We monitored American Kestrels (Falco sparverius) nesting in boxes from 1986–2006 in southwestern Idaho. The number of boxes available to kestrels each year varied from 34 to 126. Approximately 20% of boxes were on highway signs along Interstate 84, 20% were on trees in rural residential properties, and 60% were on wooden poles in rangeland, irrigated agricultural, and exurban areas. Occupancy rates increased over the 21-year period, despite increasing human populations, housing developments, and vehicular traffic near the boxes. The percent of boxes occupied each year averaged 48 ± 14, and the percent of nesting attempts that raised young to 22 d old averaged 64 ± 10. Clutch size averaged 4.8 and exhibited no significant trend over time. Nesting success and productivity also showed no trends over time. Mean number of young fledged by kestrel pairs each year ranged from 1.3 to 3.5 and averaged 2.6. Brood sizes at successful boxes averaged 4.0. Nests in boxes on trees near farmsteads and in boxes on interstate highway signs were less successful than nests in boxes on poles in less-inhabited areas, suggesting that signs on interstate highways may not be optimal nesting habitat for kestrels. Kestrel nest-initiation and hatching dates became earlier during the study period, and mean winter temperatures increased significantly during the study period, suggesting a possible effect of climate change.