Several hypotheses have been proposed to explain infanticide in animals, but the behavior has only been observed in a few bird species. I describe video evidence of infanticide by a female Red-winged Blackbird (Agelaius phoeniceus) at a nest in Valley Forge National Historical Park in Pennsylvania. A female Red-winged Blackbird was seen pecking and pulling at the sole remaining 5-day-old nestling and eventually pulled the nestling from the nest until it fell to the ground. A female Red-winged Blackbird later returned to the nest with food 1 hour after the infanticide and made several visits with food in its bill. The last visit to the empty nest occurred 7 hours after the infanticide occurred. There were not sufficient plumage differences between the feeding and attacking bird to determine conclusively that they were different birds. The occurrence of infanticide could be explained by several hypotheses and might be more common in Red-winged Blackbirds than has been reported. For example, infanticide in Red-winged Blackbirds could increase male parental investment as is seen in other polygynous species. Misplaced aggressive behavior under stressful conditions; however, is also a possible explanation for this single case of infanticide.