Geology topics

R. Michael Erwin

Publications and source records attributed to R. Michael Erwin.

21 records · Page 2Linked to original sources

USGS1985

Foraging decisions, patch use, and seasonality in egrets (Aves: ciconiiformes)

Feeding Snowy (Egretta thula) and Great (Casmerodius albus) egrets were observed during two breeding seasons in coastal New Jersey and two brief winter periods in northeast Florida. A number of tests based on assumptions of foraging models, predictions from foraging theory, and earlier empirical tests concerning time allocation and movement in foraging patches was made. Few of the expectations based on foraging theory and/or assumptions were supported by the empirical evidence. Snowy Egrets fed with greater intensity and efficiency during the breeding season (when young were being fed) than during winter. They also showed some tendency to leave patches when their capture rate declined, and they spent more time foraging in patches when other birds were present nearby. Great Egrets, in Contrast, showed few of these tendencies, although they did leave patches when their intercapture intervals increased. Satiation differences had some influence on feeding rates in Snowy Egrets, but only at the end of feeding bouts. Some individuals of both species revisited areas in patches that had recently been exploited, and success rates were usually higher after the second visit. Apparently, for predators of active prey, short—term changes in resource availability ( r esource depression ) may be more important than resource depletion, a common assumption in most OFT models.

Florida, New Jersey↗

USGS1983

Feeding habitats of nesting wading birds: Spatial use and social influences

In an effort to relate social interactions to feeding-habitat use, I observed six species of wading birds near a major colony site in coastal North Carolina. Three spatial scales of habitat use were considered: the general orientation to and from the colony (coarsest level), the habitat "patch," and (at the finest level) the microhabitat. Departure-arrival directions of Great Egrets ( Casmerodius albus ), Snowy Egrets ( Egretta thula ), Cattle Egrets ( Bubulcus ibis ), Little Blue Herons ( Egretta caerulea ), Tricolored Herons ( Egretta tricolor ), and Glossy Ibises ( Plegadis falcinellus ) were monitored at the colony site to document coarse patterns of feeding-habitat use. Added to these data were observations made at five different wetland sites to monitor between-habitat and within-habitat patterns for the five aquatic-feeding species. The results indicated a broad and variable use of feeding habitat over time. At the coarsest scale (i.e. orientation at the colony), diffuse patterns, influenced little by either inter- or intraspecific social interaction, were found for all species. At the next level (habitat "patch"), only one of five wetland sites was relatively consistent in attracting feeding birds, and its use increased from May to June. Few groups were seen at four of the five sites. At the one "attractive" site, the within-habitat patterns again were spatially variable over time, except for those of the abundant Snowy Egret, whose microhabitat preference was fairly consistent. Glossy Ibises and Snowy Egrets frequently formed mixed-species groups, Little Blue Herons were the least social, and Great Egrets and Tricolored Herons generally occurred in groups of less than 10 birds but rarely in groups larger than 30. The close association between Snowy Egrets and Glossy Ibises appeared to be based on a "beater-follower" relationship, wherein the probing, nonvisually feeding ibises make prey more available to the followers. In the study area, local enhancement appeared to play a more important role than did any "information-sharing" at the colony.

North Carolina↗

USGS1979

Multiple-factor influences upon feeding flight rates at wading bird colonies (Alias: Are flight-line counts useful?)

The temporal patterns of feeding, resting, and reproductive behavior in colonial wading birds have been studied by a number of investigators (Recher and Recher 1972, King 1974, Capen 1978, Custer and Osborn 1978, Kushlan 1978) both on a short-term (daily) and long-term (annual) basis. In coastal marine environments, activities at colonies are influenced by tides (Recher and Recher 1972, Krebs 1974, Custer and Osborn 1978), time of day (Kushlan 1978) and phase of the nesting cycle (Kahl 1964). The purpose of this paper is twofold: (1) to examine the effects of tide, time of day (physical factors), nesting phase, colony site, and spe cies identity (biological factors) on feeding flight rates at breeding col onies and, as a result of this, (2) to evaluate the usefulness of feeding flight counts as an index of the number of nests in the colony. Earlier work suggests that the relationship between the number of in dividuals flying to and from the nesting colony may be quite consistent with nest numbers. Thus, by monitoring flights from remote locations, ob servers might obtain relatively accurate census data while minimizing time and disturbance at colonies. Recent concern for the deleterious impact of humans at waterbird colonies (Buckley and Buckley 1976, Ellison and Cleary 1978) underscores the need to investigate alternative census methods.

Conference Paper↗