Bobolink Dolichonyx oryzivorus population decline in agricultural landscapes in the Midwestern USA

A computer model and data from the North American Breeding Bird Survey (BBS) were used to examine the effects of changes in the area and disturbance patterns in agriculturally associated grasslands on the demographics of bobolink Dolichonyx oryzivorus populations in a 37 000 km 2 area in northern Illinois. Input parameters for the computer model were based on documented declines in the area of hay, pasture, and oats and on changes in the timing and frequency of hay field cutting. Declines in the area of agriculturally associated grasslands strongly influenced the model population and resulted in an estimated overall bobolink population decline of 95% between 1952 and 1992. An increase of nearly two weeks in the median date of first cutting of alfalfa hay had a weaker impact on the model population. Field data for this area from the BBS suggest a bobolink population decline of over 90% between 1966 and 1992, a decline that was significantly correlated with documented declines in the regional area of pasture, alfalfa hay and oats. Although they cannot discount the possible effects of other factors, the model and empirical data I present demonstrate that declines in agricultural grassland habitat, and changes in the timing and frequency of hay field cutting, have the potential to produce significant regional population declines in grassland bird species inhabiting agricultural landscapes.