We examined nest defense behavior of the Carolina Wren (Thryothorus ludovicianus) in response to avian intruders at the nest. Freeze-dried mounts of a brood parasite (female Brown-headed Cowbird [Molothrus ater]), nest predator (Blue Jay [Cyanocitta cristata]), and a control (Swainson's Thrush [Catharus ustulatus]) were presented at wren nests to examine if wrens were able to distinguish among these threats. The primary nest defense behavior of Carolina Wrens was alarm calls. Wrens spent more time alarm calling to the Blue Jay model than the control and Brown-headed Cowbird models suggesting cowbirds were not recognized as threats to the nest. Wrens were less likely to respond during laying stage trials, probably due to a lack of nest visitations at this time. Intensity of alarm calls did not increase from the laying to nestling stage for any of the models presented. Carolina Wrens are both socially and genetically monogamous, and males should invest heavily in care of young due to their high confidence of paternity. Males and females did not differ in the amount of time spent alarm calling to any of the models, and defended their nests from intruders with equal intensity as predicted by the confidence of paternity hypothesis.