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Flight Lines: Leaf drop reveals handiwork of nesting birds

A summer nest of American goldfinches stands out in a maple tree once its leaves have fallen away. Keith Corliss / Forum Communications Co.

Remove the leaves from trees and all manner of secrets are exposed to the world. Secrets which had been hidden for the summery months in the dappled verdant bosom of leaf canopy now lay bare and vulnerable to chilly autumn winds.

Case in point is the abandoned American goldfinch nest in a tree alongside my driveway. Just weeks ago it was a shielded and shadowy place ideal for the purpose of quietly raising young. Its smallish cottony cup was well-placed in a branch crook and meant to catch the attention of nothing but doting goldfinch parents.

The birds long gone, their nest now stands boldly alone in the stark lengthening rays of late fall. It's a delicate ephemeral monument to summer and the energy-sapping effort required to procreate and pass along genes to another generation. And in some in-your-face sort of way, it serves to defiantly declare to predators that, indeed, the nest had successfully fledged three young goldfinches.

There is something magical about a bird nest even when no longer in use. This was quite recently a place where the hope of a future generation lay in the frail fragile composition of leaves, dried grass, and cattail down erected by the adult female using no other blueprint other than a primal instinct flickering on a few strands of seemingly random chromosomes. There, in the soft lined cup of a perfect nest, eggs were laid.

Eggs, those packets of promises held safely within the rigid but temporary armor of crystallized calcium carbonate, contain the product of copulation: developing embryos. With the goldfinch female solely responsible for incubation, 10 to 12 days pass until the young begin to hatch. At that point it's a total parental blitz as both male and female engage in the care and feeding of extremely fast growing young birds. After a two-week frenzy of rapid growth, the hatchlings take to the air and fly.

Consider the total timeline of the nest's useful existence. Figure a handful of days to build the nest and lay eggs, less than a fortnight later chicks emerge from their shells. Two weeks after that they fly away. In rough numbers that equals about four total weeks, give or take.

The most important element of the species' existence, the one which will determine the success or failure of this year's breeding effort, gets utilized for a mere four weeks. The only structure with any sense of place for these birds for the entire year is moved in to and out of within a short month.

It doesn't seem long enough somehow. After all, the only time goldfinches (or any other species for that matter) are tied to a particular location is while nesting. During every other moment of the year birds are free to wander. Whether it's a search for winter cover or food, a dash to evade a predator, or simply a desire to seek different territory, freedom of movement reigns. But once the decision is made to breed adults become inexorably anchored to this one structure, this one short-lived piece of architecture, this one amalgam of organic bric-a-brac where an entire year's worth of genetic hope is focused. Given the import of this moment, you'd think it should take more time.

In reality, however, the birds have it timed just right thanks to untold generations of genetic fine-tuning. Lingering any longer than necessary is dangerous. Every moment a nest remains occupied is another moment a predator may find it, another moment a passing thunderstorm may destroy the year's efforts, another moment for parasites to build up within the nest and endanger all its inhabitants.

The cavity nesters, those birds which carry out their breeding hidden from the world inside tree trunks or other structures, are through for the season too. We just can't readily see the aftermath of their efforts. No, it's the open nesting species - the robins, the goldfinches, the waxwings - whose handiwork is on display once the winds of autumn remove the leafy burden from the trees.

These nests will not be used again. These transient nurseries of yesteryear will slowly fade to dust during the course of the coming winter.

Under the influence of next spring's warmth though, leaves will once again fill the gray voids between the now-bare branches. Close behind will be songbirds, looking for the ideal spot to fashion an important but fleeting structure, one which will, for a few short weeks, become the locus of every fiber of the birds' existence.