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The Song Sparrow (Melospiza melodia) is a medium-sized American sparrow. Among the native sparrows in North America, it is easily one of the most abundant, variable and adaptable species.

Adult song sparrows have brown upperparts with dark streaks on the back and are white underneath with dark streaking and a dark brown spot in the middle of the breast. They have a brown cap and a long brown rounded tail. Their face is gray with a streak through the eye. These birds forage on the ground, in shrubs or in very shallow water. They mainly eat insects and seeds. Birds in salt marshes may also eat small crustaceans. They nest either in a sheltered location on the ground or in trees or shrubs.

The song sparrow has been the subject of several studies detailing the physiological reactions of bird species to conditions such as daylight length and differing climatic conditions. Most birds gain mass in their reproductive organs in response to some signal, either internal or external as the breeding season approaches. The exact source of this signal varies from species to species - for some, it is an endogenous process separate from environmental cues, while other species require extensive external signals of changing daylight length and temperature before beginning to increase the mass of their reproductive organs. Male specimens of M. melodia gain significant testicular mass in response both to changes in the daily photoperiod and as a result of endogenous chemical signals. Females also undergo significant ovarian growth in response to both photo-period and endogenous signals. In this way, M. melodia is amongst only a handful of birds that use both external and engodenous signalling to dictate their breeding season. Hormone levels in both males and females were found fluctuate throughout the breeding season, having very high levels in March and late April and then declining until May. These studies suggest that there are multiple factors at work that influence when and how the song sparrow breeds other than just increasing day length.

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