The Eastern Bluebird is Prince George's County's bird symbol. The male's bright iridescent blue coloring is a definite field marking, while the female's duller coloration helps her blend in with the nest as she incubates eggs and shields young. The species is not currently threatened or endangered, but in the early 20th century its population experienced a significant decline because of habitat loss and aggressive competition for nest sites from species such as European Starlings and House Sparrows. Continued watch over the population is important so steps can be taken if another decline is observed. At Patuxent River Park, more than 15 nest boxes are monitored during the nesting season.