Welcome to the first column of Birdbrained Science! In this biweekly column, I’ll be talking about a variety of scientific topics — bird-related, brain-related and otherwise.
Today’s topic, however, is indeed bird-related! Autumn brings about a lot of things: cooler temperatures, leaves falling (well, usually — I’ll cover that some other week) and a lot of pumpkin-themed food. Here in the Northern Hemisphere, it also brings about migration season.
Most of us are probably familiar with the concept of migration, but in case you aren’t, migration is a journey undertaken seasonally where species travel to another part of the world that is, at that time, better suited for their survival somehow. The new location may have more food available, better weather or better breeding conditions. Most importantly, migration means that the species will retrace their steps and return to their former location when the seasons change.
You might not think about it, but migration truly is an impressive feat. There are lots of factors that go into a successful migration, such as being in good enough condition to complete the journey, getting the timing right and having enough food sources along the way. Some migratory species travel up to thousands of miles every year. Migration takes a lot of energy and can even be fatal.
While birds are the typical candidates that spring to mind when one thinks of migration (although not all birds migrate — feral pigeons, for instance), lots of other species migrate too, including whales, bats, turtles, butterflies and salmon, to name a few.
In the Northern Hemisphere, species tend to migrate south during the autumnal months, as that’s when the weather gets colder, and thus, resources tend to decrease. They’re like retired grandparents running off to Florida during the winter.
So we’ve got the who of migration, as well as the what, the when, the where and the why. But how does migration actually work? How do animals know when it’s time and how do they know where to go?
First, before migration begins, there’s usually a preparation period. To prepare for the journey, migratory animals often spend chunks of time gorging themselves to build up fat. Fat stores more energy than carbohydrates or protein, so it’s an ideal energy source for animals about to undertake a very energetically costly journey. Some animals also undergo bodily changes in preparation for migration — birds, for example, reduce the size of some of their internal organs and increase the size of their flight muscles and heart, optimizing their body for flight.
The next question is when to actually leave. Timing is crucial for migratory animals. Leave too soon and their final destination might not have enough resources to support their journey. Leave too late and they risk freezing or starving to death.
The exact mechanisms for migration timing aren’t fully known to us and what we do know seems to vary by species. Generally, migration seems to be set off by migratory animals sensing environmental changes, such as changes in temperature, food availability and photoperiod (day length). There is also likely a genetic component involved, though it’s not well understood. Timing can also vary based on the individual and other factors, such as weather. With climate change, there also appears to be increasing difficulty for migratory species to correctly time their migrations.
After sensing that it’s time to go, some animals exhibit restless behavior. This restlessness has been especially observed in migratory birds and is called zungunruhe.
Once they know that they have to go, how do they get to where they’re going? Again, it depends on the species, and the mechanisms still aren’t fully known to us. Many species seem to be able to navigate. Some theories suggest that for some species, such as monarch butterflies, this is done at least partly through magnetoreception, or the sensing of the Earth’s magnetic field. Other species, like starlings, seem to rely on the position of the sun or nighttime stars (although this is dependent on them being raised in a typical environment). However, this kind of light-based navigation can be especially difficult for birds at night, who get confused by artificial lighting.
Apart from light and magnets, some species also seem to be able to navigate using smell. Additionally, for certain species, like the seabird Caspian tern, migratory routes seem to be at least partly taught by elders to offspring. Some animals may also be using a combination of methods to navigate.
So, there you have it: part of the who, what, when, where, why and how of migration. Although there’s still a lot we don’t know, I hope you learned something, and I hope this helped you think a little bit more deeply about migration. If you’d like to track real-time bird migration predictions, take a look at Cornell’s BirdCast.
While pondering the movement of all these other animals across the world, I leave you with the following to reflect on from migratory animals:
How do you know when it’s time to leave? What new lives could be awaiting you on the other side of the horizon?