Determination of Hatching Date for Eggs of Black-Crowned Night-Herons, Snowy Egrets and Great Egrets

J. Field Ornithol., 63(2):145-154

DETERMINATION OF HATCHING DATE FOR EGGS OF BLACK-CROWNED NIGHT-HERONS, SNOWY EGRETS AND GREAT EGRETS

THOMAS W. CUSTER U.S. Fish and Wildlife Service Patuxent Wildlife ResearchCenter P.O. Box 2618 La Crosse,Wisconsin 5,1602-2618 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 EcologicalServices Field Office 17629 El Camino Real Suite 211 Houston, Texas 77058 USA

Abstract.--Flotationof eggsin water and specificgravity of eggsof Black-crownedNight- Herons (Nycticoraxnycticorax), Snowy Egrets (Egretta thula) and Great Egrets(Casmerodius albus)were evaluatedas methodsto determinedate of hatching.Length of incubationand durationof hatchingperiod were alsodocumented for eachspecies. Although specific gravity was a better predictorof hatchingdate than egg flotation, both techniqueswere imprecise. The regressionbetween specific gravity and the number of days beforehatching differed among clutches,but not among eggswithin clutches.Specific gravity of eggs predicted hatchingdate only to within 3.8 d for Snowy Egrets,and 4.7 d for Black-crownedNight- Herons and Great Egrets. The mean incubationperiod was 27.3 d for Great Egrets, 23.7 d for Snowy Egrets 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 daysbetween hatching of A and B eggswas significantlyless (median = 1 d) than betweenhatching of B and C eggs(median = 2 d).

DETERMINACI•)N DE LA FECHA DE ECLOSIONAMIENTO PARA HUEVOS DE NYCTICORAX NYCTICORAX, EGRETTA THULA Y CASMERODIUS ALBUS Sinopsis.--Seevalu6 el m6todode flotaci6nen agua y gravedadespecifica para determinar la fechade eclosionamientode huevosde Yaboa Real (Nycticoraxnycticorax), Garza Blanca (Egrettathula) y Garza Real (Casmerodiusalbus). E1 largo del perlodode incubaci6ny duraci6ndel perlodode eclosionamiento rue tambi6n documentado para cada especie. Aunque el m6todode gravedadespecifica permiti6 predecirmejor la fechade eclosionamientoque el m6todode flotaci6n,ambos resuharon poco precisos. La regresi6nentre la gravedad especificay el nfimero de dias previosal eclosionamientodifiri6 entre camarias,pero no entre los huevosde una misma camada.La gravedadespecifica permiti6 predecirla fecha de eclosionamientoen la Garza Bianca s61opara un perlodode 3.8 d y de 4.7 d, para las otrasdos especies. E1 perlodopromedio de incubaci6nresult6 set 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 primer huevopuesto (A) resuh6con un perlodode incubaci6nmils largo que los pr6ximos dos(B y C). Para todaslas especiesel nfimerode dias entre el eclosionamientodel huevo A y B rue significativementemenor (mediana= 1 d) que el eclosionamientoentre loshuevos B y C (mediana= 2 d).

145 146] T. W. Custeret al. J.Field Ornithol. Spring 1992 The water immersionmethod (Hays and LeCroy 1971, Nol and Block- poel 1983, Schreiber1970, Van Paassenet al. 1984), the densityindex method(Collins and Gaston 1987, Furnessand Furness1981, O'Malley and Evans1980, Saundersand Smith 1981, Wooller and 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 qualitativeinformation is availableon the lengthof the incubation period and the time of hatching between first and last eggs of Black- crownedNight-Herons (Nycticoraxnycticorax, Gross 1923), SnowyEgrets (Egretta thula) and Great Egrets (Casmerodiusalbus, Harrison 1978, Palmer 1962). Quantitativestudies for otherardeids indicate that hatching of the clutchtakes severaldays and that the incubationperiod is longer in early than in later laid eggsin the clutch (Fujioka 1984, Inoue 1985, Werschkul 1979). Ardeidshave recentlybeen the focusof studiesof siblingrivalry, pri- marily becauseasynchronous hatching is widespreadin the family and results in chicksof different sizeswithin broods (Fujioka 1985, Inoue 1985, Mock and Parker 1986). In addition,because of their high trophic position, ardeids have been selectedas indicatorsof environmentalcon- taminants (Bellward et al. 1990, Custer et al. 1992, Ohlendorf et al. 1979). Therefore, methodsof assessingdates of hatchingfor thesespecies have both theoreticaland practical significancefor avian ecologists.The objectiveof this study was to evaluate the use of water immersion and densityindex methodsto estimatehatching dates of Black-crownedNight- Heron, Snowy Egret and Great Egret eggs.We also documentedthe length of the incubationperiod and the duration of the hatching period for each species.

STUDY AREA AND METHODS

We studiednesting Black-crowned Night-Herons, Snowy Egrets and Great Egretson a dredge-materialisland in Lavaca Bay 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 incubationperiod and everyday during the hatching period; some nests we visited daily during the laying and incubationperiod. Nests were flaggedwith ribbons,and eggswere indi- vidually markedwith a felt-tippedpermanent ink pen when first observed. We determinedlaying order by hatchingorder (Custer and Frederick 1990) or by observation.Date of hatchingwas determinedfrom observation or was basedon eggscracking 2 d before hatchingand pipping 1 d beforehatching (T. W. Custer, pers. obs.). Date of laying was determinedby observationor the assumptionthat oneegg was laid everyother day (Fujioka 1984, Inoue 1985, Tremblay and Ellison 1980). The in- Vol.63, No. 2 DeterminingHeron and Egret Hatching 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 5ø with a protractor.If the egg floated,the diameter of the portion above the surfaceof the water was measuredwith dial calipers(0.01 mm). Specificgravity was determinedby dividingthe mass(g) of the eggby its volume(cm3). Egg volumewas determinedin the field by subtracting the massof the suspendedegg in water from its massin air (Custer and Frederick 1990, Evans 1969, Hoyt 1979). A battery-poweredelectronic balanceaccurate to 0.1 g was usedto determinemass of 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 usedmultivariate analysisof variance(MANOVA) to testwhether regressionsbetween specific gravity and daysbefore hatching of individual eggsdiffered among clutchesor by laying order (first, secondor third eggslaid). As a result of limited data, only three-eggclutches, the most frequentclutch size (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 relationshipbetween specific gravityand daysbefore hatching. We usedPRESS methodology(Draper and Smith 1981) when evaluatingthe utility of the regressionequations for predictingdays before hatchingand to reducebias causedby using the samedata to developand test a methodology.For the PRESS tech- nique, a linear regressionbetween specific gravity and daysbefore hatching was calculatedfor each egg with more than one observation.One egg was excludedand an averageregression was calculatedfrom the remaining eggs.Prediction of days before hatching for each of the specificgravity measurementsof the excludedegg were calculatedwith the averageregression.This processwas repeatedexcluding one eggat a time. Ninety-five percentconfidence intervals of the differencebetween predicted and observeddays before hatchingwere calculatedas the remaining smallest and largestdifferences after elimination of the smallestand largest2.5% of the observed differences. Incubationperiods were comparedamong species and amongA (first egg laid), B and C eggsof three-eggclutches with two-way analysisof variance (ANOVA). Tukey's multiple comparisonprocedure was used to separatemeans. Differences in the time for all eggsin a clutchto hatch (either _<1 d or >-2d) were comparedamong species(n = 3) and laying order (n -- 3) with a three dimensionalcontingency table. Hypotheses about the proportion of observationsin each categorywere testedwith log-linear models(Bishop et al. 1975). 148] T. W. Custeret al. J.Field Ornithol. Spring 1992

RESULTS The relationshipbetween days before hatching and angle or diameter of the eggswas lessprecise than the relationshipbetween days before hatchingand specificgravity (Table 1, Figs. 1, 2). Therefore,angle and diameterof the eggswere not usedin developingpredictive equations. An almostperfect linear relationshipwas foundbetween specific gravity and days before hatching for individual Black-crownedNight-Heron, Snowy Egret and Great Egret eggs(R 2 > 0.97 for all but one Black- crownedNight-Heron, two SnowyEgret and oneGreat Egret eggs).This relationshipvaried so greatly among eggs,however, that methodsat- temptingto exploit directlythe individualegg relationships for prediction were unsuccessful(unpubl. data). For all three species,the regressions of specificgravity and daysbefore hatching differed (P < 0.0001) among clutchesbut not amonglaying order within clutches.Therefore, laying order was not usedin further predictiveequations. Ninety-five percentof the predictedhatching dates, based on specific gravity measurements,were within -2.8-3.8 d of the actual date of hatchingfor Snowy Egrets, -3.2-4.7 d for Great Egretsand -3.8-4.7 d for Black-crownedNight-Herons. These intervalswere shortenedby about 1 d at eachend when only specificgravity measurements from early in the incubationperiod were used(Table 2). Days beforehatching, based on the averageof all the specificgravity vs days before hatchingregressions from 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, where DBH -- daysbefore hatching and SG -- specificgravity (gm/cm3). The period of incubationwas longestin Great Egrets and longer in Snowy Egrets than in Black-crownedNight-Herons (P < 0.0001, Table 3). In all species,A eggswere incubated 1 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 daysbetween hatching of sequentialeggs (Table 4) did not differ amongspecies for three-eggclutches of Black-crownedNight- Herons, Snowy Egrets and Great Egrets. The number of daysbetween hatching of A and B eggs was more often -<1 (range -1-3 d) and significantlydifferent than the number of days betweenhatching of B and C eggs,which was more often _>2d (range 0-3 d, P < 0.0001). The median number 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 other species.Ninety-five percent of the predicted hatching dates were within 4.7 d of the observeddate of hatching for BLACK-CROWNED NIGHT-HERON

80 i1•{• 2422

18 16 • 40 14 12 •: 20 0 ß 10 8 i i i i i i i i i i i i i i 6

GREAT EGRET

o 26 80 TI 22

•- 60 20 • 18 •

• 40 14 •

• 20 10 0 0 • 812 i i i i I i i i i i i i i i '-6

SNOWY EGRET

26 80 24 22 •' 60 20

40 14 12 • 20 oø½ 10 8 i i i i i i i i i i i i i i 6 28 20 12 4 DAYS BEFORE HATCHING FIGUI•V.1. Meanangle (o) +_ 2 SE(open circles) between a horizontal plane and the long axisof an eggand mean diameter (ram) _+2 SE (darkenedsquares) above the water surfaceof immersedBlack-crowned Night-Heron, Snowy Egret and Great Egret eggs in relationto daysbefore hatching. BLACK-CROWNED NIGHT-HERON 1.10

1.08 1.06 1.04

1.02

1.oo 0.98

0.96

0.94

GREAT EGRET 1.10 1'081. 1.04 1.02

0.94

SNOWY EGRET

1.08

• 1.04

L9 1.02 1.00

0.94 28 20 12 4 DAYS BEFORE HATCHING FIGURV,2. Mean specificgravity (gm/cm 3) - 2 SE of Black-crownedNight-Heron, Snowy Egret and Great Egret eggsin relationto daysbefore hatching. Vol.63, No. 2 DeterminingHeron and Egret Hatching Dates [ 151

TABLE 1. Relationships,determined by linear regression,between days before hatching and angle of eggssubmerged in water, diameterof eggsexposed above water and specific gravity of eggsin Great Egrets, Snowy Egrets and Black-crownedNight-Herons. Explanatory Samplesize Species variable R 2 Nests Eggs Recordsa Great Egret Angleb 0.742 22 54 113 Diameter c 0.279 21 50 115 Specificgravity a 0.872 22 56 244 Snowy Egret Angle 0.720 28 73 245 Diameter 0.587 30 79 158 Specificgravity 0.896 30 80 431 Black-crowned Angle 0.646 26 62 217 Night-Heron Diameter 0.319 27 61 112 Specificgravity 0.830 27 68 353

Number of measurements. Angle of egg submergedin water. Diameter of egg exposedabove water. Specificgravity (g/cm3) of egg.

Black-crownedNight-Heron eggs,3.8 d for Snowy Egret eggsand 4.7 d for Great Egret eggs.The hatchingdates of 90% of Great Skua (Catharacta skua)eggs were estimated to within 4 d with specificgravity measurements (Furness and Furness 1981). Specificgravity was used to age 87% of Silver Gull (Larus novaehollandiae)eggs to within 2 d (Wooller and Dunlop 1980) and 69% of cockatoo(five speciescombined) eggs to 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-half that of the species in this study, a narrower confidencelimit is expected. Data from our studysupport the suggestionof Nol and Blokpoel(1983) that variability of incubationbehavior among adults may accountfor a large portion of the variability of specificgravity of eggs.In our study, the regressionof specificgravity and daysbefore hatch varied significantly among clutchesbut not among eggswithin clutches. Our ability to predicthatching dates decreased as the incubationperiod

T•a•LE 2. Ninety-five percent confidencelimits for differencesbetween predicted and 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 d before hatch -2.6-3.1 -1.5-2.8 -2.1-3.1 152] 7'. w. Custeret al. J.Field Ornithol. Spring 1992 TAI•LE 3. Length of incubationby laying order of eggsfor three-eggclutches of Black- crownedNight-Herons, Snowy Egrets and Great Egrets.

Number of days Laying Number of incubation Species order of eggs Mean + SE Great Egret A 6 28.3 + 0.4 B 6 27.0 + 0.3 C 6 26.5 + 0.6 Overall 27.3 + 0.3 A • Snowy Egret A 6 24.3 + 0.3 B 5 23.2 + 0.2 C 4 23.3 + 0.3 Overall 23.7 + 0.2 B

Black-crowned A 10 23.5 + 0.3 Night-Heron B 9 22.6 + 0.3 C 9 22.3 + 0.3 Overall 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 longerincubation periods than B- or G-eggs,regardless of species. increased.For example, for Snowy Egrets, 95% of the predicted days before hatching were -1.5-2.8 of observeddays before hatching when 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 differenceoccurred because of the large variance in specificgravity late in the incubationperiod. Westerskov(1950) also observedthis trend and statedthat the useof specificgravity to age Ring- neckedPheasant (Phasianus colchicus) eggs was not recommendedfor the later part of the incubationperiod. As a resultof the greaterprecision of 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 hatching date betweenlaying order of eggsin three-egg clutchesof Black-crownedNight-Herons, Snowy Egrets and Great Egrets.

Difference (d) in hatching date among laying order B-A C-B C-A Species (No. clutches) -la 0 1 2 3 0 1 2 3 0 1 2 3 4 Black-crownedNight- Heron (n = 13) 5 3 5 2 9 2 1 3 6 3 Snowy Egret (n = 13) 2 7 3 1 1 4 8 1 3 7 2 Great Egret (n = 19) 1 3 11 4 5 12 2 2 4 9 4 Overall (n = 45) 1 10 21 12 1 1 11 29 4 1 3 10 22 9

a In one case the B-egg hatched before the A-egg. Vol.63, No. 2 DeterminingHeron and Egret Hatching Dates [ 153

(1979) includedflotation of eggsin water in their key for agingEuropean Starling eggs,but placedmore quantitativeemphasis on specificgravity. Likewise,Nol and Blokpoel(1983) discountedflotation of eggsin water as an agingtechnique for Ring-billed Gull (Larusdelawarensis) eggs. In contrast,the hatchingdate of 95% of Northern Lapwing (Vanellusva- nellus)eggs were agedto within 3 d from the angle of the submergedegg in water and to within 4 d from the diameterof the egg exposedabove the surface(Van Paassenet al. 1984). Hays and LeCroy (1971) estimated age of Common Tern (Sternahirundo) eggsto within 2 d using flotation of eggsin water, but their samplesize was small (two eggsfor each of nine categoriesof embryo development). In our study,eggs within clutcheshatched over severaldays (range = 0-4 d) and the hatchinginterval betweenA and B eggswas shorterthan betweenB and C eggs.These same patterns have been observed in Cattle Egrets(Bubulcus ibis, Fujioka 1984), Little Egrets(Egretta garzetta, InDue 1985) and Little Blue Herons (Egretta caerulea,Werschkul 1979), and resultfrom increasedincubation through the egglaying period (Fujioka 1985, Gross 1923, InDue 1985, Milstein et al. 1970). We concludefrom this study that specificgravity of eggsis a better predictorof hatchingdate than angleof the eggwhen submergedin water or diameterof the egg exposedabove water. Even though the ability to predicthatching date from specificgravity of eggswas higher earlier in incubation,the confidencelimits around the predictionof hatchingdate were so large (___3-5d) that we considerthis methodto be only a rough estimateof hatchingdate in eggsof the three speciesstudied here.

ACKNOWLEDGMENTS

We thank Kirke A. King and Patricia S. McDonald for field assistance;Clementine M. Glenn for typing the manuscript;and Lawrence Blus, Brian Collins, CharlesHenny, Erica Nol, James Spann and Donald White for reviewing the manuscript.

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