Birding Today: Migration driven by food sources

Many species of birds, like the Boreal breeding Blackpoll Warblers, return to the Amazon Forest weighing less than half an ounce, making the longest overwater voyage of any songbird. It has lost an estimated 88 percent of its population, but manages to be numerous for spring migration. Better than the world’s top athletes, they get 720 miles to the gallon, making their physical limits unparalleled.

Birds have lived on the wind since the dawn of time, as migration occurs somewhere at any time, concentrated in the spring and fall. Breeding birds are migrating north as failed breeders head south, while pelagic breeders travel the most combined yearly distances, some of which can go from as far south as Tierra del Fuego to the High Arctic regions.

Migration is driven by food sources as well as the insatiable pull of zugunruhe. Night skies are full of tens of millions of migrants that are in transit from their wintering homes, and if conditions are right, their flight calls can be heard for miles. Our ancestors watched these same sights as we do hundreds of years ago, but only over the last half century have we understood the stresses and strains of these twice a year migrations. Individuals from all walks of life participate in these birding events, and we are reminded that a world without birds would be truly sterile in more ways than one.

Many species of birds, like the Boreal breeding Blackpoll Warblers, return to the Amazon Forest weighing less than half an ounce, making the longest overwater voyage of any songbird. It has lost an estimated 88% of its population, but still manages to be numerous for spring migration. Better than the world’s top athletes, they get 720 miles to the gallon, making their physical limits unparalleled.

The Arctic Tern wins for greatest distances of between 22,000 and 25,000 miles, the size of a Mourning Dove. We now have tracking devices with geolocators, which can determine that many of these breeding birds fly 51,000 miles per year, but the true limits can be even greater.

Another all-star pilot is the Bar-tailed Godwit who colonizes across from the Bering Sea. These birds gather at Bristol Bay to perform hyperphagia or binge feeding. After two weeks they double their body weight with a 55% fat content. To make their migratory journey successful, they shrink the digestive system and heart, while the pectoral muscles increase in mass to carry the fat-laden bodies. Every fall they fly nonstop from Alaska to New Zealand, over 7,000 miles, an epic journey, as they cannot glide or soar.

There are also partial migrants and non-migratory birds, as well as overlap migrants like the Bald Eagle, and irruptive northern raptors and seedeaters. When the irruptives’ food supply collapses in the cycle and there is no food, they travel to the southern part of the U.S. This year, the Red-breasted Nuthatch had a huge breeding season, so some crops were unable to support the population, as we now experience.

Those that have a visceral sense of migration when most of the birds migrate are the Willow Ptarmagin, a sub-arctic grouse that sprouts snowshoes and the intestines grow 30-50% due to the only available food sources of dwarf willow and birch buds.

Who are the real toughies of the animal kingdom?

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