With each passing week, I add more bird species to my list of those migrants that have returned for the summer.

The phenomenon of migration will forever keep me fascinated. After all, it’s something we have no firsthand knowledge of, dependent as we are on modes of transportation other than our own.

Most of us probably don’t think twice about commuting to Bangor from Old Town, or from Ellsworth to Bar Harbor. Just jump in the car, turn the key in the ignition, and depress the gas pedal. It hardly even requires thought; our minds are then occupied with the day’s work and worries. We don’t even notice the country we pass through – it just goes by in a blur, consumed by the almighty resource of gasoline.

Now stop and consider what it would be like to commute those distances under our own power, unaided and unprotected by the steel and glass shells of our automobiles.

Now everything matters. We need to notice our surroundings, taking into account the time of day, the weather, available food and cover, and the best travel routes. We need to make sure we are fit and ready for this journey, for if we run out of energy, we won’t make it to the end.

Incomprehensible, isn’t it?

Now take it a step further and imagine traveling many hundreds, even thousands of miles under our own power. As if this weren’t challenging enough, imagine facing poisoning by a deadly pesticide, such as the Swainson’s hawks were (and maybe still are) on Argentina’s pampas. The pesticide was so toxic, it killed the hawks as they were eating the grasshoppers the chemical was intended for.

Biologists estimated that between 15,000 and 20,000 hawks were killed in one winter, just in one small area of the pampas.

Or imagine being caught out on a cloudy, foggy night and breaking your neck when you run into a communications tower. This is the fate of an estimated 2 million to 4 million birds per year – in the eastern United States alone.

(The stories of the poisoned hawks and the birds killed by flying into broadcast antennae were powerfully documented in Scott Weidensaul’s book “Living on the Wind: Across the Hemisphere with Migratory Songbirds.” If you want to learn more about bird migration, I would highly recommend this book. It is available at the Fields Pond Nature Center bookstore.)

The above-mentioned scenarios are just the tip of the iceberg. Considering the increasing fragmentation and elimination of habitat, the dwindling food resources, the pollution – it’s a miracle any birds survive the journey.

Today, I heard the song of a white-throated sparrow. There’s another one that has made it – against all odds.

Known returned migrants as of April 16: osprey, bluebird, great blue heron, killdeer, tree swallow, song sparrow, white throated sparrow, red-winged blackbird, grackle, woodcock.

Chris Corio, a volunteer at Fields Pond Nature Center, can be reached at fieldspond@juno.com.