Birds chirp for a variety of reasons, but one of the most common is to signal the start of a new day. Birds have internal clocks called circadian rhythms that tell them when to sleep and when to be active. These circadian rhythms are controlled in part by changes in light, so as the sun comes up in the morning, the increasing light triggers activity in birds and they start to sing. The timing of the dawn chorus, when many birds sing at the start of a new day, helps advertise territory and attract mates.
How do birds detect changes in light?
Birds are able to detect light through special photoreceptor cells in their eyes called cones. These photoreceptor cells contain light-sensitive pigments that initiate nerve impulses in response to light. The nerve impulses are sent to an area of the brain called the suprachiasmatic nucleus (SCN). The SCN functions as the master circadian clock in birds and coordinates daily rhythms in behavior and physiology.
In addition to cones in their eyes, birds also have light-sensitive photoreceptor cells in their brains. These photoreceptor cells directly detect light and convey signals to the SCN to synchronize circadian rhythms. So birds use both external photoreceptors in their eyes as well as internal photoreceptors in their brains to regulate daily cycles.
How do circadian rhythms work?
The SCN is considered the central circadian clock in birds. It receives input from the light-sensing photoreceptor cells and then regulates daily rhythms in physiology and behavior.
On a molecular level, circadian clocks are controlled by clock genes and proteins that interact in complex feedback loops. Core clock genes like Clock, Bmal1, Period, and Cryptochrome drive oscillations in their own expression levels over a 24-hour cycle. This molecular circadian clock then regulates downstream genes involved in various processes like metabolism, physiology, and behavior.
The SCN synchronizes circadian rhythms throughout the body. In addition to the central SCN clock, peripheral tissues like the liver and lungs also have self-sustained circadian oscillations. But the SCN uses both electrical and molecular signals to coordinate these peripheral oscillations so that they remain in sync.
What external cues synchronize circadian rhythms?
The main external cue that synchronizes circadian rhythms is light. As light levels change at dawn and dusk, light-sensing photoreceptor cells in the eyes and brain relay signals to the central SCN clock, allowing it to stay in sync with the external day/night cycle.
Other external cues called zeitgebers can also help entrain circadian rhythms. For example, ambient temperature cycles caused by the rise and fall of the sun can influence circadian rhythms in some birds. Social interactions can also act as zeitgebers. Cockatoos have been observed using vocalizations at dawn as social cues to help synchronize their activity rhythms.
How does light induce chirping in the morning?
Here is how light causes birds to start chirping at dawn:
- As the sun rises, light levels increase and light enters the eyes.
- Photoreceptor cells in the retinas sense the light and send signals to the SCN.
- The SCN communicates with the pineal gland to shut down production of the sleep hormone melatonin.
- Lower melatonin levels and increased SCN activity induce wakefulness and greater neural activity.
- Circadian clock genes regulated by the SCN start to express proteins that support daytime functions.
- Increased neural activity and alertness switches on vocal centers involved in song production like the HVC and robust nucleus of the arcopallium (RA).
- Signals are sent to vocal and respiratory muscles to produce singing/chirping.
- Singing at dawn advertises territory and attracts potential mates.
So light information gets processed through the eyes, brain circadian clock, hormonal system, and vocal control centers to induce wakeful singing at dawn.
How do birds know when to stop chirping?
Just as increasing light induces morning singing, decreasing light triggers birds to stop singing in the evening. Here’s how:
- As the sun sets, ambient light levels start to decrease.
- Retinal photoreceptors detect the reduced light signal.
- They communicate light information to the SCN clock.
- The SCN induces the pineal gland to increase melatonin secretion.
- Elevated melatonin promotes sleep and reduces neural activity.
- SCN signals and melatonin shut down activity in vocal control centers like HVC and RA.
- Circadian oscillations switch gene expression profiles to support nighttime functions.
- Lower activity in vocal circuits leads to reduced singing behavior.
So the same light-sensitive molecular clockworks that stimulate morning singing also shutting singing down in response to dusk. This daily rhythmicity in vocalizations is an example of how circadian systems regulate cycles in behavior.
How do circadian rhythms continue in constant darkness?
Although external light cues help synchronize circadian rhythms, birds are able to maintain daily cycles even in constant darkness. For example, zebra finches continue to sing rhythmic morning and evening song for multiple days when kept in constant darkness.
Circadian rhythms persist without external cues because the SCN clock and molecular clockworks generate self-sustained oscillations that cycle every 24 hours. The proteins and gene expression cycles in the circadian clockwork form a feedback loop that keeps rhythms going continuously.
However, without any light cues, these endogenous circadian rhythms tend to drift slightly from 24 hours. Circadian periods range from 23-26 hours in constant darkness for most birds. But light exposure at dawn and dusk keeps recalibrating the internal circadian cycle each day.
How do seasons affect chirping?
Seasonal changes in day length modulate circadian cycles and singing behavior:
- Long days of spring/summer stimulate earlier dawn singing and later dusk singing.
- Short days of fall/winter delay the start of dawn singing and advance the end of dusk chorusing.
- Singing activity is correlated with duration of morning civil twilight.
- Some birds like sparrows also sing more overall on long days.
- Increasing day length triggers hormones that stimulate territorial and courtship behaviors.
- Photoperiodic hormones like testosterone increase vocal circuit development.
- Long days prime birds for breeding condition and induce seasonal plasticity in song.
So circadian and seasonal timing systems work together to regulate daily and annual rhythms in vocal behaviors.
How does temperature affect chirping?
Ambient temperature influences circadian rhythms and singing in birds:
- Lower temperatures often delay dawn singing in the morning.
- Birds may wait for warmer temps before getting active and vocalizing.
- High temperatures can suppress midday singing and activity.
- Heat stress may desynchronize circadian rhythms in some birds.
- Higher spring temperatures correlate with earlier onset of dawn chorusing.
- Colder winters may constrain chirping by increasing metabolic costs.
- Temperature cycles can directly entrain circadian pacemakers.
- Thermoperiods synchronize singing rhythms in some birds.
So both light and ambient temperature work together to modulate daily cycles in vocal communication signals.
How does social environment affect chirping?
The social context influences singing behavior in birds:
- Birds in larger groups start dawn singing earlier.
- Social cues help synchronize activity and chorusing rhythms.
- Some birds match singing patterns of neighbors.
- Predator calls can rapidly silence singing.
- Playback of recorded song may stimulate or inhibit vocalizations.
- Male birds sing more when fertile females are present.
- Territorial signals are highest early in breeding season.
- Social dominance can affect song output.
So both external timing cues and internal social environment modulate daily cycles in communication sounds.
How does artificial light affect chirping rhythms?
Increasing artificial night lighting is disrupting natural rhythms of daily singing:
- Light pollution can desynchronize circadian clocks.
- It alters natural light/dark cycles that entrain rhythms.
- Artificial light often causes birds to start singing earlier.
- Nighttime illumination may impair melatonin signaling.
- It can delay the end of evening chorusing.
- Some birds start singing again at midnight under artificial light.
- Long-term effects of altered rhythms are not well understood.
More research is needed on how artificial light at night impacts vocalization cycles and other aspects of avian biology.
Conclusion
Birds rely on internal circadian clocks synchronized by external light cues to control daily cycles in chirping behavior. Increasing light triggers activation of vocal circuits at dawn, while decreasing light silences singing in the evening. Circadian pacemakers and photoreception systems work together to ensure birds chirp at ecologically appropriate times of day. Social and environmental factors can also modulate daily rhythms in vocalizations. Artificial light is altering natural singing rhythms in some birds by disrupting the external cues that synchronize circadian systems. Understanding the circadian and seasonal biological timing systems underlying avian vocal behaviors provides fascinating insight into how neural mechanisms regulate rhythms in complex animal behaviors.