Automated data collection systems were used to identify periods of increased activity at estrus and to assess factors affecting the magnitude and duration of estrous activity. In total, 4,172 estrous periods in 1,454 cows across 5 herds in Europe and Canada were studied. Each herd used automated management systems manufactured by DeLaval (Tumba, Sweden) including a milk progesterone (MP4) measurement and analysis system (Herd Navigator), a body condition scoring system (BCS Camera) and an activity system (DelPro). A “heat alarm” (HA) was defined as 2 consecutive MP4 samples below 5 ng/mL following a luteal phase. Estrus was defined as an increase in activity within 7 d after HA. Peaks in activity (estrus) were detected with an algorithm and the magnitude (fold change above baseline), duration, and area were determined. Cows were classified according to parity, month, BCS (≤2.75, 2.75–3.0, 3.0–3.25, >3.25), days in milk (DIM; ≤56, 57–84, 85–140, and >140), and daily milk production (<40, 40–50, and >50 kg/d) at the time of HA. Data were analyzed using a mixed model (PROC MIXED; SAS) with animal nested within herd defined as random. There was an effect of herd (range: 65.7 to 78.7%; P < 0.001), month (range: 69.0% for May–Jun to 78.7% for Nov-Dec; P < 0.002) and BCS (range: 70.0% for ≤2.75 to 77.1% for >3.25; P < 0.004) on the percentage of cows with an activity peak after HA. Neither DIM, parity, nor milk affected peak detection. For cows with an activity peak, the peak area was affected by herd (P < 0.001), DIM (P < 0.001), month (P < 0.001) and BCS (P < 0.001). Differences in peak area were primarily associated with greater peak duration and not peak magnitude. Peak duration (h) differed for herds (range: 16.9 ± 0.3 to 19.2 ± 0.2; P < 0.001), DIM (range: 17.4 ± 0.2 for ≤56 to 19.3 ± 0.3 for >140; P < 0.001), month (range: 17.3 ± 0.3 for May–June to 19.1 ± 0.2 for Nov-Dec) and BCS (range: 17.6 ± 0.3 for ≤2.75 to 18.9 ± 0.2 for >3.25; P < 0.001). Neither parity nor milk affected peak area or duration. Conclusions were that estrous activity is affected by herd, season (lowest in summer), BCS (lowest in low BCS cows) and DIM (lowest in early lactation cows).