A night heron, a green heron, a white ibis, and a cattle egret were individually exposed overnight in 8-ft cages to densities of Culex nigripalpus ranging from 25 to 1,200 per cage. Surviving mosquitoes were examined the following morning and engorgement rates were determined as a measure of effect of antimosquito activity. In addition, the behavior of each bird was recorded for 2-hr periods at three different mosquito densities. Results indicated an inverse relationship between the density of mosquitoes and the proportion that successfully obtained blood. This relationship was ascribed to the increase in defensive activity displayed by the birds (differing in degree according to species) when exposed to increases in mosquito density. The frequency of interrupted feeding (= partial blood meals) was also related to host behavior and mosquito density.