Egg size and cotton rat predation

Although cotton rat (Sigmodon hispidus) presence at predated Northern Bobwh ite (Colinus virginianus) nests has been widely reported, it is unclear how often cotton rats are the actual predators. We presented two sizes of eggs to 35 wild-trapped cotton rats to better understand how avian egg size aff ects nest predation in the wild. Zebra Finch (Poephila gutata) eggs (16.0 x 11.4 mm) and Northern Bobwhite eggs (30.0 X 24.7 mm) were presented to cap tive cotton rats in 30 x 30 cm enclosures. Eggs were checked every 6 h for 24 h. Food was withheld from 18 of the rats in an attempt to induce egg con sumption. No cotton rats consumed bobwhite eggs (0/35 cotton rats), but Zeb ra Finch eggs were frequently consumed (29/35 cotton rats). The gapes of co tton rats used in this study were smaller than the bobwhite egg widths and may have prevented cotton rats from successfully cracking the eggshells. It is apparent that cotton rats will readily consume eggs smaller than those of the Northern Bobwhite and pose a predation threat to passerines. Based o n our data and known abundances of cotton rats throughout their range, we c onclude that cotton rats pose a significant threat to smaller birds nesting on or near the ground. However, we offer no evidence to implicate cotton r ats as important predators of bobwhite nests.