Some hosts of avian brood parasites reduce or eliminate the costs of parasitism by rejecting foreign eggs from the nest (rejecters). In turn, even acceptor hosts remove most non-egg shaped objects from the nest, including broken shells, leaves, and other detritus. Where does a potential threshold between egg-recognition and detritus-rejection lie when it comes to shape? Most previous studies applied comparisons of egg-sized objects with non-continuous variation in shapes. Here, instead, we used two series of 3D printed objects, designed a priori to vary from the natural egg shape by either reducing width or increasing edge angularity. As predicted, we detected transitions from mostly acceptance to mostly rejection in the nests of American robins along these two axes of egg shape variation. Our methods parallel previous innovations in egg rejection studies through the use of continuous variation in egg coloration and maculation contrast, to better understand the limits and thresholds of egg recognition in diverse hosts of avian brood parasites.