Once upon a time the sky...

...was filled with flocks of passenger pigeons. The empty sky in the photo marks a sad anniversary -- 100 years since the last passenger pigeon, Martha, died.

It's startling to think that a species could go from being the most numerous bird population on the planet to extinction, and all in the space of not much over a century. As late as the 1800s, the sky was said to darken, sometimes for more than a day, as masses of these birds flew overhead. 

And somehow I don't find it completely reassuring that scientists are considering ways of renewing the species, using DNA from the feet of specimens in the Royal Ontario Museum. Such birds wouldn't be the same, as they would be crossed species with another breed of pigeon. Besides, really, what's the point.

The best lesson we can take from the passenger pigeon is a cautionary one, reminding us of the fragility of life -- even when it seems to be in abundance.