Birds, The Marvels of migration
The Marvels of migration
By Jeanette Rive
Last September, I received an alert that 30 million birds were expected to be flying overhead the following few nights on their way to the southern U.S., Central and South America or further afield. Some would land in the Ottawa area to rest and feed.
How do migrants, about 20 per cent of species worldwide, know where to go? How far do they fly? How do those who spend days in the air sustain themselves?
Twice a year, close to half a billion birds, whether songbirds or shorebirds, make an epic journey – in spring to their breeding sites, in fall to escape the cold. Many of the unusual ducks and geese we see on Dow’s Lake in the spring and fall are migrants. The warblers in our gardens spend the summer here to breed before heading south. East of Ottawa, many are thrilled by the sight of tens of thousands of Snow geese spending time on ponds and farmlands on their migration path.
Some travel for thousands of kilometres, non-stop. The records seem to be held by shorebirds such as godwits and sandpipers – many godwits fly almost 12,000 km each autumn from western Alaska to New Zealand, then back again in the spring. It takes them eight or nine days of uninterrupted flight. Even though we see flocks of geese flying overhead, most birds fly solo, far above us, each bird programmed from birth to know when and where to go.
Preparations for these endurance flights are triggered by hormones reacting to changes in the day’s length. They put on a massive amount of weight by storing fat, and internal organs unused during the flight, such as the gizzard, intestines and sexual organs, shrivel up. Their lung capacity increases, and pectoral muscles double in mass. When back on land, the organs regrow until their next flight.
Most birds fly at night when it’s cooler and there is less risk of dehydration. Their bodies have adapted to burn fat efficiently, drawing on water stored in muscle and organ tissue. Sleep? Birds seem to be able to snatch little naps, resting half of their brain at a time. Just as part of the chickadee’s brain, the hippocampus, expands before winter to remember cache locations, migrant birds’ brains expand during the build-up before departure to store spatial information.
How do they know where to go? There is still much unknown about avian orientation and migration but in the last 20 to 30 years, advances in technology have taught ornithologists much. Birds have been banded for hundreds of years for identification. When small satellite transmitters were developed to attach to birds, scientists were able to track flights of specific birds. These transmitters were at first too heavy for songbirds, but there are now miniature transmitters, small enough to attach to the tiniest bird, even to the wing of a Monarch butterfly. Radar weather maps have become so refined that meteorologists and ornithologists can determine whether objects in the atmosphere are biological, such as birds, insects and bats, or non-biological. Hence the alert about the 30 million birds in the sky.
Birds are guided by cues such as the landscape, stars, the sun’s movement and bands of polarized light, but the most important tool is magnetic orientation by two primary methods. Scientists mostly now agree that rather than magnetic deposits in a bird’s bill, the trigeminal nerve running through the bill provides birds with their map sense. In addition, researchers have concluded that a bird can orient itself to its position relative to the earth as vision cells in the retina respond to photons emanating from the stars and magnetic fields arcing out of the earth.
A little bird in the sparrow family arrives in our backyards from the northern boreal forest and cold mountains from mid-September on and leaves again in mid-March. The arrival of the Dark-eyed Junco, nicknamed the “snowbird,” is a sign that winter is on its way out. Because they spend the winter in a relatively cold climate, their feathers are about 30-per-cent heavier in winter than in summer. They are ground foragers, so you are likely to see them scruffing about below your feeders – it helps if you scatter seeds on the ground so they aren’t only reliant on spillage from other birds!
Starting in March, the spring migrants will be on their way north. Looking up in the sky, if it’s very quiet, you can even hear bird sounds – a world is on the move above you. It is essential that we help where we can by not leaving lights on in buildings, helping birds that do crash into our windows and making sure their staging grounds are protected.
Jeanette Rive is a Glebe bird enthusiast and Glebe Report proofreader.