Predation on artificial nests in small grassland patches in East-Central Illinois

We measured predation on artificial ground nests in 11 grassy fields ranging from 0.8-12.6 ha. We set out 12 nests in each field baited with two Japanese quail eggs at different distances (<10 m, 25 m, 50 m) from woody edges in early June, and another 12 nests baited with one quail and one zebra finch egg in each field in mid-July. Nests were monitored at 3-d intervals for 15 d. The proportion of nests in which quail eggs were dep. redated was 33% in June and 38% in July. Including predation on finch eggs raised the proportion of nests depredated to 78% in July, indicating that small-mouthed nest predators such as mice or shrews could account for a substantial amount of predation on nests of small passerine birds. Predation on nests was negatively related to area of field only for quail eggs in July (P = 0.04). We did not detect significant relationships between numbers of nests depredated or daily predation rates and distance to woody edges. The small size of fields in our study, which are typical of grassland patches in this region, may be below a threshold at which nests in the interior of patches experience reduced predation. Alternatively, highly fragmented natural habitats in an agricultural landscape may support high densities of predators, making edge and area effects difficult to detect.