Flying Away on a Wing and Prayer

Migrating geeseFor weeks now, the air has been full of motion, full of flapping wings and rhythmic calls.  Fall has been rolling over us like an endless wave, washing down from the north along ancient streams with millions of birds at its crest.

We’re deep into migration season here in the north woods. Actually, I’m sad to say that we’re getting close to the end. When it starts in mid-August, it’s a subtle change, a gentle trickle, as species begin to disappear like lights blinking out on a Christmas tree. It begins with the little guys, warblers, shorebirds and hummingbirds that have to make the long trek to Central and South America. One of the first of the larger species to go are the Sandhill Cranes (Grus canadensis), their warbling calls ringing so high up in the sky that you often can’t see them against the clouds.

By mid-September, most of the songbirds are gone and we’re knee-deep in waterfowl, thousands of Canada Geese (Branta canadensis), Snow Geese (Chen caerulescens) and assorted ducks darken the skies and fill the farmers’ fields on their way to the southern United States. Now, in late October, the last of the waterfowl are heading off and Bald Eagles are following in their wake, on their way to places where the water bodies don’t free solid.

In Manitoba, about 85% of our birds migrate, but how do they know when it’s time to leave?

Just like the leaves on a tree and pretty much every other living organism on earth, birds are tuned into rhythms of time, the waxing and waning of day length. As the days get shorter, it triggers a response in their hypothalamus that cascades from the brain through the endocrine system, changing the cocktail of hormones coursing through their veins, resulting in an itch to move that just won’t go away. Biologists call that itch ‘migratory restlessness’ or zughenruhe.

During this period, birds get antsy, staying up well past their normal bedtimes and eating like it’s going out of style. There’s a point to this sudden change in behaviour. Most birds migrate at night and it’s important that they store as much energy as possible for their long flights. In fact, migrants will increase their fat loads to anywhere between 15 and 50% of their body weight, depending on the length of their trip.

Many have a very long way to go, some travelling thousands of kilometres to their wintering grounds. Getting there takes a good sense of direction and birds make use of a lot of different tools. Like the explorers of old, the sun and the stars play a big role in keeping birds on migratory routes that have been passed down for thousands of generation. Because many birds migrate at night, the setting sun offers a quick and easy compass to use for orienting their take-offs. Scientists have confirmed this by studying captive birds and using bring lights as a substitute celestial body. Whenever they moved the light, birds would change their take-off orientation accordingly, ensuring they were headed in the proper direction.

It’s not always possible to see the sun or the stars and once you’re in the air, they become less useful. In many cases, large landmarks, like rivers and mountain ranges serve as highways, guiding the flocks on their journey. However, when all visual cues fail, they still have one more fallback. Deposits of the mineral magnetite in their brains have been found to operate as a built-in compass, allowing individuals to pick up the earth’s magnetic field and orient themselves appropriately.

Studies are still trying to sort out how this all works from a biological level, but there’s no question of its usefulness. Work with homing pigeons have shown that messing up the electrical fields around the birds’ heads made it impossible for them to navigate accurately by scrambling their magnetic reception.

So as you watch a V of geese winging their way overhead, just take a moment to watch them. What you’re seeing is an amazing confluence of adaptations and millenia of evolution that have resulted in one of the most astonishing natural phenomena on earth that can be witnessed just about anywhere by anyone.

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Size Really does Matter

Autumn is a magical time, full of the fresh scent of fallen leaves, crisp, blue skies and the plaintive grunts of horny ungulates.

Yes, folks, it’s mating season in the boreal forest, known in the deer world as the rut. As the days get shorter, hormones start running rampant. Lean, muscular bodies have reached peak condition after a summer’s diet of green, leafy vegetation. Pheromones are being produced in vast quantities to be splashed onto every available surface. But, for deer and moose, it’s really all about the antlers, which have now been scrubbed clean of their protective velvet layer to gleam in the warm autumn light like a warriors sword.

Unlike what us human females like to tell our potential mates, size, in cervids, truly does matter. It’s all about who has the biggest rack.  Think about it, ladies, which would you be most attracted to: the young, scrawny male with little knobs that just barely make it past his ears, or the magnificent bull with antlers stretching over a meter from tip to tip? In female ungulates, the choice is ingrained: bigger is better because she’s not just seeing an impressive display, she’s seeing good genes.

Those antlers are what behaviourists call ‘honest signals’. Only males healthy enough to carry around all that weight can display them. Take moose (Alces alces), for example. Antlers of a large bull can span up to 5 ft (1.5 m) across and weigh 60 – 85 lbs (27 – 39 kg).  Male moose spend 25% of their energy in the summer just growing them, using more resources than females put into gestating young.  Antlers grow fast, starting to form in mid-summer and reaching full size by September. In fact, moose antler is the fastest growing bone tissue known. Growing it is one thing; then they have to carry them around for another few months.

It’s for that reason, that antlers are such a reliable signal of male health and why they are so attractive to females. If the male is strong and healthy enough to carry around 70 lbs on his head just to look good, he should father some healthy calves.

Another theory explaining why females choose males with larger antlers and other ornaments is known as the ‘sexy sons hypothesis’. The idea is that females choose males with the largest antlers, longest tail or brightest colours because they figure they will father sons who are equally attractive, ensuring they will pass on their genes to future generations. Either way you look at it, it has resulted in some pretty amazing looking animals.

These racks aren’t only for show. While in most cases, it can easy to determine a winner when two competing males cross paths, sometimes the match-up is too close to call on sight alone. In those situations, a fight usually breaks out. Like two hockey players, they launch at each other and lock heads, pushing and grunting until one eventually gives up. Although death is rare, fights can be dangerous. You can get cut, or gouged or, like the one pair of bull elk I watched, one can push the other into oncoming traffic.

For this reason, male ungulates spend a lot of time sizing each other up before engaging in any combat and they’ve evolved some more sophisticated ways to do that besides standing around comparing sizes. Those pheromones they splash about contain a lot of information on each individual’s  health and status. Many species, like white-tailed deer (Odocoileus virginianus), create pheromone markers all over their territory by scraping bark of small trees and rubbing scent glands at the base of their antlers onto the exposed wood, leaving their calling card. This allows other males entering the territory to decide if its worth taking this guy on without having to see him.

As the days grow shorter still, the furor eventually comes to an end. The females are with calf and the hormone levels in males peter out, leaving them exhausted, but hopefully satisfied. Having served their purpose, the antlers drop off, likely affording a great deal of relief, but it’s only a few short months before the next rack starts growing and the cycle starts again.