To everything, a season

Each morning the sun rises or the alarm rings, heralding our daily rituals, our circadian rhythms. Depending on the time of day, we care for our hunger pains, hygienic needs and necessary communications.

Humans share these daily routines with many other animal species functioning in the daylight hours. But for many animals acutely aware of the cycles of the earth, there is also a yearly or seasonal rhythm. These animals respond to a circannual rhythm, their behavior based on a specific time of year.

I've noticed this circannual behavior amongst the birds as I wander around the mountains. Having just a few months or weeks ago emerged from winter, adult birds began collecting materials for nest building. They could be seen in spring flying repetitively to a chosen site with twigs, fibers and soft materials, leaving empty-billed each time. Their behavior was wary and stealthy, to ensure privacy and protection from predators.

But now the quiet and relative peace of building, egg laying and incubation has ended. The mountains ring with cacophonous hungry, begging, fledgling birds.

What is the catalyst that begins the entire process? Each year, what prompts an adult bird to begin the mating rituals which lead to successive generations? For birds in the temperate zones of North America, the most relevant cue seems to be the lengthening day.

Receptors within birds' brains are sensitive to extremely low fluctuations in light intensity. It is through these receptors, rather than with their eyes, that birds sense the changing length of day.

In spring, these receptors are activated, resulting in hormonal changes which release an increase of testosterone in males and estrogen and other hormones in females. These hormones initiate annual behaviors such as migration, molting and reproductive activities. And it is these reproductive activities that lead us to observe their everhungry offspring.

At this time of year it may be difficult to distinguish the young from their parents, as the hatchlings grow quickly to adult size and often display the same plumage. The key feature in identifying fledglings is their behavior.

Although in an adult body, the movements of the young are ungainly, and often they look confused as to their purpose. Feathers are often puffed out and misarranged. Traces of down from the nest may still be present. Most telling though is their loud and incessant vocalization.

Once out of the nest and moving freely, juvenile birds are consummate consumers. Fledgling birds can be compared to hungry adolescent boys who are continually voracious.

Oftentimes both mother and father will provide food, unless the female is to lay another clutch, in which case the father becomes the sole food provider while the mother is incubating.

Fledgling birds are trained in a few weeks in the vast array of behaviors that are associated with feeding, beginning with searching for food. This skill requires locating the richest and most dependable food sources within a home range.

At times, food may be obvious, such as brightly colored berries, but more often the necessary nutrition is hidden. This skill may explain fidelity to one location, as birds can more efficiently find food in sites they are familiar with.

Once food has been found, it is necessary for birds to identify which foods are most beneficial, weighing nutritional gain against energy output. For example, small seeds may be easy to consume, but they are often less nourishing, and a large quantity may need to be collected.

Another consideration when identifying food is the ability to attain the nutrients.

Once food is found, it is often manipulated for consumption. Food may be eaten on the spot or cached to be eaten later, yet another tiring process for young birds to learn.

Beyond feeding skills there are lessons of flight, preening and communication, all necessary behaviors attained within the first few months of life, for soon after birds' eggs hatch, the seasons begin to change, and a new circannual rhythm begins.

As the days become shorter, birds experience a renewed sex hormone production, terminating reproductive behavior in preparation for possible migration.

Even more poignantly, longer nights signal to the fledglings that it is time to leave home and give the parents some peace. Do adolescent humans have such a cue?