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

-We studied predation by Fish Crows (Corvus ossifragus) on eggs of the White Ibis (Eudocimus albus) during the 1983 and 1984 nesting seasons at Battery Island, southeastern North Carolina. Crow predation accounted for the loss of 32% (n = 223) of ibis eggs in 1983 and 44% (n = 538) in 1984. Crows usually took all eggs in a clutch. An estimated 6 pairs of Fish Crows nested on the island each year. We believe these individuals were responsible for most egg loss. The predation rate of ibis clutches was highest in plots nearest crow nests and lowest in two plots that contained observation blinds. Results of experiments using simulated ibis nests suggested that crows were wary of the blinds. Predation declined with nest age, apparently due to increased nest attentiveness by adult ibises during the last week of incubation. The overall predation rate in 1984 was significantly higher than in 1983. Greater nest densities and less synchronous breeding by ibises in 1984 may have contributed to the higher predation rate. Ibis productivity was estimated at 1.22-1.30 fledglings per pair in 1983 and 1.05-1.12 in 1984. This level of reproduction appeared sufficient for maintenance of the population. Thus, egg predation by Fish Crows during our study did not appear to be a serious threat to the productivity of this White Ibis population. Received 24 June 1985, accepted 22 January 1986. CROWS (Corvus spp.) are well known as predators of birds' eggs. Upon finding an unguarded nest, a crow typically flies off with an egg, eats or caches it, and then returns to the same area to steal another egg (Tinbergen et al. 1967, Croze 1970, Montevecchi 1976). Because crows often return to sites of previous prey capture, their predation success increases with increasing prey density (Tinbergen et al. 1967, Goransson et al. 1975, Montevecchi 1977). Thus, predation on eggs of colonial nesters, particularly wading birds (Ciconiiformes) that do not exhibit group-mobbing antipredator behavior, may be severe (Krebs 1978). Crow predation on eggs of wading birds is documented thoroughly (e.g. Bent 1926, Baker 1940, Meanley 1955, Dusi and Dusi 1968, Rudegeair 1975, Burger and Hahn 1977, Maxwell and Kale 1977, Tremblay and Ellison 1979, Allen-Grimes 1982, Frederick 1985). We can find no detailed studies, however, of the extent of egg predation during more than one breeding season, factors affecting the rate of predation, or the impact of this egg loss on wading bird populations. 1 Present address: Wyoming Cooperative Fishery and Wildlife Research Unit, Box 3166, University Station, Laramie, Wyoming 82071 USA. The White Ibis (Eudocimus albus) reaches its northern limit of regular breeding in coastal North Carolina (A.O.U. 1983). White Ibis breeding biology was studied by Allen-Grimes (1982) at Battery Island, site of the largest White Ibis colony in North Carolina. She found nesting success to be significantly lower than in Florida populations (Rudegeair 1975, Kushlan 1977) and attributed this, in part, to high egg loss. Allen-Grimes frequently saw Fish Crows (C. ossifragus), which also nested on the island, carrying off eggs. She suggested that crow predation may have been a major reason for low nesting success of this ibis population. We initiated our study specifically to examine Fish Crow predation on ibis eggs at Battery Island. Our objectives were to determine the extent of Fish Crow predation on White Ibis eggs, to examine temporal and spatial patterns of predation, and to evaluate the impact of predation on ibis productivity. STUDY AREA AND METHODS We conducted the study from April to August 1983 and 1984 at Battery Island (33?54'N, 78?01'W), a National Audubon Society sanctuary located in the Cape Fear River estuary 1 km. southeast of Southport, Brunswick Co., North Carolina. This island has sup531 The Auk 103: 531-539. July 1986 This content downloaded from 157.55.39.59 on Sat, 15 Oct 2016 04:42:30 UTC All use subject to http://about.jstor.org/terms 532 SHIELDS AND PARNELL [Auk, Vol. 103 ported a mixed-species heronry since at least 1938 (Brimley 1938). Recently it has held the largest heronry in North Carolina (Parnell and Soots 1979, Parnell and McCrimmon 1984) and the northernmost large (>100 pairs) breeding colony of White Ibises in North America (Allen-Grimes 1982, Shields and Parnell 1983). Comprising an area of about 40 ha, Battery Island is mostly salt marsh dominated by Spartina alterniflora. Two wooded uplands, North (1 ha) and South (7 ha) colonies, provide nesting habitat for the large wading bird assemblage. White Ibises nest only in the South Colony. The South Colony site, created by deposition of dredged material (Funderburg 1960), is a grass/forbcovered dome fringed by a maritime thicket. The thicket vegetation is composed of red cedar (Juniperus virginiana), yaupon (Ilex vomitoria), Hercules'-club (Zanthoxylum clava-herculis), wax myrtle (Myrica cerifera), groundsel-tree (Baccharis halimifolia), marsh elder (Iva frutescens), and several other woody species. Trees and shrubs are also scattered in clumps across the dome. Woody vegetation on the dome is dominated by red cedar, with lesser numbers of yaupon, wax myrtle, Hercules'-club, cherry (Prunus sp.), red mulberry (Morus rubra), and buckthorn (Bumelia lycioides). In 1983 the South Colony contained an estimated 3,737 White Ibis nests and 637 nests of eight other wading bird species (Casmerodius albus; Bubulcus ibis; Egretta thula; E. caerulea; E. tricolor; Butorides striatus; Black-crowned Night-Heron, Nycticorax nycticorax; and Plegadis falcinellus). The South Colony held 4,849 White Ibis nests and 852 nests of the other eight species in 1984. At least 2 pairs of Fish Crows nested in the South Colony in 1983. We located 6 nests during extensive searches in 1984. Other potential egg predators in the South Colony included Blackcrowned Night-Herons, Boat-tailed Grackles (Quiscalus major), Norway rats (Rattus norvegicus), and snakes (Elaphe obsoleta quadrivittata and E. g. guttata). No large mammals inhabited the island. We determined the fates of White Ibis nests in ten 15 x 15 m plots selected randomly from a pre-established grid. We used the same plots in both years; White Ibises did not nest in one plot (R15) in 1984, however, and another (J28) was chosen to replace it. White Ibis nests comprised over 95% of all wading bird nests in the plots used each year. We attempted to individually mark all ibis nests and eggs in each plot. The highest nests in most plots could not be reached easily without endangering lower nests and therefore were excluded from study. Unmarked nests accounted for about 15% of all ibis nests in the study plots. We marked nests with inconspicuous plastic tags wired beneath the nest bowl. Eggs were numbered on both ends using waterproof India ink. We did not mark empty nests. We usually visited nests every 3-4 days to record contents. To minimize colony disturbance and thermal stress to eggs and young, visits were made as quickly as possible (<30 min/plot) during 0700-1200. If eggs were missing, we searched the area immediately surrounding the nest for the presence of eggshells. In addition, we regularly collected all eggshells and shell fragments within each plot. We recorded a predation loss if an egg was punctured in the manner characteristic of crows (Rearden 1951) or if the disappearance of an egg could not be attributed to the egg hatching or falling from the nest (Burger and Hahn 1977, Montevecchi 1977, Gottfried and Thompson 1978, Miller and Burger 1978). We computed a daily predation rate (DPR) by dividing the number of clutches predated (partial clutch loss was small) by the amount of nest exposure (in nest-days) (Mayfield 1961, 1975). The data were grouped by plot into 1-week periods, with the first week beginning on the date of our first visit, to examine spatial and temporal patterns in DPR each year. We compared DPRs between two groups of nests using the variance estimator and statistical test described by Hensler and Nichols (1981). When more than two groups were compared, we employed the Bonferroni multiple comparison method (Miller 1966). We estimated ibis nesting success using the Mayfield method to evaluate the impact of egg predation on ibis productivity. We calculated nest success separately for the egg and nestling stages. We defined the egg stage as the period from the day the first egg in a clutch was laid to the day before hatching of the first egg. The nestling stage extended from the day the first egg hatched to 10 days after that date, at which time nestling ibises began to leave their nests upon our approach and no longer could be associated with specific nests (cf. Custer et al. 1983). If the first clutch is destroyed, many wading birds will lay a replacement clutch, often using the original nest (Jenni 1969, Milstein et al. 1970, Maxwell and Kale 1977). Failure to take renesting into account will result in an underestimate of nesting success (Custer and Pitelka 1977). We estimated the extent of renesting by ibises to reduce this bias. Because unattended White Ibis nests are quickly (often within one day) dismantled by neighboring birds (Rudegeair 1975, Shields pers. obs.), we considered a clutch to be a replacement if the nest was not dismantled between the time of orginal egg loss and subsequent laying (Schreiber 1979). RESULTS AND DISCUSSION We individually marked 694 eggs in 262 clutches in 1983 and 1,213 eggs in 493 clutches in 1984. We marked 98% of the nests during the first week of incubation. This content downloaded from 157.55.39.59 on Sat, 15 Oct 2016 04:42:30 UTC All use subject to http://about.jstor.org/terms July 1986] Crow Predation on Ibis Eggs 533 TABLE 1. Fates of White Ibis eggs in sample clutches at Battery Island.