During migration, passerine birds typically complete a series of multi-hour flights, each followed by a period of stopover. During flight, rates of respiratory water loss are high, yet these birds show no signs of dehydration after flights. During stopover, birds become hyper-phagic to replenish fat reserves, often consuming food with high water content, such as fruit. Thus, migratory birds seem to face an osmoregulatory challenge; they must reduce water losses during flight, but retain the ability to excrete large quantities of water while maintaining osmotic balance at stopover. Our goal was to measure glomerular filtration rate (GFR) and fractional water reabsorption (FWR) of a migratory bird in free flight, at rest, and during feeding to assess the role of the kidney in maintaining water balance during migration. We used FITC-Inulin and one- and two-phase exponential decay models to first validate a technique and then measure GFR in the Swainson's thrush, a small (~30 g) songbird. Single-phase exponential decay models and the modified slope intercept method overestimated GFR by 26% compared to two-phase exponential decay models. We found no differences in GFR among fed, resting and flying birds, but fractional water reabsorption was significantly higher in resting and flying birds relative to feeding birds. There was no effect of the rate of respiratory water loss on GFR or FWR in flight. These data support the idea that birds in flight do not dramatically alter GFR, but rely on increased fractional water reabsorption to minimize excretory water losses.