Logistic-regression curves relate the probability of nest parasitism to the percent tree cover within a 2-km radius of each nest found on study areas in the Sheyenne National Grassland, North Dakota. Curves represent models with the lowest AICc values for each species or species group (see Tables 1 and 2). (a) Each curve is based on combined data for six species (see “selected species” listed in Table 1); a separate curve is given for each of the four combinations of a study and a year: the BCA study in 1999, 2000, and 2001 and the cowbird study in 2001. (b) The curve for the Clay-colored Sparrow is based on pooled data from all four combinations; the curve for the Bobolink is based on pooled data from the BCA study in 1999 and 2001 and the cowbird study in 2001. (c) For the Savannah Sparrow, separate curves are given for each of the four combinations. (d) For Red-winged Blackbirds, separate curves are given for data from the BCA study in 1999 and 2000 and from the cowbird study in 2001. Note that the scales of the axes for the first two graphs (a, b) differ from those of the second two graphs (c, d).