eggsfor eachof
nine categoriesof embryo development).
In our study,eggswithin clutcheshatchedover severaldays(range =
0-4 d) and the hatchinginterval betweenA and B eggswas shorterthan
betweenB and C eggs.Thesesamepatternshavebeenobservedin Cattle
Egrets(Bubulcus
ibis,Fujioka 1984), Little Egrets(Egrettagarzetta,InDue
1985) and Little Blue Herons (Egretta caerulea,Werschkul 1979), and
resultfrom increasedincubationthroughthe egglaying period (Fujioka
1985, Gross 1923, InDue 1985, Milstein et al. 1970).
We concludefrom this study that specificgravity of eggsis a better
predictorof hatchingdatethan angleof the eggwhen submergedin water
or diameterof the egg exposedabovewater. Even thoughthe ability to
predicthatchingdate from specificgravityof eggswas higher earlier in
incubation,the confidencelimits around the predictionof hatchingdate
were solarge (___3-5d) that we considerthis methodto be only a rough
estimateof hatchingdate in eggsof the three speciesstudiedhere.
ACKNOWLEDGMENTS
We thank Kirke A. King and Patricia S. McDonald for field assistance;Clementine M.
Glenn for typing the manuscript;and LawrenceBlus, Brian Collins, CharlesHenny, Erica
Nol, James Spann and Donald White for reviewingthe manuscript.
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j. Field
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