Feeding habitat use by colonially-breeding herons, egrets, and ibises in North Carolina

Abstract

-Nine species of herons, egrets, and ibises were followed by airplane from a nesting colony near Beaufort, North Carolina to their feeding sites. Except for Cattle Egrets, which flew exclusively to fields and dumps, the birds flew mainly to saltmarsh habitat. The selection of feeding habitats by Great Egrets and Louisiana Herons was directly related to tidal depth. The Great Egret was the only species that effectively used eelgrass beds, and its use of this habitat was restricted to between 1.5 h before and after low tide. We suspect that shorter-legged herons did not use eelgrass regularly because the water was too deep. Most Great Egrets, White Ibises, Louisiana Herons, and Snowy Egrets used areas near the colony (<4 km). Great Egrets, Black-crowned Night Herons, and White Ibises flew farther from the colony at high than at low tide. Great Egrets traveled farther from the colony when they used thermals; rate of travel to feeding sites was the same, however, whether or not they used thermals. Aggressive encounters were observed at the landing sites of Great Egrets, Louisiana Herons, Snowy Egrets, and Black-crowned Night Herons. In contrast to the other species studied, Cattle Egrets and White Ibises often flew in groups to feeding sites. Indirect evidence supports the hypothesis that colonies can act as centres, wherein unsuccessful birds follow successful ones to better feeding locations. Received 31 January 1978, accepted 7 May 1978. THE use of wetland habitats by herons, egrets, and ibises (here collectively termed herons) is not well known. Jenni (1969) found that Snowy Egrets (Egretta thula) feed in open areas, Little Blue Herons (Florida caerulea) in heavily vegetated areas, and Louisiana herons (Hydranassa tricolor) along banks or floating vegetation where the water level drops rapidly. Kushlan and Kushlan (1975) described the feeding habitat preferences of the White Ibis (Eudocimus albus) in southern Florida, and Meyerriecks (1962), Kushlan (1976a), and Custer and Osborn (1978) found that longer-legged herons feed in deeper water than shorter-legged ones. The distance herons travel from a colony to feeding sites is also poorly understood. Published estimates of the maximum distance (km) for various species are as follows: Cattle egret (Bulbulcus ibis) 28.5 (Bateman 1970), 25.8 (Hopkins and Murton 1969), 19.3 (Craufurd 1966, Skead 1966), and 29 (Siegfried 1971); Roseate Spoonbill (Ajaia ajaja) and Grey Heron (Ardea cinerea) 32.2 (Beetham 1910) and 19.3 (Nicholson 1929); Wood Stork (Mycteria americana) 40 (Kahl 1964) and 130 (Ogden et al. in press); and White Ibis, 22.9 (calculated from Bateman 1970) and 44 (Kushlan 1976b). However, the distribution of distances flown from the colony is known only for the Cattle Egret (Bateman 1970, Siegfried 1971), Wood Stork (Kahl 1964), and White Ibis (Bateman 1970). In this study we attempt to describe the area used and habitats selected by herons nesting in a colony near Beaufort, North Carolina. Unmarked herons were followed by airplane from the colony to their first landing site. Using this approach, we were able to gather information that indirectly supports the hypothesis that colonies may act as centres, wherein unsuccessful birds follow successful ones to better feeding sites (Ward and Zahavi 1973, Krebs 1974). l Present address: Office of Biological Services, National Western Energy and Land Use Team, Fort Collins, Colorado 80521 USA. 733 The Auk 95: 733-743. October 1978 This content downloaded from 157.55.39.206 on Sat, 17 Dec 2016 05:22:44 UTC All use subject to http://about.jstor.org/terms 734 CUSTER AND OSBORN [Auk, Vol. 95