Breeding Biology of Five Species of Herons in Coastal Florida

-The breeding biology of the Louisiana Heron, Snowy Egret, Cattle Egret, Little Blue Heron, and the Great Egret was studied on Riomar Island, a 2.8-ha mangrove island in Indian River County, Florida from March through May 1973. The Louisiana Heron and Cattle Egret nests were built increasingly more distant from the shoreline as the breeding season advanced. Nests of the Little Blue Heron were not horizontally stratified and the seasonal differences in siting of the Snowy Egret and Great Egret nests were not significant. The Louisiana Heron nests showed vertical stratification and were built significantly higher as the season advanced. Snowy Egret and Great Egret nests were built at lower elevations in midseason, but later nests were at the same height as the earlier nests. The heights of Cattle Egret and Little Blue Heron nests did not change during the season. The Louisiana Heron, Snowy Egret, Cattle Egret, and Great Egret completed clutches the last week of March and the first week of April. Little Blue Herons completed their clutches the second and third weeks in April. The Louisiana Heron, Snowy Egret, and Cattle Egret laid eggs on the average at slightly greater than 2-day intervals and the Little Blue Heron and Great Egret at slightly less than 2-day intervals. All five laid replacement clutches when the original clutch was destroyed. Little Blue Heron clutches were significantly larger than Snowy Egret and Great Egret clutches. Louisiana Heron clutches were significantly larger than Great Egret clutches. Incubation began after the first egg was laid in most Snowy Egret, Cattle Egret, and Great Egret nests and was delayed until after the second egg in Louisiana Heron and Little Blue Heron nests. The time interval between hatching of successive eggs in the clutch increased in all species except the Little Blue Heron, which showed a decrease in interval. Nests failures were caused mainly by Brown Pelicans, whose clumsy landings at or near their own nests knocked eggs from nearby heron nests or crushed them. Still nesting success of the Riomar herons was better than any reported in the literature. The Little Blue Herons had the highest nest success because of their relative isolation from the other nesting species. -Rice Creek Biological Field Station, State University College, Oswego, New York 13126, and Florida Medical Entomology Laboratory, Vero Beach, Flonida. Present address of second author: Division of Ornithological Research, Florida Audubon Society, 35 1st Court, S.W. , Vero Beach, Florida 32960. Accepted 19 February 1976. INTEREST in the Ardeidae has increased in recent years due, in part, to their wetland habits that make them good indicator species of man's disturbances of these habitats. Some species appear to be declining in parts of their range; others are increasing in numbers (Lowe-McConnell 1967, Belknap 1968, Maxwell and Putnam 1968, Crosby 1972, Tate 1972). The reasons for varying success among the ardeids are often unexplained and will remain so until sufficient data on breeding requirements in different habitats are available. This study provides data on the breeding biology of the Louisiana Heron (Hydranassa tricolor), Snowy Egret (Egretta thula), Cattle Egret (Bubulcus ibis), Little Blue Heron (Florida caerulea), and the Great Egret (Casmerodius albus) nesting on a mangrove spoil island in the Indian River at Vero Beach, Florida (27? 38'N, 80022'W). Spoil islands are created in the Intracoastal Waterway by dredging and pumping the spoil into piles outside the channel. Vegetation soon develops on them and provides nesting substrate for birds. Spoil islands have provided new nesting grounds for ardeids along coastal Florida. Reports in the literature on vertical and horizontal stratification of nest sites in these species are limited (Meanley 1955, Ralph and Ralph 1958, Bowen et al. 1962, Blaker 1969, Pratt 1970). Jenni (1969) sum689 The Auk 94: 689-700. October 1977 This content downloaded from 157.55. 39.120 on Mon, 05 Sep 2016 06:16:14 UTC All use subject to http://about.jstor. org/terms 690 MAXWELL AND KALE [Auk, Vol. 94 marized and presented quantitative data on vertical stratification of the Snowy Egret, Cattle Egret, Little Blue Heron, and Louisiana Heron in the inland Lake Alice, Florida heronry. This paper presents similar data for a coastal heronry. DESCRIPTION OF THE STUDY AREA Vero Beach, Indian River County, Florida, is on the Atlantic Coastal Plain about midway along the Florida peninsula. The Indian River, actually a tidal bay or lagoon, runs parallel to the coast and separates a barrier beach from the mainland. Observations were made on Riomar Island (sometimes called Crane Island), a 2.8-ha island 1.6 km south of the Merrill Barber Bridge, which crosses the Indian River at Vero Beach. This island contains one of the largest concentrations of herons and pelicans in the Indian River between the Sebastian and Fort Pierce Inlets. Riomar Island has been undergoing uninterrupted vegetational development since the last dredging of the waterway in the late 1950's. The dominant canopy species, determined by quadrat analysis by Maxwell and Kale (1974) and listed in decreasing order of abundance with relative density (Curtis and Cottam 1969) are as follows: black mangrove (Avicennia nitida) 35%, white mangrove (Laguncularia racemosa) 28%, Australian pine (Casuarina equisetifolia) 18%, Florida privet (Forestiera porulosa) 10%, red mangrove (Rhizophora mangle) 7%, button mangrove (Conocarpus erecta) 1%, and Brazilian pepper (Schinus terebinthifolius) 1%. The quantitative data on the breeding biology of the herons were collected on the south and west portions of the island. These tracts consist of thick growths of black and white mangrove and, along the waters edge, red mangrove. The northern portion of the island is similar, but Australian pine is scattered in clearings in the mangrove. The east portion is a pure stand of Australian pine fringed with small black mangroves along the shore. An open sandy space in the center of the island supports an herbaceous layer. Nest sites for all species of birds were fairly evenly distributed in the mangroves, with few nests in the eastern portion and none in the center clearing. An approximate total of 2,657 breeding pairs of 13 species nested on Riomar Island in the spring of 1973. The ardeids and their relative abundance (%) included: Louisiana Heron 37.1, Snowy Egret 29.5, Cattle Egret 12.7, Great Egret 3.9, Little Blue Heron 2.7, Blackcrowned Night Heron (Nycticorax nycticorax) 0.2, Yellow-crowned Night Heron (Nyctanassa violacea) 0. 1, Great Blue Heron (Ardea herodias) 0. 1, and Green Heron (Butorides virescens) 0.1. One other ciconiiform, the White Ibis (Eudocimus albus) 0.9, also nested on Riomar. Pelecaniforms nesting were Brown Pelican (Pelecanus occidentalis) 11.1, Double-crested Cormorant (Phalocrocorax auritus) 1.5, and Anhinga (Anhinga anhinga) 0.1 (Maxwell and Kale 1974).