Flight patterns of Great Egrets (Ardea alba) and Snowy Egrets (Egretta thula) arriving at and departing two mixed-species colonies were studied in Wichita, Kansas, in May and June 2007. For 137 short-distance flights (about 200 m) at one colony, flight duration and flight distance were recorded and wing beats were counted. For 90 longer flights (about 1,200 m) at a second colony, flight durations were recorded. From these data, wing-beat frequencies and flight velocities were calculated, then wind vector addition was used to determine air speeds and examine the effects of wind velocity and wind direction on flight patterns. Using published algorithms and data from two of our previous flight line studies, the daily energetic costs of foraging flights were estimated. Flight patterns by Great Egrets and Snowy Egrets showed some similarities. For example, mean air speeds for Great Egrets (9.2 m/s) did not differ from those of Snowy Egrets (8.7 m/s). Differences between species included lower wing-beat frequencies by Great Egrets and a much stronger effect of headwinds on Snowy Egret flight velocities. Energetic requirements for flight also differed between species, which are ascribed to differences in wing-loading and mass. The cost of an average flight for each species was estimated, and daily flight costs were compared to overall daily energy budgets. Flight comprised 25.9% of the total energy budget for Great Egrets and 27.5% for Snowy Egrets. Based on previous foraging studies, a Great Egret can meet its daily energy requirements for flight after an estimated 82 min of foraging, whereas a Snowy Egret must forage for 168 min to capture enough prey to meet the daily demands for flight. This study represents the next stage in our development of annual time-and-activity budgets for these two species of wading birds.
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Vol. 31 • No. 4