Mosquito-borne diseases can have disastrous effects on avian populations; therefore, most studies of bird and mosquito interactions have focused on the mortality and morbidity associated with the diseases. However, the effect of mosquitoes feeding on birds, independent of disease, has not been well studied. We studied Barn Owls (Tyto alba) nesting in artificial nest boxes in sugarcane (Saccharum officinarum) fields in Florida, US. To reduce mosquito effects on nestlings, we used an insecticide spray in half of the nest boxes. Mosquito suction traps were fixed to the outside of eight nest boxes (four treated and four untreated) to collect mosquitoes over a 24-h period (one trap night) once weekly, from incubation until all nestlings fledged. Collected mosquitoes were counted, sorted into blood-fed and unfed females, and identified to species when possible. The dominant mosquito species captured were Culex nigripalpus, Mansonia dyari, and Mansonia titillans. The highest total number of mosquitoes and blood-fed mosquitoes captured in a suction trap in one trap night was 3,193 and 379, respectively. Overall, significantly fewer mosquitoes were captured from treated nest boxes compared to untreated boxes. Nestling age influenced the total number of mosquitoes captured, with the highest numbers associated with fledglings 22–42 d old. The highest numbers of blood-fed mosquitoes were captured when nestlings were 22–28 d old. Nestlings in insecticide-treated boxes had higher survival rates compared to those in untreated boxes during months with high mosquito numbers. Mosquitoes can impose energetic costs on nestlings by causing stress from irritation, dehydration, and the constant regeneration of blood cells. These costs, in addition to factors such as food shortage, temperature, and overall health of the nestling, can contribute to higher mortality rates during nesting periods with high mosquito numbers.