The Role of External Cues in Circadian Rhythms

Birds rely heavily on environmental signals—primarily light—to entrain, or synchronize, their internal clocks to the 24-hour day. The central circadian clock in birds integrates these signals and coordinates physiological and behavioral processes accordingly.

What Happens When All External Cues Are Blocked?

When a bird is placed in an environment where all external cues are removed—such as in constant darkness or constant dim light—its biological clock is no longer entrained by the outside world. Instead, the bird’s internal circadian rhythm continues to operate based on its own endogenous (self-sustained) timing mechanism. This phenomenon is known as a free-running rhythm.

Scientific Evidence

• Studies show that birds kept in constant conditions (no light/dark cycle, no temperature fluctuations, no feeding cues) exhibit activity cycles that persist but gradually drift away from the exact 24-hour period, reflecting their internal circadian period.

• This free-running period is typically slightly longer or shorter than 24 hours, depending on the species and individual.

• The persistence of these rhythms, even in the absence of external cues, demonstrates the strength and autonomy of the biological clock in birds.

Why Doesn’t the Biological Clock Stop?

The circadian system in birds is robust, maintained by a network of oscillators in the brain and pineal gland that can sustain rhythmicity independently for extended periods. However, without environmental cues, the clock cannot synchronize to the natural day-night cycle, leading to a gradual “drift” in the timing of behaviors.

Conclusion

If a bird is kept in a closed cage such that all external cues are blocked, its biological clock will be free running. The bird’s internal rhythms persist, but they are no longer precisely aligned with the 24-hour day.

Correct answer: (3) It will be free running