Avian behavior and physiology are embedded in time at many levels of biological organization. Biological clock function in birds is critical for sleep/wake cycles, but may also regulate the acquisition of place memory, learning of song from tutors, social integration, and time-compensated navigation.
This relationship has two major implications. First, mechanisms of the circadian clock should be linked in some way to the mechanisms of all these behaviors. How is not yet clear, and evidence that the central clock has effects is piecemeal.
Second, selection acting on characters that are linked to the circadian clock should influence aspects of the clock mechanism itself. Little evidence exists for this in birds, but there have been few attempts to assess this idea. At its core, the avian circadian clock is a multi-oscillator system comprising the pineal gland, the retinae, and the avian homologs of the suprachiasmatic nuclei, whose mutual interactions ensure coordinated physiological functions, which are in turn synchronized to ambient light cycles (LD) via encephalic, pineal, and retinal photoreceptors.
At the molecular level, avian biological clocks comprise a genetic network of “positive elements” clock and bmal1 whose interactions with the “negative elements” period 2 (per2), period 3 (per3), and the cryptochromes form an oscillatory feedback loop that circumnavigates the 24 h of the day.
We assess the possibilities for dual integration of the clock with time-dependent cognitive processes. Closer examination of the molecular, physiological, and behavioral elements of the circadian system would place birds at a very interesting fulcrum in the neurobiology of time in learning, memory, and navigation.
“The Bird of Time has but a little way
To fly—and Lo! the Bird is on the Wing.”Omar Khayam, The Rubaiyat
Although all free-living organisms express biological rhythms in many, if not all, aspects of their biochemistry, cell biology, physiology, and behavior, biological clock function is particularly obvious in birds (Cassone et al., 2009), so much so that temporal metaphors of birdlife pervade our poetic and collective consciousness.
The cock crows at sunrise. The nightingale sings in the evening. The early bird happens to get the worm. We recognize autumn by the sounds and splendor of geese flying south in their regiments of “vees,” and spring springs eternal with the coming of spring migrants and the advent of birdsong in our gardens.
As such, it is not surprising that some of the earliest scientific analyses of biological clock function and organization arise from work in birds. In some poetic sense, as is stated in The Rubaiyat, birds are time, and, scientifically, much of the study of birds is an exercise in the study of biological time-keeping.