How Energy Efficiency and Other Factors Influence Bird Migration

bird migration

Story at-a-glance -

  • An impulse guided by instinct, bird migration has been hypothesized by scientists to be more than just a quest for balmier weather and an abundance of food; the instinct is based on energy efficiency
  • One driving force behind migration is the balance of energy cost to a bird flying long distances with the energy savings of settling in places where food is abundant and competition for it is less keen
  • Researchers set up a study to observe bird movement in a simulated planet to exhibit how birds populate areas such as the tropics due to lush vegetation, where they don’t need to work very hard to survive
  • Only about 15 percent of the total bird population worldwide are migratory birds, but experts say most migratory bird species are in decline, due in part to industrialization and subsequent habitat loss
  • Festivals are formed around the migration of birds in numerous places across the U.S., especially where different migrating bird species overlap

By Dr. Karen Shaw Becker

Bird migration may be one of the true wonders of the world. One reason for this is that there are 10,600 known species of birds, although most don’t take flight for the purpose of relocating in one fantastic commute for thousands of miles. Only about 15 percent of the total bird population worldwide commit themselves to these annual aerial expeditions.

How do they know when to go? How do they know where to go? One simple explanation for why they fly is that, like creatures confined to the ground, food is a driving compulsion, but also, in the sustenance-seeking pursuit, wise energy consumption is paramount. Interestingly, birds can sleep as they fly, which explains how many are able to travel so far in such short periods.

Like those whose search depends largely on where sustainable food sources are located, it often depends on the weather, or more specifically, the season. In the fall, when dipping temperatures and shorter days are harbingers of the impending expiration of abundant or even available food, there’s a sort of bird call, if you will, for survival and the quest for plenty that can only be answered by relocating.

One author called it “seasonal ecological adjustment on a gigantic scale.”1 It’s also nature’s answer to global bird distribution — or redistribution — accomplished through an impulse guided by instinct. When the roughly 5 billion birds enter U.S. air space every year, they often come from points as far south as the tropics, but it depends on the migratory species and where each needs to go to find a place to spend the winter.

Another factor is the nesting instinct, as many bird species are flying to find safe breeding grounds and a place to rear their young. However, a recent study published in the journal Nature Ecology & Evolution put the whole phenomenon down to an interesting premise — that for every migrating bird species, whether large or small, and as widely as their behaviors may vary, all that flying is really all about energy efficiency.2 The Washington Post notes:

“The energy cost to a bird of flying long distances is balanced out by the energy savings of being in a place where, in summer, there are lots of mosquitoes, flies, insect larvae and other avian delicacies, and there is relatively little competition for food.

The focus on energy acquisition and energy expenditure explains not only individual bird behavior — say, why one warbler chooses to fly from the Yucatan to Upstate New York — but also the geographical distribution of all birds collectively.”3

What Factors Influence Migration?

Ornithologists, who are experts on birds, understand the pattern known as the “green wave” that birds follow in the spring in their collective quests to seek food. The study calls it a “follow-the-energy model” that explains the way birds seem called to distribute themselves the way they do on a global scale. After all, there’s evidence that all birds may have had a common ancestor 170 million years ago.

Marius Somveille, Ph.D., the study’s lead author, set up the study to observe bird movement in a simulated planet created to exhibit how birds populate areas such as the tropics because of the availability of lush vegetation.

But it also shows how the food supply of an ecosystem is so readily available that birds don’t need to work very hard to survive. A problem appears in the scenario, though, due to the simple fact that so many birds end up settling there at the same time and for the same reason. Subsequently:

“Billions of birds are crammed together, competing for the same food. Meanwhile, the planet moves around the sun, winter turns to spring, the snow melts in the temperate zones. Viewed from space, there's all this enticing, suddenly efflorescent terrain far from the tropics.”4

Before long, Somveille says, it becomes so crowded that some bird species move to a new place where the competition for resources isn’t so keen. The theory of natural selection is never more evident than in areas of the world where food is abundant, temperatures are balmy and migrating birds find that producing healthy offspring has never been so easy.

In addition, the instinct that pushes birds to migrate is an inherited one, “switched on” by hormones that are in tune with changes in day length. One of the most puzzling aspects of bird migration is why some birds migrate and some do not, but while it seems to remain an “ornithological mystery,” the research is said to supply a much better understanding of the ecological phenomenon.

Looking at negative costs such as harsh winters seems easy enough to understand; even faced with flying thousands of miles, it’s worth it. The study observes:

“Our analysis provides a unified mechanistic explanation of bird migration as a behavior that allows highly mobile avian species to optimize their energy budget in the face of fluctuating resources and interspecific competition. Indeed, in the early parts of the simulation, virtual species are resident in the tropics …

Migration is a progressively more favorable strategy as interspecific competition reduces local energy supply, making it more energetically efficient to migrate to areas with unexploited seasonal surpluses of energy. In our increasingly crowded virtual world, species progressively start exploiting more extreme pockets of seasonally available energy supply, often migrating longer distances.”5

What Constitutes ‘Fall Migration?’

For avid bird-watchers, fall migration is something to look forward to, but seasons vary, so the exact times that birds take to the air en masse varies, too. For savvy “birders,” there are plenty of clues regarding when certain species may be in your vicinity if you know what to look for, depending on your local geography and a few other dynamics. Here’s when fall and subsequent bird flight takes place, generally speaking:

  • In the Northern Hemisphere, autumn falls between early to mid-September and mid- to late November or early December.
  • In the Southern Hemisphere, autumn is from mid- to late March extending through late May or early June.

Needless to say, many factors can influence when birds feel the urge to move from where they breed to where they’ll spend the winter, such as altitude and climate, but birds rely on considerations that often scientists can only hypothesize about. Here are five of the most common:

  • Light — The way the slant of the sun and overall light levels as it changes from day to day tells birds it’s time to fly
  • Temperature and climate — Cooling trends are an important indicator, as well as more rain, which in some areas is known as the “rainy season”
  • Food — Birds looking for food find there’s less available when it’s either been harvested or other birds have eaten it, so they move on, but sooner sometimes when unrelated issues like drought change the landscape
  • Location — Birds start out from where they are, which may seem counter-intuitive, but it makes a difference where they spend their summer; in the Arctic, for instance, they might start in July, as opposed to areas of South America, where they might take wing in late September
  • Offspring — If their babies are too young to take care of themselves properly, mature birds won’t migrate without them, so species that typically breed later in a season also migrate later

How the Modern World Impacts Bird Migration

Because migratory birds fly at night, such as those traveling by air from Mexico and South America, they’re forced to cross the Gulf in a single flight, and many are too exhausted to make it.

Another study describes that, rather than making their final destination of barrier islands and swamp areas surrounding the coast, they’ve fallen by the thousands, only to become easy prey for sharks, according to Pete Marra, Ph.D. of the Smithsonian Migratory Bird Center.6 And The Washington Post observes:

“The instinct to migrate can be overwhelmed by changes in the environment. The white stork — known to all students of science as the bird that delivers babies — normally commutes seasonally between Africa and Europe, but many members camp out permanently at landfills in Spain and Portugal, their migratory instinct switched off by the availability of Early Bird Specials every day of the year.”7

During the most prevalent times of bird migration, usually spring and fall, bird conservationists are ever conscious of the dangers modern life poses for birds, and for several different reasons. Artificial lights disorient birds as well as attract them, and they’re everywhere — on cell towers, stadiums, streetlamps and high-rise office windows. Birds also often collide with tall buildings in their nocturnal flights.

It could be one reason why most migratory bird species are in decline, says Susan Elbin, Ph.D. who led a study published by the Proceedings of the National Academy of Sciences (PNAS).8

The American Bird Conservancy9 lists felled rainforests, wooded areas becoming parking lots, and mining operations and wind turbines as examples of how industrialization has changed not just the landscape, but the air as a habitat for birds, as well as the loss of places where they rest and refuel along the way.

If survival of the fittest is part of the conversation, there’s also the fact that about 2 billion birds are killed by cats on an annual basis in the U.S. alone, according to a professor of ornithology at Cornell University, Amanda Rodewald, Ph.D. who stated:

“For birds that are migrating, they really need to do two things. They need to stay alive — not get eaten by predators, not fly into buildings, communication towers. They also need to find habitats that are suitable for them to gain weight. Fat — the energy in fat — is the currency in migration.”10

How You Can Observe Bird Migration

If you’re thinking about following bird migration in your area, keep in mind that some bird species can fly amazing distances in a single day, and if you’re not familiar with migration patterns, once-a-year foray sightings are easy to miss. Fortunately, there are several clues you can begin watching for, and you’ll soon get better at it knowing what to look for, such as:

  • Plumage — Did you know that the feathers of some bird species can signify the time for migration is close at hand, even if that particular species isn’t one that picks up and leaves? American goldfinches are one example of songbirds that begin molting so that their bright-colored plumage starts becoming dull. In addition, as young birds become less distinguishable from their parents, it’s another indicator.
  • Backyard bird species — As you watch the birds of your area and even your own backyard, you’ll notice specific habits some of them exhibit from year to year. You might think about taking notes on a calendar to journal your observations, including when you don’t see a certain bird type anymore — they’ve gone. Next year, you’ll know what to anticipate.
  • Genders — Sometimes the male and female birds of a species have different migration habits. For example, you may notice in the rufous hummingbird, which hail from North America, that the more colorful-throated males (meaning they’re dimorphic) will leave a few weeks before the females.
  • Flocks — Swifts and swallows are examples of migratory birds that form truly enormous flocks in the fall just before they leave on their migration journey. When they start congregating on wires or in trees, they’re “mustering the troops,” as it were, for the group flight.
  • Fall festivals — Whole festivals are formed around the migration of birds in certain areas, especially when the occurrences of different migrating bird species overlap, and the opportunities are everywhere across the U.S.

A wide array of rare birds such as white-collared seedeaters, red-billed pigeons, red-crowned parrots and Muscovy ducks are examples of what avid birders cross off their sighting lists during the Rio Grande Valley Birding Festival in Harlingen, Texas, usually held in November.

The Whooping Crane Festival takes place in Princeton, Wisconsin, and Migration Celebration in Ithaca, N.Y., offers interactive exhibits for kids, according to All About Birds.11

Some ways to help migrating birds on their flights include planting flowers and berry bushes and trees with birds in mind, The Spruce12 advises. Later, allow blooming flowers to fade away naturally instead of dead-heading them to give birds more feeding opportunities, even though it may be unsightly in some peoples’ eyes. Also avoid pruning trees and bushes, as they provide shelter for migrating birds.

Click Here and be the first to comment on this article
Post your comment