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Fish Crow Predation on Eggs of the White Ibis at Battery Island, North Carolina

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Fish Crow Predation on Eggs of the White Ibis at Battery Island, North Carolina FISH CROW PREDATION ON EGGS OF THE WHITE IBIS AT BATTERY ISLAND, NORTH CAROLINA MARK A. SHIELDS• AND JAMESF. PARNELL Departmentof BiologicalSciences, University of North Carolina, Wilmington,North Carolina28403 USA ABSTRACT.--Westudied predation by Fish Crows (Corvusossifragus) on eggsof the White Ibis (Eudocimusalbus) during the 1983 and 1984nesting seasonsat BatteryIsland, southeastern North Carolina. Crow predation accountedfor the lossof 32% (n = 223) of ibis eggsin 1983 and 44% (n = 538) in 1984.Crows usually took all eggsin a clutch. An estimated6 pairs of Fish Crows nested on the island each year. We believe these individuals were responsible for mostegg loss.The predation rate of ibis clutcheswas highest in plots nearestcrow nests and lowest in two plots that containedobservation blinds. Resultsof experimentsusing simulatedibis nestssuggested that crowswere wary of the blinds. Predationdeclined with nest age,apparently due to increasednest attentivenessby adult ibisesduring the last week of incubation.The overall predation rate in 1984 was significantly higher than in 1983. Greaternest densities and lesssynchronous breeding by ibisesin 1984may have contributed to the higher predationrate. Ibis productivitywas estimatedat 1.22-1.30fledglings per pair in 1983and 1.05-1.12 in 1984.This level of reproductionappeared sufficient for maintenance of the population.Thus, egg predationby Fish Crows during our study did not appearto be a seriousthreat to the productivityof this White Ibis population.Received 24 June1985, accepted22 January1986. CROWS(Corvus spp.) are well known as pred- The White Ibis (Eudocimusalbus) reaches its ators of birds' eggs.Upon finding an unguard- northern limit of regular breeding in coastal ed nest, a crow typically flies off with an egg, North Carolina (A.O.U. 1983). White Ibis eats or caches it, and then returns to the same breedingbiology was studiedby Allen-Grimes area to stealanother egg (Tinbergen et al. 1967, (1982)at BatteryIsland, site of the largestWhite Croze 1970, Montevecchi 1976). Because crows Ibis colony in North Carolina. She found nest- often return to sites of previous prey capture, ing successto be significantly lower than in their predation successincreases with increas- Florida populations (Rudegeair 1975, Kushlan ing prey density (Tinbergen et al. 1967, Gfr- 1977) and attributed this, in part, to high egg ansson et al. 1975, Montevecchi 1977). Thus, loss.Allen-Grimes frequently saw Fish Crows predation on eggs of colonial nesters,particu- (C. ossifragus),which alsonested on the island, larly wading birds (Ciconiiformes)that do not carryingoff eggs.She suggestedthat crow pre- exhibit group-mobbingantipredator behavior, dation may have been a major reasonfor low may be severe(Krebs 1978).Crow predation on nesting successof this ibis population. We ini- eggsof wading birds is documentedthorough- tiated our study specifically to examine Fish ly (e.g. Bent 1926, Baker 1940, Meanley 1955, Crow predation on ibis eggsat Battery Island. Dusi and Dusi 1968,Rudegeair 1975, Burger and Our objectiveswere to determine the extent Hahn 1977, Maxwell and Kale 1977, Tremblay of Fish Crow predation on White Ibis eggs, to and Ellison 1979, Allen-Grimes 1982, Frederick examine temporal and spatial patterns of pre- 1985). We can find no detailed studies, how- dation, and to evaluatethe impact of predation ever, of the extentof egg predationduring more on ibis productivity. than one breeding season,factors affecting the rate of predation, or the impact of this egg loss STUDY AREA AND METHODS on wading bird populations. We conductedthe studyfrom April to August1983 and 1984 at Battery Island (33ø54'N,78ø01'W), a Na- • Present address:Wyoming Cooperative Fishery tional Audubon Societysanctuary located in the Cape and Wildlife ResearchUnit, Box3166, University Sta- Fear River estuary !km southeastof Southport, tion, Laramie, Wyoming 82071 USA. Brunswick Co., North Carolina. This island has sup- 531 The Auk 103:531-539. July 1986 532 S•ELr•sAND PARNELL [Auk,Vol. 103 ported a mixed-speciesheronry since at least 1938 We usually visited nestsevery 3-4 days to record (Brimley 1938).Recently it has held the largesther- contents.To minimize colony disturbanceand ther- onry in North Carolina (Parnell and Soots1979, Par- mal stressto eggs and young, visits were made as nell and McCrimmon 1984) and the northernmost quicklyas possible (<30 min/plot) during 0700-1200. large (>100 pairs) breeding colony of White Ibises If eggswere missing,we searchedthe area immedi- in North America (Allen-Grimes 1982, Shields and ately surroundingthe nest for the presenceof egg- Parnell 1983). shells. In addition, we regularly collected all egg- Comprisingan area of about40 ha, BatteryIsland shells and shell fragments within each plot. We is mostlysalt marsh dominated by Spartinaalternifiora. recordeda predation lossif an egg was punctured in Two woodeduplands, North (1 ha) and South(7 ha) the manner characteristicof crows (Rearden 1951) or colonies,provide nesting habitat for the large wad- if the disappearanceof an eggcould not be attributed ing bird assemblage.White Ibises nest only in the to the egg hatchingor falling from the nest (Burger South Colony. and Hahn 1977, Montevecchi 1977, Gottfried and The South Colony site, createdby depositionof Thompson1978, Miller and Burger 1978). dredgedmaterial (Funderburg 1960), is a grass/forb- We computeda daily predationrate (DPR) by di- covered dome fringed by a maritime thicket. The viding the numberof clutchespredated (partial clutch thicket vegetationis composedof red cedar(Juniperus losswas small) by the amount of nest exposure(in virginiana),yaupon (Ilex vomitoria),Hercules'-club nest-days)(Mayfield 1961, 1975). The data were (Zanthoxylumclava-herculis), wax myrtle (Myrica ceri- groupedby plot into 1-weekperiods, with the first fera),groundsel-tree (Baccharis halimifolia), marsh eld- week beginning on the date of our first visit, to ex- er (Ira frutescens),and severalother woody species. aminespatial and temporalpatterns in DPR eachyear. Trees and shrubsare also scatteredin clumps across We comparedDPRs between two groupsof nestsus- the dome. Woody vegetationon the dome is domi- ing the variance estimator and statistical test de- nated by red cedar, with lessernumbers of yaupon, scribedby Hensler and Nichols (1981). When more wax myrtle, Hercules'-club,cherry (Prunussp.), red than two groupswere compared,we employed the mulberry (Morusrubra), and buckthorn (Bumelialy- Bonferroni multiple comparison method (Miller cioides). 1966). In 1983 the South Colony containedan estimated We estimatedibis nesting successusing the May- 3,737 White Ibis nestsand 637 nestsof eight other field methodto evaluatethe impactof egg predation wading bird species(Casmerodius albus; Bubulcus •bis; on ibis productivity. We calculatednest successsep- Egrettathula; E. caerulea;E. tricolor;Butorides striatus; arately for the egg and nestling stages.We defined Black-crowned Night-Heron, Nycticorax nycticorax; the egg stageas the period from the day the first egg and Plegadisfalcinellus). The SouthColony held 4,849 in a clutch was laid to the day before hatching of the White Ibis nests and 852 nests of the other eight first egg. The nestling stageextended from the day speciesin 1984.At least 2 pairs of Fish Crows nested the first egg hatched to 10 days after that date, at in the South Colony in 1983. We located 6 nestsdur- which time nestling ibisesbegan to leave their nests ing extensivesearches in 1984. Other potential egg upon our approachand no longer could be associated predators in the South Colony included Black- with specificnests (cf. Custeret al. 1983). crowned Night-Herons, Boat-tailed Grackles (Quis- If the first clutchis destroyed,many wading birds caiusmajor), Norway rats (Rattus norvegicus), and snakes will lay a replacementclutch, often using the origi- (Elapheobsoleta quadrivittata and E.g. guttata).No large nal nest (Jenni 1969, Milstein et al. 1970, Maxwell mammals inhabited the island. and Kale 1977).Failure to take renestinginto account We determined the fates of White Ibis nests in ten will result in an underestimate of nesting success 15 x 15 m plots selectedrandomly from a pre-estab- (Custer and Pitelka 1977). We estimated the extent of lished grid. We used the same plots in both years; renesting by ibises to reduce this bias. Becauseun- White Ibisesdid not nest in one plot (R15) in 1984, attendedWhite Ibis nestsare quickly (often within however, and another (J28)was chosento replaceit. one day) dismantled by neighboring birds (Rude- White Ibis nests comprisedover 95% of all wading geair 1975,Shields pers. obs.), we considereda clutch bird nestsin the plots used each year. We attempted to be a replacementif the nest was not dismantled to individually mark all ibis nests and eggs in each between the time of orginal egg lossand subsequent plot. The highest nests in most plots could not be laying (Schreiber1979). reachedeasily without endangeringlower nestsand thereforewere excludedfrom study.Unmarked nests RESULTS AND DISCUSSION accountedfor about 15%of all ibis nestsin the study plots. We marked nestswith inconspicuousplastic We individually marked 694 eggs in 262 tags wired beneath the nest bowl. Eggs were num- clutchesin 1983and 1,213eggs in 493 clutches bered on both ends using waterproofIndia ink. We in 1984. We marked 98% of the nests during did not mark empty nests. the first week of incubation. July1986] CrowPredation onIbis Eggs 533 TABLE1. Fatesof White Ibis eggsin sampleclutches
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