Nest initiation and end dates for eight grassland bird species in Wisconsin and Illinois

We used nest records from published grassland bird studies done in Wisconsin and Illinois to compile a dataset for 8 obligate grassland bird species Study sites were clustered in southwestern Wisconsin near Mt. Horeb (43.0167°N, 89.7500°W); sites were located in Dane, Grant, Green, Iowa, Lafayette, Monroe, and Rock counties. In northern Illinois, study sites were located in Lee, Ogle, Will, Grundy, Carroll, and Jo Daviess counties (40.9822 to 42.2356°N, -87.5433 to -90.3489°W).The dataset consists of 3257 nests with observations on nest initiation date (date first egg laid), nest fate (success, failure), number of fledglings, and nest end date (date the nest either failed or fledged at least one nestling). The species were Bobolink, Dickcissel, Eastern Meadowlark, Grasshopper Sparrow, Henslow's Sparrow, Savannah Sparrow, Vesper Sparrow, and Western Meadowlark. 

Methods
Nest data from all studies were put into a common format of nest initiation date (date first egg laid), nest fate, number of nestlings, and nest end date (date the nest either failed or fledged at least one nestling). We used the fate (success or failure) as assigned by the original researchers. We used the nest initiation and failure dates if recorded by the original researchers. In general, nest initiation dates were estimated based on field notes or digital video (i.e., video-surveillance of nests). In some instances, nests failed within the same nesting stage (laying, incubation, or nestling) in which they were first found. For these cases, nest initiation dates were estimated following Cox et al. (2012. Species and temporal factors affect predator-specific rates of nest predation for forest songbirds in the Midwest. Auk 129:147–155) using average species-specific incubation times from Ehrlich et al. (1988. Birder’s handbook. Simon and Schuster Inc. New York, New York, USA). Nest end dates (date when nest failed or fledged at least one nestling) were either known (e.g., forced fledging during nest checks, digital video, field observations) or estimated. Nest end date was estimated using the mid-point between the dates when the nest was last observed with eggs/nestlings present and when the nest was next observed empty. We used the data to estimate the potential species-specific impacts of disturbance (e.g., mowing, fire) conducted at different dates in the breeding season on nests and fledglings.