Bird song peaks sharply in the period around sunrise and falls away within a couple of hours. Several separate advantages coincide in that window, which is why the pattern appears in unrelated species worldwide.
Sound travels better in still air
Air near the ground is calm and thermally uniform before sunrise, so sound propagates with less scattering and reaches further than it will later.
Once the sun warms the surface, rising air creates turbulence that breaks up sound waves, and background noise from wind and insects increases.
Measurements of song transmission through habitat consistently show greater range at dawn, which means the same effort reaches more listeners at that hour.
Early light is poor for feeding
Most songbirds hunt insects visually, and light levels before sunrise are too low for efficient foraging.
Singing costs energy but not much, and it can be done from a perch, so the pre-dawn period is time that would otherwise be unproductive.
As light improves, the balance shifts and birds move to feeding, which is why the chorus fades rather than stopping abruptly.
Territory has to be reclaimed daily
Territorial boundaries are maintained by advertisement rather than by physical barriers, and overnight some neighbours will have died or moved.
Singing early establishes that the holder is present and healthy before any prospecting bird has a chance to move in during the day.
Males that fall silent are tested quickly by neighbours, which is why an early and vigorous chorus functions as a daily statement of occupancy.
Females are listening
In many species females are most fertile in the early morning, so song at that hour reaches them at the point of greatest relevance.
Song quality carries information about condition, since sustained loud singing after a cold night indicates a bird that fed well the previous day.
Experiments in which males are temporarily silenced show reduced pairing success, indicating that the timing and vigour of song both matter to outcomes.
Why the chorus is changing
Artificial light advances the start of singing in urban birds, which begin earlier where street lighting is bright and continue later into the evening.
Traffic noise pushes some species to sing at higher frequencies or shift their peak activity earlier, avoiding the loudest part of the commuting period.
These adjustments have costs. Earlier singing extends the active day, and altered frequencies may carry less well or be less attractive to females tuned to the original form.