Like other animals such as salmon, whales, and wildebeest, migratory birds appear to instinctively know which direction to go.
Why birds migrate has been studied for a long time, and it’s now universally accepted that they do so as the result of seasonal changes in resources like warmth and food. Like other animals such as salmon, whales, and wildebeest, migratory birds appear to instinctively know which direction to go, and for how long, and this is proving to be one of the greatest puzzles that people the world over are racing to solve.
Bird migration has been observed by humans since time began, with one of the earliest recorded mentions in The Iliad, written in the 8th century BC. Several passages in the Bible remark upon birds disappearing en masse, and of course with many birds being a source of food, it’s no surprise their absence was noted, so that, in time, it can be prepared for.
During World War II, pilots began to refer to mysterious patterns on their aircraft radar, calling them “angels” or “phantoms”. These patterns were eventually discovered to be flocks of migrating birds, and this realisation would pave the way for great advances in using radar to track migration pathways, known as flyways. Each species has their own flyway pattern, and that pattern is repeated every single year. How?
During World War II, pilots began to refer to mysterious patterns on their aircraft radar, calling them “angels” or “phantoms”.
Academic studies to discover these flyway mechanisms have been carried out all over the world since the early 1900s, but even with today’s huge advances in technology, we still have a lot to learn. Methods such as attaching transmitters, ringing birds by attaching coded bands to their legs, or even spraying them with dyes, have all contributed to a global amassing of knowledge about where a certain bird is in the world, how long it took to get there, and which way it went.
There are some excellent resources online that show you the flyways of many different species of bird, like this one from National Geographic, showing historical flyways, or this one, produced by the Cornell Lab of Ornithology and NASA, providing real-time tracking.
Whilst there has been no one singular study that has definitively cracked the mystery of migration, the science world mostly agrees that there are probably three major factors involved: using the sun or stars to navigate, landscape recognition, and perhaps the most intriguing of all, magnetic fields.
Landmarks of the universe
Some birds will only migrate by day, the Eurasian starling being one of them. In the 1950s, a study was performed that involved observing starlings close to their known migration date, who would align themselves with their migration direction when the sun was visible.
Then, using mirrors, the starlings were “tricked” by moving the sun into a different position. Amazingly, the birds would re-align with what they thought was the direction of their flyway, but were in fact facing the wrong way.
Finally, a cloudy sky was mimicked, and then the starlings would not face any particular direction. The conclusion was that these birds used the sun as a compass for their direction of travel, but there was no escaping the fact that there had to be another mechanism at play to help them on cloudy days.
After the sun compass experiment, the same thing led researchers to study birds who were known to migrate at night, using a planetarium, and flipping constellation patterns through 180 degrees – this did indeed cause those birds to realign, as with the starlings.
However, this study also failed to address the fact that the constellations changed on the horizon. This led scientists to believe, and this is a commonly held theory to this day, that birds can and do memorise the positions of the stars, and that they can quite literally keep that second star on the right, and straight on ‘til morning.
Down to earth
Looking out a window as the aeroplane takes off, the outside world soon becomes more two-dimensional looking, with roads and fields and towns splayed out beneath you like an intricate patchwork quilt. Birds of course also see this, and it has been shown that they use landscape recognition to find their way.
Homing pigeons don’t migrate, but they do find their way back to the exact same wooden loft-house every time they are released. People have tried many times to confuse them, boxing them up and driving them miles out of the way and then releasing them, only to watch them circle in the sky for a few minutes, heads flicking this way and that, and then suddenly, they’ve oriented themselves in a specific – and correct – direction, and off they go.
Migrating birds are thought to use this method when they are closer to their destination. Birds have excellent eyesight, and simply recognising a mountain range, coastline, a river, or even a church spire can keep birds heading in the right direction. This is why habitat destruction, urban development and even just natural events like landslides, avalanches and flooding can disorient a bird.
Reading between the lines
One of the most fascinating theories involves the discovery of magnetite in bird’s eyes, beaks, and ears. Magnetite is an oxide form of iron, and there are even traces of it in us. It has been suggested that, using this compound, birds can detect the Earth’s magnetic field, and “see” it as they fly, figuring out where north and south are, and therefore which direction home is.
Many scientists also believe that, along with this magnetic orienting system, the beak uses olfactory stimuli to keep the bird pointed in the right direction – it can smell where it should go, following clues borne on the winds, like the salt of the sea, or the pine of the forests.
When birds do arrive, there is one thing you can guarantee, and that is they are going to be hungry. Making sure arriving migrant birds don’t perish after all that effort is one of the best things you can do for them, and you can read about the different types of food that birds need here.
Planting trees or bushes that can act as a temporary home until they’ve managed to build a nest is a great idea, or if you have any outbuildings that you know are used each year, learn the time of year birds arrive, and leave those doors or windows open.
Most birds will need to repair or completely construct a nest, so familiarise yourself with what materials they need and leave those wind-blown twigs on the ground, or don’t completely clear off that moss from your shed roof, or pour some water into that dried up mud.
There's no place like home.
It’s not just the same type of bird that returns, it’s also the same bird. If you think you’re feeding the same blackbirds as last year, you may well be right.
All of the above theories about the physical how of migration have never actually been proven; evidence just appears to suggest this is what’s happening, that’s all. The most puzzling thing of all, though, is not how they get there, although that is pretty fascinating, but more to the point how do birds know that “there” is there.
How does that flock of geese know that particular salt marsh is home, and not a near identical one fifty miles up the coast? Ringing birds has shown that it’s not just the same type of bird that returns, it’s also the same bird. If you think you’re feeding the same blackbirds as last year, you may well be right.