Successful conservation of grassland bird populations requires basic information on their breeding biology; in particular, information from undisturbed native prairie over an extended period of time. We present data collected at Bowdoin National Wildlife Refuge in northcentral Montana on the reproductive biology of six grassland bird species that breed in mixed-grass prairie: Sprague's Pipit (Anthus spragueii), Savannah (Passerculus sandwichensis), Grasshopper (Ammodramus savannarum), and Baird's (A. bairdii) sparrows, Chestnut-collared Longspur (Calcarius ornatus), and Western Meadowlark (Sturnella neglecta). Basic measures of reproductive biology are presented, including estimates of nest density, phenology, fecundity, parasitism rates, and nest success, and how these and other characteristics varied across years. Nests (n  =  1,494) of the six focal species accounted for 98% of all passerine nests found during 1997–2007; Chestnut-collared Longspurs (51%) were the dominant breeding species. Total nest density across years ranged from 20 to 41 nests per 40 ha (CV  =  26%) on unburned sites. Mean clutch initiation date and clutch size varied little across years; however, clutch size tended to decrease over the course of a season, regardless of bird species. Daily nest survival rates did not differ markedly among bird species, but did vary substantially among years, suggesting that year-dependent factors were affecting nest success among all species similarly.