A Billion Bird Flock: The magic, mystery, and biology of bird migration

sandpipers
A flock of sandpipers in Canada

Imagine a billion birds passing overhead during your morning commute. This isn’t the plot of some cheap Hitchcock knock off; it’s the exact situation John J. Audubon (yes, that Audubon) found himself in during the autumn of 1813 in Kentucky:

The air was literally filled with Pigeons; the light of noon-day was obscured as by an eclipse, the dung fell in spots, not unlike melting flakes of snow; and the continued buzz of wings had a tendency to lull my senses to repose…Before sunset I reached Louisville…. The Pigeons were still passing in undiminished numbers, and continued to do so for three days in succession… The banks of the Ohio [river] were crowded with men and boys, incessantly shooting at the pilgrims… For a week or more, the population fed on no other flesh than that of Pigeons, and talked of nothing but Pigeons.

This excerpt describes the Passenger Pigeon’s extraordinary fall migration from the Northern US and Canada to the Southern United States. Audubon’s words begin to describe the enormity of this species. During their heyday, colonies of Passenger Pigeons were so large that their combined weight broke tree limbs, and the amount of poop they produced influenced the composition of forest vegetation. During migration, a single flock of these birds over Ontario, Canada was estimated to be over 1 mile wide and 300 miles long, and to contain 3.5 billion birds.

Sadly, you and I will never see this awe-inspiring event in real life or on an episode of Planet Earth. As of 1914, over-hunting and habitat destruction killed all 3-5 billion Passenger Pigeons in the world.

While this story reminds us of the importance of conservation and sustainable practices, it also provides a stunning example of the scale and grandeur of bird migrations. It’s estimated that 1,800 species of birds (about 20% of all species) migrate in some form each year, many of them right under our noses (or more accurately, above our heads)! Migration is especially evident during fall in the United States when over 40-50% of all bird species migrate south to the Caribbean, and Central and South America.

In honor of the spring migration, I’m hatching this three-part segment of The Birdphiles to answer some common questions about bird migration. In the spirit of my previous article on Darwin’s finches pigeons, I’ll dispel some bird myths along the way. So, without further ado, lets make like penguins and dive right in!

Diversity in bird migration

Birds are some of the most conspicuous migratory animals in nature. For many people, the word “migration” immediately conjures the image of thirty geese flying in a “V” formation through the midday sky. Yet this one example can’t encapsulate the magnitude, beauty, and diversity of bird migrations. Individual species differ drastically in many aspects of migration including how far they travel, which route they choose, and their general migration strategy. Even within a species there is variability in which individuals migrate and when. Below, I’ve included an info-graphic to help illustrate some of the amazing diversity and beauty in bird migration.

Bird Migration 1[1]

Why migrate in the first place?

Let’s get one thing straight: migration is tiresome, dangerous, James Bond level business. Long bouts of flight require large amounts of energy and can damage flight feathers. Birds that migrate during the day fall prey to predatory birds like the Eleonora’s Falcon, while nighttime migrants are picked off in mid-flight by killer bats (at least in parts of Europe). Even without predation, large concentrations of birds may increase the spread of pathogens and parasites between individuals.

Eleonora’s Falcon

If I had to worry about killer bats and infection, I wouldn’t leave home, let alone fly an 11,000 mile round trip journey from Europe to China (like the Pied Wheatear). With all of these caveats, why do birds migrate in the first place?

Research and convention point toward both food availability and baby making as the driving forces behind bird migration. Birds need energy to defend a territory, attract a partner, build a nest and produce eggs. Moreover, once those eggs hatch, some species invest a huge amount of energy in feeding their chicks.

Many birds migrate to the Northern Hemisphere during spring when days become longer and food is more abundant. These conditions provide the fuel for reproduction and may provide parents more time to feed their hungry chicks.

As winter approaches, those hungry hatchlings become fully-fledged birds that compete with their parents for increasingly limited resources. These conditions make it advantageous for birds to migrate back to the Southern hemisphere where spring is just beginning.

It’s important to note that as with many aspects of migration, research on this topic is incomplete at best. It is likely that other factors such as habitat availability (for nest building) or avoidance of predators (which may be present in some places but not others) provide additional motivation to migrate, at least in some species.

Migration myth busting

While there are many things about bird migration we don’t know, there are a couple things we do know. For instance, some common “facts” about bird migration are complete bupkis. There are two myths in particular I’d like to address:

Myth 1: Birds in North America migrate south for the winter. This is a bit like thinking the universe revolves around the earth. Just because you live in North America doesn’t mean everyone else does. Most migratory species in North America are tropical birds that, over time, evolved to migrate north for their winter (that is, winter in the Southern hemisphere). Additionally, many species you see during winter have actually flown down from Canada or other Northerly regions. Not exactly what you traditionally think of as “south”.

Myth 2: Birds migrate because it’s cold outside. Don’t push your hatred of the cold off on my avian peeps! The fact is, while some birds migrate because they aren’t adapted to low temperatures, other birds have a variety of physical and behavioral adaptations to handle the cold.

Before being co-opted for flight, feathers were thought to serve a different purpose: research suggests that feathers were likely used by dinosaurs to regulate their body temperature! When a pigeon in NYC puffs up to twice its normal size, it traps air between its feathers, and if you can recall from high school physics class, air is a great insulator. The big challenge for a bird may actually be staying cool during the summer heat, both because of their fantastic insulation and because birds have the metabolism of Michael Phelps (though, of course, birds can handle heat as well).

A warm pigeon

Coming up next

So birds are eager beavers who migrate yearly to help with baby making, but how do they actually do it (the migration that is)? Want to know how a bird’s navigation abilities put Garmin to shame? I’ll give you a hint: if you can’t smell where this article is going, I guarantee a pigeon could (assuming where at least 300 miles near our final destination).

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Image Credits

Michael Wheelock is a birder, basketball player, and graduate student at The Rockefeller University. Catch him on twitter @MSWheelock and follow his series #TheBirdphiles.



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