1. Thirteen mixed-species heronries (10 in Florida, two in Virginia, one in North Carolina) were studied in 1980 as part of a project begun in 1979 aimed at evaluating the 'flight-line' census method..2. Standardized counts of Snowy and Cattle Egrets, Louisiana and Little Blue Herons flying to and from the nesting colony were made for three hr periods, followed by a nest count of the colony. 3.Significant differences were found in the flight rates (number of birds per nest x hour) of the four species at the Chincoteague colony. However, when Cattle Egrets and Louisiana Herons were compared at all 13 colonies, their respective flight rates were in opposite rank to those at Chincoteague. Colony differences, then, may mask species differences. 4. A linear regression model showed a strong fit (R2=0.92) between the hourly flight number (3 hr means) and the nest number, but point estimates (single colony) had very large confidence limits. A given colony might be over-or underestimated by a factor of 2, using the regression equation as a predictive model. 5. A more appropriate application of the method would be to determine regionwide (e.g., state), rather than colony-specific, population estimates. 'Total' estimates for all (n= 13) colonies were within 10% of the actual nest number.