A newly devised nightlighting technique was used to capture breeding adult common loons (Gavia immer) at the Seney National Wildlife Refuge in northern Michigan in 1989. The behaviors of 6 pairs of known-sex, color-marked common loons were subsequently quantified during the breeding cycle in 1990. Collected observational data indicate that foraging, resting, locomotion, and preening were frequent throughout the breeding cycle. Time spent foraging was greatest during the pre-nesting period (53 to 57%), but declined significantly during the nesting and post-nesting periods (p<0.05). Time spent foraging during the pre-nesting period was similar to that of fall and winter studies. During the pre-nesting period adult loons spent about 15% of the time in locomotion; this was significantly greater than the other time periods (p<0.05) and is attributed to selecting a nest site. During the nesting cycle, almost half of each bird's time spent was nest-sitting. Sexual differences were negligible during nest-sitting. Resting and chick-rearing were the predominant behaviors during the post-nesting period and were responsible for the biggest difference in parental duties. Time spent preening declined from 8% during the pre-nesting period to 4 to 5% during the post-nesting period. Time spent by nesting pairs to produce chicks is approximately 10% during pre-nesting, 48 to 49% during nesting, and between 38 to 44% during post-nesting. By quantifying and establishing behavioral standards, subtle abnormalities or changes can be detected to better manage for viable common loon populations.