Editing Song sparrow (section)

== Physiology ==
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 &ndash; 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.<ref name="controlof"/> Females also undergo significant ovarian growth in response to both photo-period and endogenous signals. Hormone levels in both males and females fluctuate throughout the breeding season, having very high levels in March and late April and then declining until May.<ref name="environcontroll"/> 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.

Due to the myriad subspecies of the song sparrow and the extremely varied climate of southern [[California]], where many of these subspecies make their homes, physiological studies were undertaken to determine how climatic conditions and local environment influenced the bill size of ''M. melodia'' subspecies. The bill of a bird is highly important for thermoregulation as the bare surface area makes a perfect place to radiate excess heat or absorb solar energy to maintain homeostasis.<ref name="environphys"/> Knowing this, comparisons of bill length between individual song sparrows collected in different habitats were made with regard to the primary habitat type or microclimate that they were collected in. Larger beaked subspecies were strongly correlated with hotter microclimates - a correlation that follows from the conditions of [[Allen's rule|Allen's Rule]].<ref name="influence"/>