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

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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 in- cubationperiod. 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 obser- vation or was basedon eggscracking 2 d before hatchingand pipping 1 d beforehatching (T. W. Custer, pers. obs.). Date of laying was deter- minedby 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 hatch- ing 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 averageregres- sion.This processwas repeatedexcluding one eggat a time. Ninety-five percentconfidence intervals of the differencebetween predicted and ob- serveddays 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-
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