Optimizing the use of electronic aids for estrus detection in dairy heifers will have direct implications for increased reproductive efficiency as well as more sustainable reproductive management, as hormonal interventions can be minimized. The objective was to describe estrus-related changes in rumination, eating, and high-active time in Holstein heifers using an ear-attached movement sensor whose application has been previously described in lactating cows. Starting at 6 mo of age, outdoor group-housed Holstein heifers (n = 22) received an ear-attached movement sensor (CowManager Sensor, Geryerscop, UT, the Netherlands), and were subjected to weekly ovarian ultrasonography until 2 consecutive ovulations were confirmed. Starting 1 wk after the second ovulation, ovaries were scanned every 2 d until 2 subsequent ovulations were confirmed, monitoring ovarian dynamics throughout 1 complete estrous cycle. Data were evaluated keeping days of estrus (defined as day of highest activity preceding ovulation) and ovulation as reference points, and analyzed using the GLIMMIX procedure of SAS. Mean (±SE) daily rumination time was decreased (295.2 ± 9.6 vs. 427.2 ± 12.0 min/d; P < 0.01) while eating time was unexpectedly increased (386.4 ± 14.4 vs. 338.4 ± 14.4 min/d; P = 0.02) on day of estrus compared with the mean activity over the 5 d preceding. As expected, high-active time was increased (487.2 ± 16.8 vs. 247.2 ± 16.8 min/d; P < 0.01) on day of estrus compared with the 5 d prior. Furthermore, high-active time was markedly increased (376.8 ± 33.6 vs. 216.0 ± 9.6 min/d; P < 0.01) during the 2 d preceding day of ovulation. In summary, rumination time was decreased on day of estrus relative to the 5 d preceding, while high-active time was increased on day of estrus relative to the 5 d preceding estrus as well as during the 2 d preceding ovulation. Significant changes in rumination, eating and activity were documented in Holstein heifers by an ear-attached movement sensor, which can be a useful tool for reproductive management.