Colony and Nest Site Selection in White-Faced and Glossy Ibises

-The factors determining colony and nest site selection in the White-faced and Glossy Ibises (Plegadis) were examined from 1972 through 1975. White-faced Ibises were studied in Texas and Argentina, Glossy Ibises in New Jersey and New York. White-faced Ibises generally nest in tule marshes, infrequently on dry land. In selecting a dry land site, White-faced Ibises preferred to nest on the ground among low shrubs and in mixed forbs rather than in habitats containing grass and cactus. Glossy Ibises' habitat varies from Phragmites, Iva, and Smilax to other low shrubs. They did not always nest on the ground. White-faced Ibises nested next to conspecifics, whereas in Glossy Ibises the choice of nearest neighbor was random with respect to the species present. The mean distance to the nearest neighbor was generally less for Glossy Ibis when compared to White-faced Ibis. The space around nests that was devoid of any other nest was examined graphically. Nearest neighbors were farther away when they were on the same level than when the nearest neighbor was above the nest being examined for both species. The visibility index from the nests of Glossy and White-faced Ibises was low, and was less than the mean visibility index for heron and egret species nesting in the same colonies. Visibility was less in the direction of the closest nest compared to that in other directions for all colonies examined. In some colonies, the visibility index in the direction of the closest nest was directly related to the distance to the closest nest. Glossy Ibises nested in denser habitats than did White-faced Ibis, and this was reflected in lower visibility indices for Glossy Ibis. Possible reasons for the recent range changes in the two Plegadis species are discussed. Department of Biology, Livingston College, Rutgers University, New Brunswick, New Jersey 08903. Accepted 15 January 1976. MOST studies of the breeding biology, behavior, and ecology of the family Threskiornithidae have only mentioned or described the nesting habitat briefly. We know of no study of the factors determining colony and nest site selection in any of the ibises or spoonbills. And, with the exception of Jenni's (1969) study on four species of herons during the breeding season, little work had been done on nesting preferences of herons and egrets, which often nest with ibises and spoonbills. We studied the factors determining colony and nest site selection in 2 colonies of White-faced Ibises (Plegadis chihi) and 5 colonies of Glossy Ibises (P. falcinellus), with a view to comparing and contrasting colony and nest site selection in these 2 closely related species, which may help explain the causes of recent range changes. Until recently in the Western Hemisphere the Glossy Ibis bred in the Eastern United States and White-faced Ibis bred in the Western United States, Mexico, and southern South America (Palmer 1962, A.O.U. Checklist 1957). Both species have been reported to breed in Florida (Brewster 1886) and in Louisiana (Ryder 1967). Historically the White-faced Ibis has had a more extensive, although discontinuous range in the New World (Ryder 1967). Until fairly recently, the breeding range of Glossy Ibis in the United States was limited primarily to Florida (Bull 1974), and the total breeding population was estimated to be 400 pairs (Palmer '1962). Oberholser (1974) attributed the restricted range of Glossy Ibis to the fact that its marsh habitat niche was already occupied by White-faced Ibises. The extension of the breeding range of Glossy Ibis was first noted in the 1940's and 50's (Pearson 1942, Stewart 1957, Stepney and Power 1973). Hailman (1959) summarized the breeding expan664 The Auk 94: 664-676. October 1977 This content downloaded from 157.55.39.113 on Sun, 15 May 2016 06:06:58 UTC All use subject to http://about.jstor.org/terms October 1977] White-faced and Glossy Ibises 665 sion, which reached north to Brigantine National Wildlife Refuge in New Jersey by 1957. Post et al. (1970) recorded the first breeding in New York at Jamaica Bay in 1961. By 1973 they had increased to 711 breeding pairs on Long Island (Buckley and Davis 1973). In 1973 Glossy Ibises' breeding range jumped to Maine (Finch 1972, Bull 1974), and in 1975 they spread into Massachusetts (Finch pers. comm.). Sight records of stragglers have been reported as far north as St. Pierre off Newfoundland (Finch 1973) and at Bradley's Marsh near Brandon, Ontario (Goodwin and Bollinger 1972). Glossy Ibises have also undergone a breeding range expansion into the West Indies and northern South America (Gochfeld 1970, 1973). The White-faced Ibis, on the other hand, seems to be contracting its breeding range, though such range contractions are poorly documented in the literature. Ryder (1967) summarized the breeding colonies in the United States and estimated that in 1965 there were 10,000 breeding pairs. He reported a decline in breeding numbers in California and attributed it to loss of habitat. Further decline in the numbers breeding in California was noted by Small (1975). White-faced Ibis are also declining in Nevada (Zimmerman 1975) and Utah (Ryder 1967, Zimmerman 1975). A decline in Texas was due to pesticides (Kirke King pers. comm.). Of the Ibis colonies Ryder (1967) lists west of the 89th meridian, 27% were no longer active in 1965. STUDY AREAS AND METHODS We selected study areas to include varied habitats in different geographical locations. Colonies ranged from tules (Scirpus) to small shrubs. In general, colonies were homogeneous physiognomically. The vegetation either consisted of all one species (e.g. tules or Phragmites) or contained several species of vegetation intermixed so as to appear similar in structure. Thus the latter type did not contain large patches of one plant species surrounded by tracts of other species. Two heterogeneous colonies (Danger Island, Aransas Pass, Texas for the White-faced Ibis, and Meadowbrook on Long Island, New York for the Glossy Ibis) were selected for an examination of general habitat preferences, whereas the homogeneous colonies were selected to study specific nest site selection preferences. Colonies examined were: 1. Laguna de Burgos, Azul, Province of Buenos Aires, Argentina. This colony, located in a pure stand of Scirpus sp. (maximum height 3 m) contained 1,500 pairs: 3% Great Egret Casmerodius albus, 8% Roseate Spoonbill Ajaia ajaia, 45% Snowy Egret Egretta thula, 18% White-faced Ibis, and 26% Brownhooded Gull Larus maculipennis. Eggs were laid in November. 2. Danger Island, Aransas Pass, Texas. The colony, in Borichiafrutescens, mixed forbs, and Opuntia cactus (maximum height 1.12 m) contained 1,500 pairs: 6% Great Blue Heron Ardea herodias, 2% Reddish Egret Dichromanassa rufescens and Little Blue Heron Florida caerulea, 37% Louisiana Heron Hydranassa tricolor, 30% Snowy Egret, and 25% White-faced Ibis. Mixed forbs on Danger Island included Oenothera drummondii, Ambrosia artemisiifolia, Gaillardia pulchella, Thelesperma filifolium, Machaeranthera phyllocephala, Gavra spp. Helianthus debilis (Jones et al. 1961). Laughing Gulls Larus atricilla nested nearby. Egg-laying extended from February to April. 3. Barrel Island, Brigantine National Wildlife Refuge, New Jersey. This colony, in Iva frutescens (maximum height 1.15 m), contained 291 pairs: 12% Great Egret, 13% Louisiana Heron, 36% Snowy Egret, 14% Black-crowned Night Heron Nycticorax nycticorax, and 25% Glossy Ibis. Herring Gulls Larus argentatus nested in Spartina around the heronry. Egg-laying was in April-May. 4. Meadowbrook in Jones Beach, New York. This colony, in Phragmites communis and Myrica pensylvanica (maximum height 3 m) contained 90 pairs: 64% Black-crowned Night Heron and 36% Glossy Ibis. Herring Gulls nested nearby. Egg-laying was April-May in all the New York colonies. 5. Zach's Bay, Jones Beach, New York. This colony. similar in plant species to Meadowbrook, contained 500 pairs: 2% Louisiana Heron and Little Blue Heron, 37% Snowy Egret, 7% Black-crowned Night Heron, and 54% Glossy Ibis. 6. Loop, Jones Beach, New York. This colony, similar to 4 and 5 in vegetation, contained 139 pairs: 52% Snowy Egret, 7% Black-crowned Night Heron, and 41% Glossy Ibis. 7. Seganus Thatch, Long Island, New York. This colony, in Smilax sp. and Iva (maximum height 3.2 m) contained 300 pairs: 21% Great Egret, 8% Little Blue Heron, 25% Snowy Egret, 19% Black-crowned Night Heron, and 27% Glossy Ibis. This content downloaded from 157.55.39.113 on Sun, 15 May 2016 06:06:58 UTC All use subject to http://about.jstor.org/terms 666 BURGER AND MILLER [Auk, Vol. 94