A five-year study was carried out to test a sensitive, nondestructive, new method for determining sublethal and long-term consequences of fenitrothion applications on forest songbirds. I used censuses and territory mapping of singing males to provide indices of relative abundance on sprayed and control plots. Mist-netting was used to colour-band individuals of 3 indicator species (the chestnut-sided warbler, magnolia warbler and white-throated sparrow), to determine "their breeding condition, and to follow their fates after a fenitrothion application. Time-budget observations were made of the behaviour of some marked individuals. Fenitrothion was applied aerially in 1985, and by ground in 1986. I first studied the effect of fenitrothion on the songbird community; comparing treatment and control plots, before and after the application. Data on the bird populations showed that fewer young were caught in 1985, and birds returned at a lower rate the year following treatment; a lower proportion of the 1986 catch in mist nets were males. Nonetheless, other factors (budworm cycles, for example) had a greater influence on population trends than the treatment. Behavioural observations indicated that individuals of the 3 indicator species did not abandon the treated area after either application, although white-throated sparrows and magnolia warblers moved away from foliage regions with higher deposits of spray. There was no significant change in the allotment of time to social, maintenance, or feeding behaviours, for any of the indicator species. This observation is not consistent with the known symptoms of organophosphate poisoning (i.e. increased time spent sitting, bill-wiping, and preening). Chestnut-sided warblers and white-throated sparrows continued to forage actively on sprayed plots or nearby. Magnolia warblers decreased their foraging effort after both applications. In part 2 of the thesis, I studied the effect of fenitrothion on the invertebrate food of forest songbirds. Branch samples were taken as a measure of the food available to foliage-gleaning birds. Drop trays were placed under sample trees to measure the amount of food fallen from the tree after the application. Following treatment, there was a lower density of invertebrates on white birch and balsam fir branches than on control trees. This reduction was not observed until 5 days after the application, although drop tray samples indicated an immediate kill of invertebrates. These techniques sampled different types of invertebrates such that a large proportion of the drop tray samples were not associated with the tree (eg. flying Diptera). In addition to a decreased abundance, a large portion of the remaining invertebrates on white birch treated trees were dead. This suggests a further decrease in food availability to birds like warblers that specialize on moving prey. The depression of food available to birds was probably short-lived, assuming movement of invertebrates into depopulated areas from unsprayed foliage nearby. Behavioural responses of birds to spraying were also short-lived, apparent only within 5 days of the application. At the maximum allowable single dose, fenitrothion appears to have little effect on forest songbirds, even when tested with a highly-sensitive method.