sentences:  The efforts required for raising the cowbird chick often exhaust the parent and lead to either the death of the parent or the death of the other chicks within the nest, thus decreasing the chances of reproduction for that songbird.In a study conducted by the University of British Columbia, cowbird parasitism was shown to be a direct cause of population declines in black-capped vireos, Bell's vireo, and the southwestern willow flycatcher (Levy, 2004).Avian species have expanded their range since long before humans.