Incubation of a foreign object by a Common Nighthawk (Chordeiles minor) in Florida

Ground-nesting birds typically maintain a nest free of foreign materials but occasionally are known to incubate a foreign object, often in the presence of other eggs. In May 2012, we discovered a Common Nighthawk (Chordeiles minor) incubating a turkey oak (Quercus cerris) acorn along with 2 eggs within a mesic flatwoods forest in Clay County, Florida, USA. Herein, we report this observation and include 3 possible hypotheses for this behavior.