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Factors Affecting Reproductive Success of Wood Storks (Mycteria Americana) in East-Central Georgia

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Factors Affecting Reproductive Success of Wood Storks (Mycteria Americana) in East-Central Georgia The Auk 112(1):237-243, 1995 FACTORS AFFECTING REPRODUCTIVE SUCCESS OF WOOD STORKS (MYCTERIA AMERICANA) IN EAST-CENTRAL GEORGIA MALCOLM C. COULTER• AND A. LAWRENCEBRYAN, JR. SavannahRiver Ecology Laboratory, Drawer E, Aiken,South Carolina 29802, USA ABS?R•cr.--From1984 through 1989, we examinedthe reproductivesuccess of WoodStorks (Mycteriaamericana) at the Birdsvillecolony in east-centralGeorgia. Average fledging success rangedamong years from 0.33to 2.16fledglings per nest.For neststhat producedfledglings, prey availabilitywas an importantfactor affecting reproductive success. Yearly averageprey densitiesat foragingsites were significantlycorrelated with the averagenumber of fledglings producedfrom successfulnests. Among 243 nestsobserved, all eggsor chickswere lostfrom 104(43%) nests. Five factorswere associatedwith the lossof entire clutchesor broods.During the two driest years, 1985 and 1988, raccoon(Procyon lotor) predation eliminated almost all chicks.Many nestswere abandonedearly in 1989, following periodsof cold weather when the parentsappeared to be under stress.In 1985, the birds desertedthe colony before egg laying when the area experiencedfreezing weather. Following nest abandonmentswithin the colony,paired adultsthat presumablyhad abandonedtheir nestswere involved in nest takeoversthat alsocaused the lossof eggsand chicks.Three stormsduring the studycaused the lossof a few nests.Some losses were due to unknown factors.The importanceof these mortality factorsvaried from year to year. Nest abandonmentsand subsequentaggression seemto be related to cold periodsearly in the season.Raccoon predation seems to be related to drying out of the water under the colony.This suggeststhat the storkshave a window in time when it is bestto breed--afterthe winter and earlyspring cold weather and beforethe water dries under the colony in the summer.Received 4 December1991, accepted 15 November 1992. THE FACTORSTHAT affect reproductive success 1986, Bryan and Coulter 1991). Birds may also in birds have been a main topic in studies of abandon their nests (Frederick and Collopy avian biology(Lack 1966, 1968, Perrins and Moss 1989b). Moreover, the relative importance of 1975, DeStevens 1980, Winkler and Walters 1983, the various factorsis likely to differ between and referencescited within these papers). Re- years. productivesuccess of wading birds may vary The reproductive successof Wood Storks considerablyamong colonies and betweenyears (Mycteriaamericana) was studied at a colony in (Rodgers1987b, Frederick and Collopy 1989a). east-centralGeorgia from 1984 through 1989 The importanceof food is one factor affecting (Cou,lter 1988).We examinedthe relativeim- reproductivesuccess of wading birds(Kahl 1964, portance of different factors affecting repro- Clark 1979). In southern Florida, a positive re- ductive successand how thesevaried from year lationshiphas been shown between the drying to year. We evaluated whether environmental rate of wetlands and both numbers of nest at- conditions influence these processes and temptsand reproductivesuccess (Frederick and whether, by influencing reproductive success, Collopy 1989a).Some wading birds had greatest these conditionscould affect aspectsof breed- reproductivesuccess or beganbreeding in large ing biology suchas phenology. numbersduring yearswith fasterdrying rates than in yearswith slowerdrying rates(Kushlan et al. 1975). Other factorsalso affect reproduc- ]•ftETHODS tive success:predation (Rodgers 1987a, Fred- We studiedthe breedingbiology of Wood Storks erick and Collopy 1989b), weather (Rodgers at the Birdsvillecolony (32ø52'N,82ø03'W) in Jenkins 1987b),and intraspecificaggression (Frederick County,east-central Georgia from 1984through 1989. The Birdsvillecolony was located in Big DukesPond in all years, except 1985 when the birds nested in •Present address: P.O. Box 48, Chocorua, New Little DukesPond about1 km from Big DukesPond. Hampshire 03817, USA. During theseyears, the colonyhas varied in sizefrom 237 238 COULTERAND ]3RYAIq [Auk, Vol. 112 a minimum of about 100 pairs in 1984 to a maximum spring followed cold weather, when the storks ap- of 193 pairs in 1987. We followed the fate of those peared to be under stress.We attributed these losses neststhat we could observewell eachyear (n = 26 to to stressinduced by the cold weather. In casesin 65) fromthe periodof egglaying (late March) through which nestswere foundempty with no apparentcause, dispersalof young(early July through early Septem- the failure was attributed to unknown causes. ber, varyingamong years). We madeobservations in It was not possibleto examinedirectly the impor- the colonyfrom 0630to 1730EST five daysper week. tanceof food availability to reproductivesuccess be- Observationsin Big Dukes Pond were made from a causeour estimatesof the numbersof fledglingspro- tower 18 m high in 1984and from a 20-m tower in duced were made visually, and we were unable to 1986-1989. In 1985, observations were made in Little assessbody condition.Therefore, we examinedthe Dukes Pond from a tree blind about 7 m high. We relationshipbetween the availabilityof potentialprey beganfollowing nestsonly after the nestscontained at foragingsites (average yearly density and biomass) eggs.The eggs and chickswere countedeach day, and averagefledglings per nest through regression and the causesof losswere determined when possi- and correlationanalyses. In theseanalyses we con- ble. sideredonly nestsfrom which at least one young We were able to count eggsin the nestsonly in fledged.Although somemortality in the successful 1987-1989. We determined the clutch size for each nestsmay have been due to predation, intraspecific nest and considered the maximum number of chicks aggression,or harshweather, any effects of foodavail- countedduring all observationsas a measureof initial ability on reproductivesuccess would be most ap- brood size. We calculatedhatching successas initial parent among these nests.We determined the avail- brood size/clutch size. We determined the number ability of potential prey by sampling their density of chicksfiedging in eachnest as the numberof chicks and biomassat foragingsites visited by storksof the alive at the time that the young couldfirst fly. Birdsvillecolony (Depkin et al. 1992,Coulter unpubl. In someyears, we believed that the reproductive data). successamong the neststhat we followed intensively Data are summarized as œ _+ I SD. In most cases, was not representativeof the entire colony due to parametric statistical tests were used. ANOVA was greaterpredation in someareas of the colonythan in usedto compareparameters among multiple years. otherareas. For all yearswe estimatedthe total num- When ANOVA results indicated significant differ- ber of nestslost to predation,and calculatedan overall ences,Scheff&'s multiple-comparison tests were used fledgingrate for the colony. to examinedifferences between specific years. When When one or two chicks in a nest were lost but not sample sizes were less than 10, nonparametrictests the entire brood,it wasusually difficult to determine were used:Wilcoxon matched-pairs signed-ranks tests, the causeof mortality.On two occasions,we observed chi-squaretest and Spearmanrank correlations.All chicks thrown from their nestsby intruding adults, statistical tests were two-tailed and were considered and on one occasionwe observeda raccoon(Procyon significantat P < 0.05.Analyses were performedwith lotor)taking a chick. In mostcases we were unable to the STATA computerstatistical package of StataCor- determine the causes of these losses. poration. More often we could determine the cause of mor- tality when an entire broodwas lost than when less than the entire brood was lost. Five sources of mor- RESULTS tality were identified.We found entire nestsmissing CLUTCH SIZE AND HATCHING SUCCESS immediatelyfollowing two heavythunderstorms, and attributed theselosses to storm damage.We observed eggsand chicksthrown from nestsby intrudingstorks The storkslaid clutchesof one to five eggs, (Bryan and Coulter 1991),and tallied theseas intra- with most clutchesconsisting of two or three specificaggressions. During dry yearswhen the swamp eggs.From 1987 through 1989, the only years under the nesttrees became dry, raccoonsentered the in whichwe couldcount eggs, the averageclutch colony(usually at night) and killed nestlings.After size was 2.9 + 0.74 eggsper nest (n = 48; Table these invasions,the carcassesof dead young often 1). Significantdifferences existed between years were left in the nests. We attributed these losses to (ANOVA, F:,45= 5.99, P < 0.01). The average raccoonpredation. To corroborateour identification clutch size in 1987 was significantlydifferent of raccoonpredation and examinethe extentthat these from those in 1988 and 1989, but clutch sizes animalsintruded into the colony, F. C. Depkin spent in 1988 and 1989 were not different (Scheff&'s three nights observingthe nestswith a night scope from the tower. Somenests were occupiedone day, multiple-comparisontest). but not occupied the next. There was no indication An averageof 2.6 + 0.71 (range 0-5) chicks that the contentsof the nestshad been disturbedby hatched in 1987-1989 (Table 1). The numbers predators, intraspecific interactions, or inclement of chicksthat hatchedwere significantlydif- weather. Some of these abandonments in the early ferent between 1987 and 1988 (ANOVA, F2•5= January1995] ReproductiveSuccess of Wood Storks 239 TABLE1. Clutch size and hatching success(œ + SD) brood sizes between other years were not sig- of Wood
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