Mysterious flights

The migration of birds has long been one of the great mysteries of nature. In Long Hops, Mark Denny adds to the mystery even as he tries his best to solve it.

Not that Denny fails in his task. Rather he points out so many variables in bird migration that simple solutions do not suffice.

We normally think of birds flying south for the winter and north for summer, at least in the northern hemisphere. Yet some birds migrate east-west. Others go up and down — up the mountain in warm weather, down again when it turns cooler. Robins have been known to change direction in mid-migration as temperatures change.

Blue grouses can’t fly. They migrate on foot.

Some birds migrate by sight, following coastlines or other markers. Others migrate by smell or by infrasound. Some follow their favorite foods. Others find their way by the stars. Others by internal compasses. Some seem to have been hatched with the knowledge of where they are supposed to be at a certain time of year.

As his title suggests, Denny is primarily interested in birds that travel long distances, sometimes thousands  of miles. He lives in Hawaii, and birds that migrate to and from there must fly a long way without stopping to rest or to refuel. He estimates that a high percentage of birds die simply because they lack the energy for such distances.

Denny’s book, published in 2016, is probably too basic for experts in this field, yet often too technical for us amateurs. There is enough here, however, for anyone with even the slightest interest in bird migration to find fascinating. And mysterious.